Fluke 845AR User manual

845AR

Notwithstanding any provision of any agreement the following warranty is exclusive:
The JOHN FLUKE MFG. CO., INC., warrants each instrument it manufactures to be free from defects in material and
workmanship under normal use and service for the period of 1-year from date of purchase. This warranty extends only
to the original purchaser. This warranty shall not apply to fuses, disposable batteries (rechargeable type batteries are
warranted for 90-days), or any product or parts which have been subject to misuse, neglect, accident, or abnormal
conditions of operations.
In the event of failure of aproduct covered by this warranty, John Fluke Mfg. Co., Inc., will repair and calibrate an
instrument returned to an authorized Service Facility within 1year of the original purchase; provided the warrantor's
examination discloses to its satisfaction that the product was defective. The warrantor may, at its option, replace the
product in lieu of repair. With regard to any instrument returned within 1year of the original purchase, said repairs or
replacement will be made without charge. If the failure has been caused by misuse, neglect, accident, or abnormal
conditions of operations, repairs will be billed at anominal cost. In such case, an estimate will be submitted before
work is started, if requested.
THE FOREGOING WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY IMPLIED WARRANTY OF MERCHANTABILITY, FITNESS,
OR ADEQUACY FOR ANY PARTICULAR PURPOSE OR USE. JOHN FLUKE MFG. CO., INC., SHALL
NOT BE LIABLE FOR ANY SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, WHETHER IN
CONTRACT, TORT, OR OTHERWISE.
li any failure occurs, the following steps should be taken:
1.Notify the JOHN FLUKE MFG. CO., INC., or nearest Service facility, giving full details of the difficulty, and
include the model number, type number, and serial number. On receipt of this information, service data, or
shipping instructions will be forwarded to you.
2. On receipt of the shipping instructions, forward the instrument, transportation prepaid. Repairs will be
made at the Service Facility and the instrument returned, transportation prepaid.
SHIPPING TO MANUFACTURER FOR REPAIR OR ADJUSTMENT
All shipments of JOHN FLUKE MFG. CO., INC., instruments should be made via United Parcel Service or "Best Way"
prepaid. The instrument should be shipped in the original packing carton; or if it is not available, use any suitable
container that is rigid and of adequate size. If asubstitute container is used, the instrument should be wrapped in paper
and surrounded with at least four inches of excelsior or similar shock-absorbing material.
CLAIM FOR DAMAGE IN SHIPMENT TO ORIGINAL PURCHASER
The instrument should be thoroughly inspected immediately upon original delivery to purchaser. All material in the
container should be checked against the enclosed packing list. The manufacturer will not be responsible for shortages
against the packing sheet unless notified immediately. If the instrument is damaged in anyway, aclaim should be filed
with the carrier immediately. (To obtain aquotation to repair shipment damage, contact the nearest Fluke Technical
Center.) Final claim and negotiations with the carrier must be completed by the customer.
The JOHN FLUKE MFG. CO., INC, will be happy to answer all applications or use questions, which will enhanceyour
use of this instrument. Please address your requests or correspondence to: JOHN FLUKE MFG. CO., INC., P.O. BOX
43210, MOUNTLAKE TERRACE, WASHINGTON 98043, ATTN: Sales Dept. For European Customers: Fluke (Holland)
B.V., P.O. Box 5053, 5004 EB, Tilburg, The Netherlands.
‘For European customers, Air Freight prepaid.
John Fluke Mfg. Co., Inc., P.O. Box 43210, Mountlake Terrace, Washington 98043
Rev. 4/80

