HP 3465B Service manual

OPERAT ING AND SERVICE MANUAL
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MULTIMETER
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HEWLETT^ PACKARD

OPERATING AND SERVICE MANUAL
MODEL 3465B
MULTIMETER
Serial Numbers; 1530A00101 and greater
IMPORTANT NOTICE
Any changes made in instruments manufactured after this printing
will be found in a "Manual Changes" supplement supplied with this
manual . Be sure to examine this supplement, if one exists for this
manual, for any changes which apply to your instrument and record
these changes in the manual.
If the Serial Number of your instrument is lower than the one on
this title page, the manual contains revisions that do not apply to
your instrument. Backdating information given in the manual adapts
it to earlier instruments.
Where practical, backdating information is integrated into the text,
parts list and schematic diagrams. Backdating changes are denoted
by a delta sign. An open delta (A) or lettered delta (A/i^) on a given
page refers to the corresponding backdating note on that page.
Backdating changes not integrated into the manual are denoted by
a numbered delta (A-j) which refers to the corresponding change In
the Backdating Section (Section VI I I).
A This symbol is an international symbol meaning "refer to the
Operating and Service Manual." The symbol flags important
operating instructions in Section l it.
WARNING
1
To prevent potential fire or shock hazard, do not expose
equipment to rain or moisture.
Manual Part No. 03465-90010
Microfiche Part No. 03465-90060
Copyright Hewlett-Packard Company 1976
P.O. Box 301, Loveland, Colorado 80537 U.S.A.
Printed: August 1976

HEWLETT [hp: PACKARD
"0
CERTIFICATION
Hewlett-Packard Company certifies that this instrument met its published
specifications at the time of shipment from the factory. Hewlett-Packard
Company further certifies that its calibration measurements are traceable to the
United States National Bureau of Standards, to the extent allowed by the
Bureau's calibration facility, and to the calibration facilities of other
International Standards Organization members.
WARRANTY AND ASSISTANCE
This Hewlett-Packard product is warranted against defects in materials and
workmanship for a period of one year from the date of shipment, except that
in the case of certain components, if any, listed in Section I of this operating
manual, the warranty shall be for the specified period. Hewlett-Packard will, at
its option, repair or replace products which prove to be defective during the
warranty period provided they are returned to Hewlett-Packard, and provided
the proper preventive maintenance procedures as listed in this manual are
followed. Repairs necessitated by misuse of the product are not covered by this
warranty. NO OTHER WARRANTIES ARE EXPRESSED OR IMPLIED,
INCLUDING BUT NOT LIMITED TO THE IMPLIED WARRANTIES OE
MERCHANTABILITY AND EITNESS EOR A PARTICULAR PURPOSE.
HEWLETT-PACKARD IS NOT LIABLE EOR CONSEQUENTIAL DAMAGES.
If this product is sold as part of a Hewlett-Packard integrated instrument
system, the above warranty shall not be applicable, and this product shall be
covered only by the system warranty.
Service contracts or customer assistance agreements are available for
Hewlett-Packard products.
Eor any assistance, contact your nearest Hewlett-Packard Sales and Service
Office. Addresses are provided at the back of this manual.

Model 3465B
Table of Contents
TABLE OF CGIMTEIMTS
Section
I. GENERAL INFORMATION
1-1. Introduction
1-3. Description
1-5. Specifications
1-7. Instrument and Manual Identification . . ..
1-9. Options
1-11. Warranty Exceptions
1-13. Accessories
1-15. Safety Considerations
Page
1
Section Page
II. INSTALLATION 2-1
2-1. Introduction 2-1
2-3. Initial Inspection 2-1
2-5. Power Requirements 2-1
2-7. Grounding Requirements 2-1
2-10. Environmental Requirements 2-1
2-12. Repackaging for Shipment 2-1
2-16. Power Cords and Receptacles 2-1
Section Page
III. OPERATING INSTRUCTIONS 3-1
3-1. Introduction 3-1
3-3. Front Panel Features 3-1
3-5. Turn-on and Warm-up 3-1
3-7. Internal Battery Voltage Measurement
and Recharging 3-1
3-9. Low Battery Voltage Detection 3-1
3-11. Overload Indication 3-1
3-13. AC Voltage Measurements 3-1
3-14. AC Voltage Ranges 3-1
3-16. DC Voltage Measurements 3-2
3-17. 10 mV Range Zero Adjust 3-2
3-19. DC Voltage Ranges 3-2
3-21. Current Measurements 3-3/3-4
3-23. AC Current Ranges 3-3/3-4
3-25. DC Current Ranges 3-3/3-4
3-27. Ohms Measurements 3-3/3-4
3-28. Ohmmeter Ranges 3-3/3-4
3-30. Ohmmeter Reference Current 3-3/3-4
Section Page
IV. THEORY OF OPERATION 4-1
4-1. Introduction 4-1
4-3. Description 4-1
4-6. Simplified Theory 4-2
4-8. Signal Conditioning 4-2
4-18. Analog-to-Digital (A-D) Converter 4-3
4-26. Logic Section 4-4
4-35. Display 4-4
4-37. Power Supply 4-5
4-39. Detailed Theory 4-5
4-41. Precision Resistor Pack (R7 5) 4-5
Section Page
4-43. Ohms Converter 4-5
4-50. AC-to-DC Converter 4-6
4-56. A-D Conversion Using a
Monopolar Reference 4-6
4-64. Data Accumulator 4-8
4-69. Display 4-8
4-72. Power Supply 4-10
4-78. -t 10 V Series Voltage Regulation. . 4-11/4-12
4-80. Battery Low-Voltage Detection . . . 4-11/4-12
Section Page
V. MAINTENANCE 5-1
5-1. Introduction 5-1
5-3. Test Equipment Required 5-1
5-5. Performance Tests 5-1
5-6. DC Voltmeter Accuracy Test 5-1
5-8. DC Ammeter Accuracy Test 5-1
5-10. Ohms Accuracy Test 5-2
5-12. AC Voltage Accuracy Test 5-2
5-14. AC Ammeter Accuracy Test 5-2
5-16. Alternate AC Ammeter Accuracy Test
(200 mA/2000 mA, 40 Hz to 1 kHz) .. 5-4
5-19. AC Normal Mode Rejection Test 5-5
5-21. AC Effective Common-Mode
Rejection Test 5-6
5-23. DC Voltmeter Input Resistance Test. ... 5-6
5-25. AC Voltmeter Input Impedance Test . . . 5-6
5-27. Adjustment Procedures 5-8
5-29. Disassembly Procedure 5-8
5-30. Power Supply Adjustment 5-9
5-34. Input Amplifier Adjustments 5-9
5-37. Ohms Converter Adjustments
(R58 and R69) 5-10
5-39. AC-DC Converter Adjustments 5-10
Section Page
VI. REPLACEABLE PARTS 6-1
6-1. Introduction 6-1
6-4. Ordering Information 6-1
6-6. Non-Listed Parts 6-1
6-8. Parts Changes 6-1
6-10. Proprietary Parts 6-1
Section Page
VII. TROUBLESHOOTING AND CIRCUIT
DIAGRAMS 7-1
7-1. Introduction 7-1
7-3. Schematic Diagrams 7-1
7-5. Preliminary Check 7-1
7-6. Visual Inspection 7-1
7-8. Preliminary Troubleshooting 7-1
7-11. Analog/Digital Isolation 7-1

Table of Contents
Model 3465B
Section Page
7-13. Analog Troubleshooting 7-1
7-14. Analog Isolation 7-1
7-18. Power Supply Faults 7-2
7-20. Signal Conditioning Faults 7-3
7-26. Analog-to-Digital Converter Faults 7-3
7-29. Integrator/Slope Amplifier/
Comparator/Auto-Zero 7-3
7-31. Digital Troubleshooting 7-4
7-33. Display and Display Driver Verification. . 7-4
Section
7-35.
7-37.
7-40.
7-42.
7-44.
Page
Display and Display Driver Quick Test .. 7-4
Display and Display Driver Verification
and Troubleshooting Test 7-5
Polarity, Zero Detect and
Clock Circuit Verification 7-5
Analog Switch Lines and Control State
Counter Verification 7-5
Data Accumulator Input/Output
Verification 7-5
LIST OF TABLES
Table Page
Table
1-1. Specifications
1-2
5-6.
1-2. General Information . . .1-3/1-4
3-1. Ohmmeter Current Through Unknown ...
. .3-3/3-4
5-7.
4-1. Display Interface Connections
4-9
5-1. Test Equipment Required
5-0
5-8.
5-2. DC Voltmeter Accuracy Test
5-1
5-3. DC Ammeter Accuracy Test
5-2
5-9.
5-4. Ohms Accuracy Test
5-2
6-1.
5-5. AC Voltage Accuracy Test
5-3
6-2.
6-3.
Page
AC Ammeter Accuracy Test, 200 /rA
through 20 mA Ranges 5-4
AC Ammeter Accuracy Test, 200 mA and
2000 mA Ranges 5-5
AC Ammeter Accuracy Test 200 mA and
2000 mA Ranges 5-5
Power Supply Jumpers 5-10
Standard Abbreviations 6-1
Code List of Manufacturers 6-2
Replaceable Parts 6-3
LIST OF ILLUSTRATIONS
Figure
2-1. Power Receptacles 2-2
3-1. Front Panel Features 3-2
4-1. Basic Block Diagram and Measurement
Sequence 4-1
4-2. Simplified Diagram, Ohms Converter 4-2
4-3. Block Diagram, AC-to-DC Converter 4-3
4-4. Block Diagram, Power Supply 4-5
4-5. Over-Voltage Protection Circuit 4-5
4-6. Basic Diagram, AC Converter Amplifier 4-6
4-7. Functional Diagram, A-D Converter 4-7
4-8. Data Accumulator 4-9
4-9. Basic Diagram, DC-to-DC Converter 4-10
Figure Page
4-10. Simplified Diagram, DC-to-DC Converter 4-10
5-1. DC Ammeter Accuracy Test 5-1
5-2. Ohms Accuracy Test 5-2
5-3. AC Ammeter Accuracy Test 200 /lA through
20 m A Range 5-3
5-4. AC Ammeter Accuracy Test
200 mA and 2000 mA 5-5
5-5. AC Normal-Mode Rejection Test 5-6
5-6. AC Kffective Common-Mode Rejection Test. .. . 5-6
5-7. DC Voltmeter Input Resistance Test 5-7
5-8. AC Voltmeter Input Impedance Test 5-7
5-9. Multimeter Adjustment Location 5-8

Model 3465 B
Seelion 1
SECTION I
GENERAL INFORIVIATION
1-1. IIMTRODUCTIOW.
1-2. This section contains general information concerning
the -hp- Model 3465B Multimeter. Included is an instru
ment description, specifications, information about instru
ment and manual identification, option and accessory in
formation and safety considerations.
1-3. DESCRIPTION.
1-4. The -hp- Model 3465B Multimeter is a 4-1/2 digit, five
function digital multimeter. The five functions are dc volts,
ac volts, dc current, ac current and ohms. Measurements
can be made to four significant digits with a sample rate of
2-1/2 readings per second. Throughout this manual, the
3465B Multimeter will be referred to as Multimeter.
1-5. SPECIFICATIONS.
1-6. Instrument specifications are listed in Table 1-1. These
specifications are the performance standards or limits
against which the instrument is tested. Any change in the
specifications due to manufacturing, design or traceability
to the U.S. National Bureau of Standards will be covered by
a change sheet. Additional information describing the oper
ating characteristics are not specifications but are supple
mental information for the user.
1-7. INSTRUMENT AND MANUAL IDENTIFICATION.
1-8. Hewlett-Packard uses a two-section serial number. The
first section (prefix) identifies a series of instruments. The
last section (suffix) identifies a particular instrument within
the series. If a letter is included with the serial number, it
identifies the country where the instrument was manufac
tured. This manual is kept up-to-date with the instrument
at all times by revision. If the serial prefix of your
instrument differs from the one on the title page of this
manual, refer to Section VIll for backdating information
that will adapt this manual to your instrument. All corres
pondence with Hewlett-Packard should include the
complete serial number.
1-9. OPTIONS.
1-10. The following is a list of the options available for the
multimeter. Multimeter options are available to allow oper
ation from various line voltages.
Option
100
115
210
230
910
Description
86 - 106 V ac 48 - 440 Hz
104 - 127 V ac 48 - 440 Hz
190 — 233 V ac 48 — 440 Hz
208 - 250 V ac 48 - 440 Hz
An additional Operating and
Service Manual
1-11. Warranty Exceptions.
1-12. Batteries are warranted for 90 days.
1-13. ACCESSORIES.
1-14. The following accessories are available to extend the
usefulness of your Multimeter:
a. Model 11096B RF Probe, 100 kHz to 500 MHz
(down 3 dB at 10 kHz and 700 MHz), for use on the 10 V
and 100 V ranges in the DCV function only.
b. Model 11002A Test leads, dual banana to dual alli
gator.
c. Model 11003A test leads, dual banana to probe and
alligator.
d. Model llOOOA dual banana to dual banana, 44 in.
e. Model 341 lOA soft vinyl carrying case.
f. Model 3411 lA HV Probe, 40 kV dc.
g. Model 34112A Touch — Hold Input Probe.
1-15. SAFETY CONSIDERATIONS.
1-16. This operating and service manual contains cautions
and warnings alerting the user to hazardous operating and
maintenance conditions. This information is flagged by a
camion or warning heading and/or the symbol A . The
A symbol appears on the front panel and is an inter
national symbol meaning "refer to the Operating and
Service Manual' . This symbol flags important operating
instructions located in Section 111. To ensure the safety of
the operating and maintenance personnel and retain the
operating condition of the instrument, these instructions
must be adhered to.
1-1