845AR
1-1. INTRODUCTION
1-2. The Fluke Model 845AR High Impedance Volt-
meter-Null Detector allows measurement of dc voltages
from one microvolt to 1000 volts dc in 19 ranges. When
used as anull detector on the 100 millivolt range and
below, the input impedance is an excellent lj/ megohms?
Alinear recorder output allows the instrument to be
used for production testing, and also as a dc amplifier
with amaximum gain of 120 db.
1-3. The instrument may be wired to operate from
aline power source of 115 volts ac or 230 volts ac, as
desired. The instrument is designed to be mounted
directly in astandard EIA 19 inch relay rack. Resilient
feet are also provided for bench top use.
grounding. Better than 10*0 ohms up to 80% relative
humidity and 35° C. With driven guard, isolation im-
proves by at least one order of magnitude up to 1013
ohms. Any input terminal may be floated 1100 volts
off chassis ground.
yDC COMMON MODE REJECTION
ftbetter than 160 db, input short-circuited, 80% relative
humidity; better than 140 db, open-circuited, 50% rela-
tive humidity; better than 120 db, open-circuited, 80%
relative humidity.
AC COMMON MODE REJECTION (below 100 kHz)
100 volts rms or 120 db greater than end scale, which-
ever is less, will effect reading less than 2% of end
scale. Input open-circuited.
1-4. ELECTRICAL SPECIFICATIONS
INPUT VOLTAGE RANGE
1microvolt to 1000 volts dc end scale in nineteen ranges,
using XI and X3 progression.
INPUT RESISTANCE
-
100 megohms onllOO millivolt range and above/ 10 meg
ohms on 100- millivolt r-ange-and-below-. Vw/
'1 (00 Igv\e*»
accuracy \-
±(3% end scale +0. 1microvolt).
AC NORMAL MODE REJECTION (60 Hz and above)
AC voltages 60 db above end scale will effect reading
less than 2% of end scale. Maximum voltage not to ex-
ceed 750 volts rms.
RECORDER OUTPUT
0-1 volt, one side at chassis ground; linear to 0. 5% of
end scale. Source impedance, 5k to 7. 5k.
OF ZERO
Better than 0. 15 microvolt/hr, better than 0, 3micro-
volt/day.
MAXIMUM NOISE (input shorted)
Range Noise (peak-peak)
TEMPERATURE COEFFICIENT OF ZERO
Less than 0. 1microvolt/0Cfrom 15° Cto 35° C. Less
than 0. 2microvolt/0Cfrom 0°C to 50° C.
1microvolt 0. 20 microvolt
3microvolt 0. 25 microvolt
10 microvolt -1000 volt 0. 30 microvolt
METER RESPONSE TIME (to 90% of reading)
Range Time
1microvolt 5seconds
3microvolt 3seconds
10 microvolt -1000 volt 1-1/2 seconds
INPUT ISOLATION
Better than lO1^ohms at less than 50% relative humidity
and 25° Cregardless of line, chassis, or recorder
ZERO CONTROL RANGE
±5 microvolt minimum.
OVERLOAD PROTECTION
Up to 1100 volts dc may be applied on any range. Typical
recovery time is 4seconds.
INPUT POWER
115/230 volts ac ±10%, 50 to 440 Hz, approximately
3watts.
1-5. ENVIRONMENTAL SPECIFICATIONS
OPERATING TEMPERATURE RANGE
Within all specifications from 15° Cto 35° C.
X-l

845AR
Within all specifications from 0° to 50° Cexcept:
Derate by afactor of two —*
Maximum Noise and Meter Response Time.
DC Common Mode Rejection —
Derate by 20 db.
STORAGE TEMPERATURE RANGE
-40° Cto +70° C.
RELATIVE HUMIDITY RANGE
0to 80%.
SHOCK
Meets hammer blow requirements of MIL-T-945A and
MIL-S-901B.
VIBRATION
Meets 10 Hz to 55 Hz tests of MIL-T-945A.
1-6- MECHANICAL SPECIFICATIONS
MOUNTING
Standard EIA relay rack. Resilient feet provided for
bench use.
WEIGHT
9pounds.
SIZE
3. 47 inches high x19 inches wide x 8. 26 inches deep.
1-2