Section I
Model 3465 B
Table 1-1. Specifications.
DC VOLTMETER
Ranges: 20 mV, 200 mV, 2 V, 20 V, 200 V, 1,000 V
Maximum Input: 1,000 V (DC + Peak AC)
Accuracy (1 year + 23°C ± 5°C):
Range Specification
20 mV
200 mV through 200 V
1000 V
± (0.03% of reading + 2 counts)
± (0.02% of reading + 1 count)
± (0.025% of reading + 1 count)
Temperature Coefficient (0°C to 50°C): ± 0.003% of Read-
ing/°C
Effective Common-Mode Rejection (with 1 k£7 imbalance in
either lead):
AC: > 120dBat 50/60 Hz ± 0.1%
AC Normal-Mode Rejection:
> 60 dB at 50/60 Hz + 0.1 %
Input Resistance:
20 mV through 2 V ranges: (80% R.H.) ^ 1o' ^ rz
20 V through 1,000 V ranges: 10 Mfi ± 1 %
AC VOLTMETER
Ranges: 200 mV, 2 V, 20 V, 200 V, 500 V (500 V Max)
Qverrange: The maximum reading decreases linearly from
19,999 at 10 kHz to 10,000 at 20 kHz.
Accuracy: 1 year + 23°C ± 5°C)
500 V
(500 V rms)
MAX
± (0.5% of reading + 5 counts)
200 V
20 V
2 V
200 mV
± (0.1 5% of reading
-I- 5 counts)
± (0.5% of reading
+ 1 5 counts)
-+■
40 IK 2 K 10 K 20 K
INPUT VOLTAGE FREQUENCY (Hz)
Temperature Coefficient (O^C to 50^0: ± (0.005% of Reading
+ .2 counts)/°C
Input Impedance: 1 M ± 1 % shunted by < 100 pF
DC AMMETER
Ranges: 200 nA. 2 mA, 20 mA, 200 mA, 2,000 mA
Maximum Input: 2 A from < 250 V source
Protection: 2 A/250 V fuse (normal blow)
Voltage Burden:
Range
200 mA - 200 mA
2,000 mA
Max Burden at Full Scale
< 250 mV
< 700 mV
Accuracy: 1 year-i-23°C ± 5°C)
Range Specification
200 mA, 2 mA
20 mA
200 mA, 2000 mA
± (0.07% of reading + 1 count)
+ (0.11 % of reading + 1 count)
± (0.6% of reading 1 count)
Temperature Coefficient (0®C to 50*^0:
Specification
Range
± (%of Reading)/°C
200 mA
± 0.006%
2 mA, 20 mA
± 0.004%
200 mA, 2,000 mA
± 0.01%
AC AMMETER
Ranges: 200 mA, 2 mA, 20 mA, 200 mA, 2,000 mA
Qverrange: The maximum reading decreases linearly from
19,999 at 10 kHz to 10,000 at 20 kHz.
Accuracy: (1 year,+ 23°C ± 5°C)
(% of Reading + Counts)
2000 mA
^ 200 mA f ± (0.8%
+ 5)
UJ
20 mA
2 mA
cc
a:
g 200 mA
u
<
± (0.4%
+ 5)
± (0.65%
- + 5)
± (0.25% + 5)
± (0.6%
-•-15)
40 IK 2 K 10 K 20 K
INPUT CURRENT FREQUENCY (Hz)
Temperature Coefficient (0°C to 50°C); ± 0.01% of Read-
ing/°C.
Protection: 2A/250 V fuse (normal blow)
Voltage Burden:
Range
Max Burden at Full Scale
200 mA - 200 mA
< 250 mV
2,000 mA
< 700 mV
OHMMETER
Ranges: 200 2 kn, 20 kfi, 200 kfi, 2,000 kJ2, 20 Mr2
Accuracy: (1 year + 23°C ± B'^C)
Range
200 n
2 kf2 through 2 MS7
20 Mn
Specification
± (0.02% of reading -t- 2 counts)
+ (0.02% of reading + 1 count)
+ (.1 % of reading + 1 count)
Temperature Coefficient {0°C to 50®C)
Range
Specificatio
200 n through 2 Mil
20 MQ
n
± (% of Reading)/°C
± 0.0015%
± 0.004%
1-2

Model 3465 B
Section 1
Table 1-2. General Information.
Maximum Input Voltages:
Between Input HIGH (V, H.) and COM:
Function Max Voltage
DC Volts
1000 V (dc -1- peak ac)
AC Volts 600 V dc; 500 V ac rms;
800 V peak ac
Ohms 350 V (dc + peak ac)
Between AMPS (A), HIGH (V, and COM terminals and
ground:
± 500 V (dc + peak ac}
Reading Rate: 2.5 samples per second
Overload Indication: Display Blanks except for overrange "1"
and decimal point (also polarity sign on DCV or DCA FUNC
TIONS).
Ohms Terminal Characteristics:
Configuration: 2 wire
Open-circuit voltage: < 5 V max.
Overload protection; 350 V (dc + peak ac)
Nominal current through unknown resistance:
Range
Current
200 n 1 mA
2 kJ2 1 mA
20 kii
10 mA
200 ka
IOmA
2000 kn
1 juA
20 Mn 0.1 mA
Power Requirements:
Power: AC Line; 48 — 440 Hz
86 - 106 V Option 100
104 - 127 V Option 115
190 -233 V Option 210
208 -250 V Option 230
Battery (Rechargeable NiCad): 6 hours minimum continuous
operation
Recharge Time: 8 hours (instrument off)
Total Instrument Power Dissipated:
Instrument on, Battery Operation: < 1 watt
Instrument on, Line Operation: < 10 VA
Battery Test: Depress DCV and 10 Mf2: Recharge NiCad bat
teries if the display reading is < 0.380.
Environmental Considerations:
Operating temperature: 0®C to 40°C (32°F to 104'^F)
Humidity range: 95% at 40°C
Storage temperature: - 20°C to + 50°C (-4°F to 122°F)
1-3/1-4

Model 3465B
Section II
SECTION II
INSTALLATION
2-1. INTRODUCTION.
2-2. This section contains information and instructions for
the installation and shipping of the Multimeter. Included
are initial inspection procedures, power and grounding
requirements, environmental information and instructions
for repackaging for shipment.
2-3. INITIAL INSPECTION.
2-4. This instrument was carefully inspected both mechani
cally and electrically before shipment. It should be free of
mars or scratches and in f)erfect electrical order upon
receipt. To confirm this, the instrument should be inspec
ted for physical damage in transit, and the electrical
performance should be tested using the performance tests
outlined in Section V. If there is damage or deficiency, see
the warranty inside the front of this manual.
2-5. POWER REQUIREMENTS.
2-6. This Multimeter will operate on ac line voltage or from
internal rechargeable NiCad batteries. AC line voltage
options are described in Table 1-2.
lOd'^F) or stored outside the ambient temperature range of
- 20OC to + SQOC (- 40F to 1220F).
A
Verify that the ac power source matches the
power requirement of the instrument as marked
on the option label affixed to the rear of the
instrument.
2-7. GROUNDING REQUIREMENTS.
2-8. To protect operating personnel, the National Electrical
Manufacturer's Association (NEMA) recommends that the
instrument panel and cabinet be grounded. Multimeters are
equipped with a three-conductor power cable which, when
plugged into an appropriate receptacle, grounds the instru
ment. The offset pin on the power cable is the ground wire.
2-9. To preserve the protection feature when operating
from a two-contact outlet, use a three-prong to two-prong
adapter and connect the green pigtail on the adapter to
power line ground.
2-10. ENVIRONMENTAL REQUIREMENTS.
2-11. The Multimeter should not be operated outside the
ambient temperature range of O'^C to 40°C (32°F to
WARNING
To prevent potential electrical or fire hazard,
do not expose equipment to rain or moisture.
2-12. REPACKAGING FOR SHIPMENT.
2-13. The following paragraphs contain a general guide for
repackaging the instrument for shipment. Refer to Para
graph 2-14 if the original container is to be used; 2-15 if it
is not. If you have any questions, contact your nearest -hp-
Sales and Service Office (see back of Manual for office loca
tions).
NOTE
If the instrument is to be shipped to Hewlett-
Packard for service or repair, attach a tag to
the instrument identifying the owner and indi
cating the service or repair to be accomplished.
Include the model number and full serial
number of the instrument. In any correspon
dence, identify the instrument by model num
ber and full serial number.
2-14. Hace instrument in original container with appropri
ate packing material and seal well with strong tape or metal
bands. If original container is not available, one can be
purchased from your nearest -hp- Sales and Service Office.
2-15. If original container is not to be used, proceed as
follows:
a. Wrap instrument in heavy paper or plastic before
placing in an inner container.
b. Place packing material around all sides of instrument
and protect panel face with cardboard strips.
c. Place instrument and inner container in a heavy
carton or wooden box and seal with strong tape or metal
bands.
2-16. POWER CORDS AND RECEPTACLES.
2-17. Figure 2-1 illustrates the plug cap configurations that
2-1

Section II
Model 3465B
are available to provide ac power to the Multimeter. The
-hp- part number shown directly below each plug cap draw
ing is the part number for the power cord set equipped with
the appropriate mating plug for that receptacle. The appro
priate power cord should be provided with each instrument.
However, if a different power cord set is required, notify
the nearest -hp- Sales and Service Office and a replacement
cord will be provided. The instrument ac power input
receptacle and cord set appliance coupler meet the safety
specifications set by the International Commission on Rules
for the Approval of Electrical Equipment (GEE 22).
8120-1348 8120-0698
126V-6A- 260V-6A*
■UL LISTED FOR USE IN THE UNITED STATES OF AMERICA
Figure 2-1. Power Receptacles.
2-2

Model 3465B
Section ill
SECTION III
OPERATING INSTRUCTIONS
3-1. INTRODUCTION.
3-2. This section contains instructions for using the Multi
meter for making dc voltage, ac voltage, dc current, ac
current and ohms measurements. The section also contains
a description of the front and rear panel features.
WARNING
To prevent potential electrical or fire hazard,
do not expose the Multimeter or its accessories
to rain or moisture.
3-3. Front Panel Features.
3-4. An illustration and description of the front panel is
provided in Figure 3-1. All controls and connectors are
identified and briefly described.
3-5. Turn-on and Warm-up.
3-6. For specified measurement accuracy, allow the instru
ment to warm-up for at least 10 minutes.
Before connecting the instrument to ac power,
verify that the ac power source matches the
power requirement of the instrument as marked
on the option label affixed to the rear of the
instrument.
3-7. Internal Battery Voltage Measurement and Recharg
ing.
3-8. The Multimeter contains a feature allowing the user to
check battery strength to determine the need for battery
recharging. The procedure is to place the Multimeter in the
DCV function and depress the 10 megohms range switch. If
the absolute value of the front panel display is .380 or less,
recharge the batteries. Recharging of the NiCad batteries is
performed by operating the Multimeter on an ac source.
Measurements can be made with the Multimeter operated
from the ac source during the recharging period.
NOTE
After 8 hours, a completely discharged battery
will be fully charged with ac line voltage con
nected and the POWER switch off. Shorter
charge periods will allow reduced battery
operating time. There is no danger of over
charge. For convenience, overnight charging is
recommended.
3-9. Low Battery Voltage Detection.
3-10. A battery source safety feature of the Multimeter is
low battery voltage detection circuit which turns the instru
ment off when battery voltage reaches a low level. This
protects against cell reversal of the NiCad batteries. If
during operation the display disappears or immediately
after turn-on the display appears and disappears after
several seconds, low battery voltage is indicated. To verify
low battery voltage, the procedure described in the pre
ceding paragraph can be used or verify by placing the OFF/
ON switch to OFF and to ON again. The display will appear
and again disappear. Operation from an ac line source and
recharging of the NiCad batteries is required when this
occurs.
NOTE
In protecting batteries and circuitry, the low
battery voltage detection circuit may shut
down the instrument if the power switch is
momentarily turned off then back on. To
restore normal operation, the instrument
should be turned off with the front panel
power switch for a minimum of 3 seconds.
3-11. Overload Indication.
3-12. The Multimeter is capable of displaying 19999 for all
functions and ranges. There are maximum voltage limita
tions in DCV and ACV, however (see ac and dc voltage
measurement paragraphs). In an overload condition where
the input exceeds 19999, the last four digits blank and the
overrange "1" and decimal point will be displayed. The
polarity sign is also displayed in the dc volts and dc current
functions in the overload condition.
3-13. AC VOLTAGE MEASUREMENTS.
A
Maximum input voltage in the ACV FUNC
TION is 500 V rms, 800 V peak and 600 V dc.
Do not exceed these voltages or damage to the
instrument will occur.
3-14. AC Voltage Ranges.
3-15. The ACV FUNCTION has five ranges from 200 mV
to 500 V. Each range has a maximum display reading of
3-1

Section III
Model 3465B
FUNCTION
RANGE
TtV ;4fKSI
POWER rnv '^-V ~A -vA «
□□□□□□□
A
Do no! apply a vohugc greater than ± 500 V de or 500 I' /teuA between
any terniinal atid eliassis ground or damage to the iiisiruntent will
oceiir.
©
©
©
©
OFF/ON Switch — Pushbutton push on/push off switch.
FUNCTION Switch — Function markings are located
above each pushbutton switch.
— V - DC Volts
^ V - AC Volts
= DC Amps
~ A = AC Amps
n = Ohms
Display — Indicates the measured value and polarity of dc
volts or amps.
RANGE Switch. Range markings are located above each
pushbutton switch. Color bands identify the range
switches associated with each function switch.
©
©
©
©
DCV/ACV/OHMS — High input terminal.
A Symbol — This symbol is an international symbol
meaning "refer to the Operating and Service Manual".
This symbol will appear in this section of the manual flag
ging operating instruction information.
COM Input Terminal — This terminal is connected to
circuit ground for all measurements except ohms. In the
ohms function, the COM terminal is disconnected from
circuit ground.
DC/AC AMPS — High input terminal. 2 amp fuse located
behind removable "A" terminal cap.
Figure 3-1. Front Panel Features.
19999. However, the 500 V range is limited to a maximum
ac input voltage of 500 V.
3-16. DC VOLTAGE MEASUREIVIENTS.
A rv>"v>>nnnnrvvj
KautionJ
Do not exceed a maximum input voltage of
1000 V (dc + peak ac) on the 500 V range or
damage to the instrument will occur.
3-17. 10 mV Range Zero Adjust.
3-18. When using the Multimeter on the 20 mV range in
DC volts, short the input terminals and zero the Multimeter
display with the rear panel ZERO ADJ control. The display
should indicate 0.000 before proceeding with measure
ments.
3-19. DC Voltage Ranges.
3-20. DC Voltage measurements can be made from 20 mV
to 1000 V full-range. Each range has a maximum display
reading of 19999. However, the 1000 V range is limited to
maximum input of 1000 V dc and peak ac (see DC Voltage
measurements caution in Paragraph 3-16).
3-2

Model 3465B
Section III
3-21. CURRENT MEASUREMENTS.
rWWWYVWVJ
A f CAUTIONS
Do not exceed a maximum input voltage of
250 V dc + peak ac or a maximum dc or ac
rms input current of 2 A or the amps fuse,
located directly behind the "A" terminal, will
open. See the following paragraph for replace
ment instructions.
3-22. The Multimeter is protected from the application of
excessive current by a 2 A fuse located directly behind the
front panel "A" terminal. If it is necessary to replace this
fuse, use the side slots on the "A" terminal to rotate the
terminal. The terminal and fuse will protrude from the
front panel. Remove the terminal and fuse, replace fuse
with a 2 A rated fuse as listed in Table 6-3 Miscellaneous
Parts General, and designated Fl.
3-23. AC Current Ranges.
3-24. AC current measurements are specified over a fre
quency range of 40 Hz to 20 kHz. There are five current
ranges from 200 ph to 2000 mA. See current measure
ments Caution in Paragraph 3-21.
3-25. DC Current Ranges.
3-26. DC Current measurements can be made on five cur
rent ranges from 200 pA to 2000 mA. See current measure
ments caution in Paragraph 3-21.
3-27. OHMS MEASUREMENTS.
A
Do not apply voltage greater than ± 250 V dc -r
Peak AC between the ohms and common input
terminals in the ohms function or damage to
the instrument will occur.
3-28. Ohmmeter Ranges.
3-29. Resistance measurements can be made on six ranges
from 200 ohms to 20 megohms. Both input terminals (f2
and COM) are floating with respect to circuit ground.
3-30. Ohmmeter Reference Current.
3-31. The ohmmeter reference current through the un
known resistance for each range is shown in Table 3-1.
Table 3-1. Ohmmeter Current Through Unknown.
Current
Range
Through Unknown
200 a
1 mA
2 ka
1 mA
20 ka
10 mA
200 kn
10 mA
2000 kn
1 mA
20 Mn
0.1 mA
Maximum open-circuit voltage at the ohms input terminals
is less than 5 V.
3-3/3-4

Section IV Model 3465B
4-6. SIMPLIFIED THEORY.
4-7. A simplified theory of operation of the Multimeter is
presented in the following paragraphs. The simplified
theory describes each section of the functional block
diagram, Figure 7-1. These sections are the signal condi
tioning section, analog—to-digital section, logic section and
the display section. Also presented is a simplified descrip
tion of the power supply. Refer to Figure 7-1, Functional
Block Diagram, and Figure 4-1, Basic Block Diagram and
Measurement Sequence, for this discussion.
4-8. Signal Conditioning.
4-9. Signal conditioning consists of attenuating and/or
converting the input signal to a dc voltage within the
working limits of the input amplifier. For full-scale inputs,
this voltage can vary from 20 mV dc to 2 V dc depending
on the function and range.
4-10. The signal conditioning section consists of current
shunts, an input attenuator, ohms converter and an
ac—to—dc converter. The output from the signal condi
tioning section is applied to the input amplifier during the
run—up interval of the measurement sequence. The Input
Amplifier Gain Table located on Figure 7-3 indicates the
full-scale input level applied to the input amplifier for each
function and range. This signal is the output of the signal
conditioning section.
4-11. Ohms Converter. The ohms converter is a high gain
integrating amplifier. A simplified diagram of the ohms
converter is presented in Figure 4-2. The blocks of the
ohms converter are the integrating amplifier, protection
diodes, over-voltage protection circuit and the overload
loop. An integrating amplifier is used because this type of
amplifier is less susceptible to oscillations. The protection
diodes clamp the Q terminal to a voltage of about 1.2 V
in the positive direction or 0.7 V in the negative direction.
With the terminal clamped, protection against excessive
voltages applied to the ohms terminals is provided by an
over-voltage protection circuit located between the ohms
amplifier and the terminal. For excessive voltages, this
circuit isolates the COM terminal from the ohms amplifier.
4-12. Figure 4-2 shows two outputs of the ohms converter
being applied to the input amplifier. The ohms output is
the ohms converter measurement signal and the auto-zero
output is the ohms amplifier dc offset signal which is called
the auto-zero (AZ) signal. This AZ signal is applied to the
input amplifier during the auto-zero interval of the mea
surement sequence and establishes the reference for the
analog—to—digital converter. An AZ signal greater than
± 1 mV causes the instrument readings to be invalid. This
condition (AZ signal > ± 1 mV) is present when the
unknown resistance, R^, is removed and an open loop is
present on the ohms amplifier. To maintain the AZ signal at
< ± 1 mV when an open loop is present, an overload
feedback circuit is used.
4-13. The ohms output, (LO terminal of the ohms con
verter) is applied to the input amplifier. This output is a dc
voltage, the level of which is dependent on the ratio of the
unknown resistance, R^, to the variable resistance, 10", and
the ohms reference supply. The variable resistance, 10", is a
resistor string located in the precision resistor pack R75.
The value of 10" is selected by the range switches shorting
those resistors in the string that are not required. The value
of 10" can range from 10 kn to 10 Mf2. A discussion of the
precision resistor pack R75 can be found in the detailed
theory.
4-14. The formula for the ohms converter output voltage
is:
Ohms
Output '
Voltage
10"
Reference Supply Voltage -t Voffset
* 10"
i-lOV vw-
*+IV FOR
lOM RANGE
029
-{(r-
f1
PROTECTION
DIODES
OVERLOAD
PROTECTION
OVER-
VOLTAGE
PROTECTION
CIRCUIT
AUTO-ZERO
OUTPUT
AV IS
PROPORTIONAL
TO Rx
\ TO
>INPUT
AMP
OHMS
'OUTPUT
3465-B-4167
V
Figure 4-2. Simplified Diagram, Ohms Converter.
4-2

Model 3465B
Section IV
The reference supply is + 10 V for all ranges except the
20 M range. For this range the reference supply is + 1 V.
The full-scale output of the ohms converter is 2 V dc. On
the 20 M range with a Rx of 20 MFi (full-scale), an output
of 2 V dc is needed. From the formula for the ohms
output, it can be seen that 10" would have to equal
100 Mf2. Since the range of 10" is 10 kfi to 10 Mf2, a 10"
of 10Mf2 combined with a reference supply of IV
provides the desired 1 V dc full-scale ohms converter
output.
4-15. AC—DC Converter. The ac—dc converter is an average
responding ac converter. It measures the average value of a
sine wave and multiplies this by a fixed scale factor to
convert it to an rms value. The output of the converter is a
dc voltage equal to the rms value of the sine wave.
4-16. Figure 4-3 is a block diagram of the ac -dc converter.
The blocks consist of an impedance converter, an ac
converter and a filter. The impedance converter has a high
input impedance to prevent loading of the input signal. It
also provides the gain necessary to drive the ac converter.
An impedance converter gain of unity, 9.964 or 10 is
selected by the function and range switching. The gain of
9.964 is used with the ac current function and the gain of
10 is used with the 200 mV, .2 mA, 200 and 20 V,
20 mA, 20 kfl ranges.
4-17. The ac converter amplifies the signal from the
impedance converter by the scale factor. This converts the
average value of the sine wave to the rms value. Half-wave
rectification of the sine wave is also performed by the ac
converter. This rectified signal is filtered to provide the
proportional dc output which is applied to the analog—to-
digital converter.
4-18. Analog-to-Digital (A-D) Converter.
4-19. The A—D converter block is comprised of an input
amplifier, reference supply, integrator, slope amplifier,
comparator and auto-zero circuit. It makes an analog—to-
digital conversion using the dual-slope integrating tech
nique. Four control state signals from the logic section (10,
IZ, II and 12) regulate the measurement sequence. 10 and
IZ regulate the input amplifier and auto-zero switching
respectively while 11 and 12 select the reference supply
required during the run-down interval.
4-20. Input Amplifier. The first stage of the A—D con
verter is the input amplifier. During the run-up interval of
the measurement sequence, control state signal 10 switches
the output of the signal conditioning block to the input
amplifier. The output of the signal conditioning block is a
dc voltage which varies between 20 mV and 2 V for
full-scale inputs, depending on the function and range
selected. The gain of the input amplifier is adjusted by the
function and range switching to provide an output of 2 V
dc for any full-scale input signal. See Input Amplifier Gain
Table on Figure 7-3.
4-21. Reference Supply. The A—D converter uses a mono-
polar reference supply of -tlOV. A reference voltage is
applied to the integrator during the run-down interval to
discharge the integrating capacitor. Since the discharge rate
is constant, the time required for the integrator to reach a
zero reference is proportional to the input signal. This time
period is the run-down interval and is processed to
determine the display. A positive and negative reference
voltage is required since the input signal can be either
polarity. A detailed discussion of the operation of the
monopolar reference supply can be found in the detailed
theory.
4-22. Integrator. The integrator output is a result of a
current summation at the integrator summing junction
(inverting input). A positive current summation (current
flowing into the integrator input) will cause the integrator
to ramp negative. A negative current summation (current
fl owing out of the integrator input) will cause the integra
tor to ramp positive. The integrator sums currents from the
input amplifier, reference supply, - 7 V supply and the
auto-zero loop during designated times.
4-23. Slope Amplifier. Following the integrator is a X4000
amplifier. This amplifier is divided into two stages; the first
with a gain of 40 and the second with a gain of 100. The
slope amplifier amplifies the integrator output to provide a
more vertical crossing of this output with the reference
level. This provides greater accuracy of the voltage—to-
time conversion during the run-down interval.
4-24. Comparator. The comparator provides two logic
outputs; a high output of 0 V or a low output of - 7 V. The
comparator output is high when the integrator output is
greater than the reference level. The comparator is low
when the integrator output is less than the reference level.
AC
INPUT-
SIGNAL
IMPEDANCE
CONVERTER
X!
X 9.964
orXIO
AC
CONVER
TER
FILTER
DC VOLTAGE TO
►ANAUDG-TO-
DIGITAL CONVERTER
Figure 4-3. Block Diagram, AC—to—DC Converter.
4-3

Section IV
Model 3465B
This logic level is sensed by the logic section to determine
polarity and zero-detect.
4-25. Auto-Zero Circuit. During the measurement se
quence, the auto-zero loop is closed except for the run-up
and run-down intervals. This loop includes the slope
amplifier and the integrator but does not physically include
the input amplifier although the loop does compensate for
the input amplifier offset. When the auto-zero loop is
closed, the input of the input amplifier is grounded. If the
summation of currents at the integrator summing junction
is not zero, the integrator begins to ramp up for a negative
summation or ramp down for a positive summation. The
integrator output is applied through the X4000 slope
amplifier to the auto-zero capacitor, C4. The voltage on the
auto-zero capacitor causes a current to flow at the summing
Junction that returns the summation to zero. This auto-zero
configuration compensates for the analog offset of the
input amplifier and integrator by providing a current at the
summing junction that cancels the currents resulting from
the offset.
4-26. Logic Section.
4-27. The Logic Section is comprised of combinational and
state logic. This section processes the comparator output to
determine the polarity of the input signal and to make a
voltage—to—time conversion of the input signal. Time
accumulated during the conversion is proportional to the
input signal and is stored. The display is derived from this
accumulated time. A voltage-to-time conversion with the
accumulated time being stored occurs once each measure
ment sequence.
4-28. Seven blocks make up the logic section. These blocks
are:
a. Clock
b. State Clock
c. Polarity and Zero Detect
d. Data Transfer and Reset
e. Control State Counter
f. Control State Decode
g. Data Accumulator
The HIGH and LOW logic levels used in the logic section
are 0 V and - 7 V respectively. The following discussion
describes the basic operation of the logic section.
4-29. Clock and State Clock. The timing of the logic
section is derived from the clock circuit. The clock operates
at 100 kHz and is crystal-controlled. A state clock, driven
by the clock output and the count extend line from the
data accumulator, drives the control state counter to
initiate each measurement interval.
4-30. Polarity and Zero Detect. The polarity and zero-
detect circuit monitors the comparator output. The state of
this output at the beginning of the run-down interval
determines the polarity of the input signal. Zero-detect is
determined at the point the comparator output changes
states during the run-down, overrange or overflow intervals.
If the integrator ramps positive (negative input signal)
during run-up, the comparator output goes HIGH and
returns to LOW at the zero-detect point. If the integrator
ramps negative (positive input signal) during run-up, the
comparator output goes low and returns to high at the
zero-detect point. These comparator output logic states are
stored in a D flip—flop. At the beginning of the run-down
interval, this state identifies the polarity of the input signal.
The outputs of the D flip—flop provide the signals needed
to select the correct polarity display and the correct
reference supply signal (11, 12) during the run-down
interval. An EXCLUSIVE OR and latch processes the
comparator output to provide the zero-detect signal.
4-31. Data Transfer and Reset. The data transfer and reset
circuits provide logic signals to the data accumulator
required to load the storage latches and reset the decade
counters. A detailed description of the data accumulator is
provided in the detailed theory section. While the TXFR
input of the data accumulator is low, data in the decade
counters is transferred to the static storage latches. The
RESET input resets the decade counters to zero when low.
This must occur after the transfer to the storage latches has
taken place. To ensure that reset occurs after termination
of transfer, an RC delay circuit precedes the reset gates.
4-32. Control State Counter. The control state counter
provides the timing for the measurement sequence intervals.
The output from the counter establishes the timing of the
analog control signals (IZ, 10, 11 and 12) which are applied
to the A—D converter. The state clock and reset inputs to
the control state counter determine the outputs of the
counter.
4-33. Control State Decode. The control state decode
converts the polarity, zero-detect and control state counter
inputs to the correct analog control signals. These signals,
applied to the A—D converter, perform the measurement
sequence switching. This switching consists of the input
amplifier switch, the auto-zero switch and the reference
supply switches.
4-34. Data Accumulator. The data accumulator consists of
a counter, data latches, a multiplexer, digit select decoder
and output buffers. At the beginning of the Run-Down
interval of the measurement sequence, the data accumula
tor begins to count clock pulses until zero-detect occurs.
This count is proportional to the input signal and is the
time conversion used to generate the display. The digit
select decoder scans the display digits from the most
significant digit to the least significant digit while the
multiplexer provides the corresponding BCD outputs for
each digit. A detailed discussion of the data accumulator is
presented in the detailed theory.
4-35. Display.
4-36. The multimeter display contains four full digits with
an overrange "1" and polarity sign. All segments and indi
cators are light-emitting diodes. A BCD-to-seven segment
decoder receives BCD information from the data accumu-
4-4

Model 3465 B
Section IV
lator and applies the seven-segment code to the display
drivers. The display drivers apply the seven-segment code to
all digits simultaneously. Digit strobe lines activate the digit
corresponding to the seven-segment code at that point in
time. Scanning of the digits is from the most significant to
the least significant digit. To complete the display, the
proper decimal point is enabled by range switching.
4-37. Power Supply.
4-38. Figure 4-4 is a block diagram of the power supply.
The power supply develops three output voltages from a
single dc output voltage (+ Vb)- This dc input voltage is
applied to a dc-to-dc converter which develops output
voltages of -t 11 V dc and - 7 V dc. A series regulated
+ 10 V output is developed from the + 11 V converter
output. This + 10 V is used as the reference voltage in the
A-D converter and to develop the reference current in the
ohms converter and as the reference voltage for the
converter regulator. The converter regulator controls the
converter and regulates the - 7 V and -t 11 V outputs of the
converter. A discussion of the operation and regulation
process of the dc-to-dc converter is presented in the
detailed theory.
SERIES
VOLTAGE
REGULATOR
dc-to-dc
CONVERTER
REGULATOR
CONVERTER
Figure 4-4. Block Diagram, Power Supply.
4-39. DETAILED THEORY.
4-40. This portion of the theory of operation provides a
detailed discussion of the circuits in the Multimeter. The
circuits described here are the ohms converter, ac—dc
converter, monopolar reference supply, data accumulator
of the logic section, display and the power supply. A
discussion of the precision resistor pack (R75) is also
provided. The detailed discussion makes use of the schema
tics in Section Vll.
4-41. Precision Resistor Pack (R75).
4-42. The precision resistor pack, R75, is a laser trimmed
substrate providing high precision resistances. A diagram of
R75 is shown on Figure 7-2. The input attenuator, power
supply, ohms reference supply. A—D reference supply and
the input amplifier require highly accurate resistances to
maintain the accuracy of the Multimeter. These resistances
are part of the resistor pack. The advantage to the resistor
pack is high precision resistors and good temperature
tracking. As resistance values of the resistor pack change
due to temperature changes, the ratio of the resistors
remains the same.
4-43. Ohms Converter.
4-44. Refer to Figure 7-2 for this discussion. Both ends of
the ohms converter are floating with respect to the
instrument ground. The unknown resistor, Rx, becomes the
feedback loop of the ohms amplifier. The ratio of Rx to
10" determines the gain of the ohms amplifier, Q25 and
U15. 10" is a variable resistance between 10 kfl and 10 MS2
selected by the range switching. The ohms converter input
is the reference voltage provided by the ohms reference
supply. This reference voltage times the amplifier gain is the
ohms converter output supplied to the input amplifier
during the run-up interval. Full-scale ohms converter gain
and output values are provided in the ohms converter table
located on Figure 7-2.
4-45. The HI LEAD of the ohms converter is connected
to the reference supply through 10" of the resistor pack
R75. The fl HI LEAD is clamped by protection diodes
CR15 and CR25 to prevent the destruction of FET Q25
and R75 by the application of large voltages. These diodes
clamp the HI LEAD to about 1.2 V positive or 0.7 V
negative.
4-46. With the SI HI LEAD clamped, over-voltage protec
tion must be provided to protect the ohms amplifier from
excess voltage. The over-voltage protection circuit is located
between the ohms amplifier and the LO terminal point and
is shown in Figure 4-5. During normal operation < 2 mA of
current flows through Q30, R94 and Q32. If a large voltage
is applied to the ohms terminals, the current through this
circuit will try to exceed 2 mA. This current will cause a
large enough voltage drop across R94 to turn on Q31. When
Q31 is on, it removes the base drive from Q30, which turns
off, disconnecting the LO terminal point from the ohms
converter. Since Q30 is a high voltage transistor, large
voltages are not applied to the ohms converter.
iOV
R82
LO TERMINAL POINT
FROM OHMS_
AMPLIFIER
T
1
OHMS
OUTPUT
.y.CR28
V
.?465-B-4l72
Figure 4-5. Over-Voltage Protection Circuit.
4-5

Section IV
Model 3465B
4-47. In the event of open loop (R^ = °°), the ohms
amplifier output begins to drive negative. The input
(negative port), which is the auto-zero output, could exceed
± 1 mV under an open loop condition due to the lack of
negative feedback through an R^. This auto-zero output
must be maintained at < ± 1 mV for accurate operation of
the A-D converter. To satisfy this requirement, an over
load protection circuit consisting of CR23, CR24 and R86
is used. When the ohms amplifier output goes below
approximately -H.5 V, the zener diode (CR23) turns off.
The overload loop, CR24 and R86, is introduced by the
turn-on of CR24 when CR23 is off. This loop provides the
negative feedback required to maintain an auto-zero output
< ± 1 mV. When an R^ is introduced, CR23 turns-on,
CR24 turns-off, and the overload loop is inoperative.
4-48. A maximum output by the ohms converter of < 5 V
is guaranteed by a voltage divider composed of R93 and
R95. Additional protection components of the ohms
converter are: A) CR29 which prevents Q32 junction
breakdown due to fast transients, B) CR28 which blocks
negative transients that could come in via the LO terminal
point and C) R91 and C27 which suppress high voltage,
high frequency transients.
4M-9. Degradation of accuracy in the ohms function due to
changes in the ohms reference with respect to the A—D
reference is minimized since both reference voltages are
derived from the same -t 10 V reference supply. If the
reference supply voltage changes, both the ohms reference
and the A—D reference are affected alike and any change is
effectively cancelled.
4-50. AC-to-DC Converter.
4-51. The AC—to—DC converter is an average responding
ac converter. It has a bandwidth of 40 Hz to 20 kHz. The
converter is composed of two stages (see Figure 7-2). The
fi rst stage, U19, is an impedance converter. The purpose of
this amplifier is to provide a high impedance to the input so
loading of the input signal does not occur. It also provides
high drive capability for the ac converter stage, U18. The
input of the impedance converter is protected against large
voltage swings by diodes CR35 and CR37. Voltages in
excess of -t 10 V or - 7 V peak ac will forward bias these
diodes, returning excess current to the power supply.
4-52. The impedance converter, U19, has a selection of
three gains; the 200 mV, .2 mA, 200 and 20 V, 20 mA,
20 kf2 ranges select a gain of 10. The ac current function
selects a gain of 9.964, while the remainder of the ranges
and functions select a gain of unity (see U19 Gain Table,
Figure 7-2).
4-53. The second stage of the AC—to—DC converter is the
ac converter, U18. A basic diagram of this stage is shown in
Figure 4-6. The amplifier has three feedback loops. These
loops are the ac negative feedback loop, the dc negative
feedback loop, and the positive feedback loop. The ac
negative feedback loop is divided into two branches; one
branch for the positive half cycle and the second branch for
the negative half cycle. Diodes CR33 and CR34 switch
,V(jc TO
INPUT AMP
-•—wv—4-
RI25
220K
CR33
AC
NEGATIVE
FEEDBACKt
LOOPS
10 K
CR34
RI27
220K
RI26
lOK
FROM
IMPEDANCE
CONVERTER
UI8
FILTER
POSITIVE
FEEDBACK
LOOP
DC
NEGATIVE
FEEDBACK
LOOP
Figure 4-6. Basic Diagram, AC Converter Amplifier.
between the positive and negative half-cycles to introduce
the correct loop for its respective half-cycle.
4-54. During switching of the diodes, little negative feed
back is present. During the switching transition, the positive
feedback loop (C45, R120 and R123) boosts the amplifier
gain. This boost in gain speeds the switching transition of
the diodes which gives a good frequency response at low
signal levels. Once the switching transition has occurred,
negative feedback is again present. The negative feedback
overrides the effects of the positive feedback loop at all
times other than the diode switching transition period.
4-55. The output of the AC-to-DC converter is derived
from the positive half-cycle, negative feedback loop. The
positive half-cycle developed across the load resistor R118
is the half-wave rectified signal of the ac converter amplifier
output. This rectified signal is filtered to provide the dc
output that is applied to the input amplifier during the
run-up interval of the measurement sequence. For full-scale
inputs, the AC-to-DC Converter output is 1.6 V dc. This
output is kept relatively free of the dc offset present on the
inverting input of U18 (pin 2) by the voltage divider R125
and R118. The portion of the offset appearing across the
load resistor R118 is attenuated by a factor of 23.
4-56. A-D Conversion Using a Monopolar Reference.
4-57. Before preceeding with this discussion, review the
4-6

Model 3465B
Section IV
A—D converter description of the integrator, slope ampli-'
fier and auto-zero circuit in the simplified theory. Figure
4-7, Functional Diagram, A—D Converter, illustrates these
circuits in relation to the monopolar reference supply, the
input amplifier and the comparator. It also illustrates the
integrator output and the four control state signals, IZ, 10,
11 and 12, with respect to the measurement sequence
intervals.
COMPARATOR
INPUT.
FROM
SIGNAL
CONDITIONING
SECTION
20 mVdc to 2 Vdc
FULL—SCALE)
MONOPOLAR
REF SUPPLY
+ IOV
INTEGRATOR
INPUT AMP
SLOPE AMP
lOOK
vs/v
LOGIC
X4000
FULL-SCALE
SECTION
2 Vdc
42 K 500k|
AUTO-ZERO
OOFF
-NOTE-
A-D CONVERTER
SHOWN IN AUTO
ZERO MODE
C4
AUTO-ZERO
CAPACITOR
3465-B-4I69
, AUTO-ZERO ,
I 0.1 SEC I
RUN-UP
0.1 SEC
INTEGRATOR
OUTPUT
RUN-DOWN
0.1 SEC
NEGAT VE
/INPUTS
OVERRANGE
0.1 SEC
, OVERLOAD ,
I 0.1 SEC I
ONS OV
OFFS -7V
/
/VTs POSITIVE
\ / INPUTS
IZ
10
11
12
ON-
OFF
ON
OFF-
ON-
OFF
ON
OFF -
MEASUREMENT
INTERVAL
INPUT
SIGNAL
POLARITY
STATE OF
CURRENT SWITCH
11
12
AUTO-ZERO
+
CLOSED
CLOSED
OPEN
OPEN
RUN-UP
+
CLOSED
CLOSED
OPEN
OPEN
RUN-DOWN
+
OPEN
CLOSED
OPEN
CLOSED
Figure 4-7. Functional Diagram, A—D Converter.
4-7

Section IV
Model 3465B
4-58. The A—D converter of Figure 4-7 is shown in the
auto-zero mode. The input amplifier is grounded at the
input, control state switch II is closed, 12 is open and the
auto-zero loop is closed. Note that the auto-zero loop does
not include the input amplifier but is connected to the
integrator summing junction (integrator inverting input).
Also connected to the summing junction are the input
amplifier output, two current paths from the monopolar
reference supply and the - 7 V supply through R59 and
R43.
4-59. The auto-zero loop uses a current balancing tech
nique at the integrator summing junction to establish the
reference. The basic principle is that the algebraic sum of
currents at the integrator summing junction must be equal
to zero. When the sum is zero, the output of the integrator
will not change. If the sum is not zero, the integrator will
ramp up for a negative current or ramp down for a positive
current because of the inverting input.
4-60. When the auto-zero loop is closed, the currents
summed at the integrator summing junction come from
four sources; 1) the output of the input amplifier with its
input grounded, 2) one current path of the monopolar
reference supply (switch II closed), 3) the - 7 V supply
through R43 and R59 and 4) the auto-zero loop. The input
amplifier output is the analog offset of this amplifier. The
current due to the - 7 V supply Is roughly the negative of
the current from the monopolar reference supply. The
auto-zero loop then stores a voltage on the auto-zero
capacitor that produces a current through R28 and R42 of
the correct magnitude to force the summation of currents
at the integrator summing junction to zero. Forcing the
summation of currents to zero compensates for the analog
offset of the input amplifier and integrator.
4-61. During the run-up interval, the auto-zero loop is
opened by control state switch IZ. The voltage stored on
the auto-zero capacitor is still applied to the integrator
summing junction and the summation of currents remains
zero. At the time the auto-zero loop is opened, the output
of the signal conditioning section is switched to the input
amplifier by control state signal 10. The output of the
input amplifier causes the algebraic summation of currents
at the integrator summing junction to deviate from zero.
This causes the integrator to run-up.
4-62. At the end of the run-up interval, the input amplifier
is switched back to ground by control state signal ID. The
summation of currents at the integrator summing junction
is again zero and if no other action were taken, the
integrator output would not change. The integrator output
is positive at the end of run-up for negative inputs and
negative for positive inputs. At the end of the run-up
interval, the polarity of the integrator output is determined
by the logic section. This also identifies the polarity of the
input signal.
4-63. At the beginning of the run-down interval, the logic
section selects the appropriate reference to return the
integrator output to zero. Run-down uses the summation of
currents principle at the summing junction of the integra
tor. The two current paths (II and 12) of the monopolar
reference supply provide the means of changing the
summation of the currents. The summation of currents at
the summing junction can be made negative by opening
switch 11 and removing this current flow to the junction.
The summation can be made positive by closing switch 12
in addition to II, and providing twice the current from the
monopolar reference supply. Opening switch II with 12
open, runs the integrator up which is required for positive
inputs (see Figure 4-7). Closing II and 12 runs the
integrator down which is required for negative inputs. The
time required for the integrator to reach zero-detect during
the run-down interval is proportional to the input voltage
which caused run-up and determines the display.
4-64. Data Accumulator.
4-65. Refer to Figure 4-8, Data Accumulator Diagram, for
this discussion. The data accumulator processes the logic
signals from the logic section and provides the BCD output
and the scan signals that determine the dsiplay. The data
accumulator consists of a counter, data latches, a multi
plexer, digit select decoder and output buffers. At the
Ireginning of the measurement, the reset signal (RESET)
goes to a logic 0 to initialize the counter and digit select
decoder. At the beginning of the run-down interval of the
measurement sequence, the counter begins to accumulate a
count proportional to the run-down time.
4-66. The counter consists of four divide by 10 circuits.
The output of each circuit is a BCD number representing
one digit of the input signal. At the end of the run-down
interval, the transfer signal (TXFR) is set to a logic 0. This
stores the counter outputs in the data latches.
4-67. The scan signal will gate each BCD signal from the
latches, beginning with the most significant digit first,
through the multiplexer to the output. At the same time
that the scan gates the digits through the multiplexer, the
gating signal is output to the display as a digit activation
pulse.
4-68. The BCD output of the multiplexer is applied to the
display section (see Figure 7-4). The BCD is applied to quad
NAND gates in the display section where the BCD logic is
converted to BCD logic. The BCD is applied to the seven
segment decoder where it is transformed to a seven bit
binary number and applied to each numeral in the display.
As the digit activation pulse from the data accumulator
sequentially activates each numeral from most significant to
least significant, the seven bit binary data will be displayed.
4-69. Display.
4-70. Refer to Figure 74 for this discussion. The display
segments are powered by a + 3 V supply. This voltage Is
4-8