AMessage From
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Some semiconductors and custom IC’s can be
damaged by electrostatic discharge during
handling. This notice explains how you can
minimize the chances of destroying such devices
by:
1. Knowing that there is aproblem.
2. Learning the guidelines tor handling them.
3. Using the procedures, and packaging and
bench techniques that are recommended.
The Static Sensitive (S.S.) devices are identified in the Fluke technical manual parts list with the symbol
The following practices should be followed to minimize damage to S.S. devices.
1. MINIMIZE HANDLING
2. KEEP PARTS IN ORIGINAL CONTAINERS
UNTIL READY FOR USE.
3. DISCHARGE PERSONAL STATIC
BEFORE HANDLING DEVICES
4. HANDLE S.S. DEVICES BY THE BODY

a
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845AR
SECTION li
OPERATING INSTRUCTIONS
2-1. RECEIVING INSPECTION
2-2. This instrument has been thoroughly tested and
inspected before being shipped from the factory. Im-
mediately upon receiving the instrument, carefully in-
spect for damage which may have occurred during ship-
ment. If any damage is noted, follow the instructions
outlined in the warranty page at the back of this manual.
2-3. CONTROLS, TERMINALS, AND INDICATOR
2-4. The location and function of the front-panel
controls are described in Figure 2-1. Detailed oper-
ating descriptions are given in the following para-
graphs.
2-5. PRELIMINARY OPERATION
2-6. Connect the Model 845AR line plug to a115 volt
ac power outlet or to 230 volts ac if the instrument is so
wired.
WARNING1
.
The round pin on the polarized three-prong
plug connects the instrument case to power
system ground. Use athree-to-two pin
adapter when connecting to atwo-contact
outlet. For personnel safety, connect the
short lead from the adapter to ahigh-quality
earth ground.
METER
Indicates the voltage applied
to the input terminals.
OPR ZERO CONTROLS
ZERO -Opens input terminals
and shorts amplifier input alio'
ing zeroing of the instrument
with the lower zero control.
OPR -Instrument is ready for
operation as avoltmeter or
null detector.
POWER SWITCH
ON -Line power applied
to instrument. ISOLATED OUTPUT
LEVEL CONTROL
For full-scale meter
deflection, this control
allows adjustment of the
isolated output voltage
from 0to 1volt dc.
INPUT AND
COMMON TERMINALS
Provide connection to vol-
tage being measured. .
POWER LIGHT
Indicates line
power is applied
to the instrument.
'T
*m9
Mtamm
WWW-
GUARD TERMINAL
This terminal is usually strapped to the common terminal
The terminal is connected directly to an inner chassis
shield. By removing the strap, aguard potential equal
to the common terminal potential may be applied to the
guard terminal, shunting the leakage current from the
common terminal to ground.
MECHANICAL ZERO
Screwdriver adjustment usee
to set the meter to zero.
ISOLATED OUTPUT
TERMINALS
Provide connection to
an external recorder.
RANGE CONTROL
Provides selection of the desired full-scale sensitivity
Figure 2-1. CONTROLS, TERMINALS, AND INDICATOR
2-1

845AR
a. Place the Model 845AR controls as follows:
POWER ON
RANGE 10 MICROVOLTS
OPR/ZERO ZERO
b. Adjust the zero control for an initial zero meter
deflection. Place the RANGE switch to the 1
MICROVOLT RANGE and re-zero with the zero
control.
2-7. MECHANICAL ZEROING
2-8, It may become necessary to adjust the mechani-
cal zero control of the Model 845AR at more frequent
intervals than complete calibration. To mechanically
zero the instrument proceed as follows:
a. Place the RANGE switch to 1000 VOLTS and the
POWER switch to ON.
b. Adjust the mechanical zero adjustment screw for
zero meter deflection.
c. Place the RANGE switch to 10 MICROVOLTS and
electrically zero the instrument as outlined in
paragraph 2-5.
d. Repeat steps aand b.
2-9. OPERATION AS AHIGH IMPEDANCE
VOLTMETER
2-10. To operate the Model 845AR as a High Impedance
Voltmeter perform the preliminary operations according
to paragraph 2-5 and proceed as follows:
a. Place the controls as follows:
POWER ON
OPR/ZERO OPR
RANGE 1000 VOLTS
Tiotef
When measuring voltages in the microvolt
ranges, use copper wire having low thermal
EMF's.
b. Connect the voltage to be measured to the Model
845AR INPUT terminal and connect the common
point of the voltage being measured to the COMMON
terminal.
c. Deflection of the meter indicates the polarity and
magnitude of the measured voltage. Increase the
sensitivity of the Model 845AR for maximum on-
scale deflection.
2-11. OPERATION AS ANULL DETECTOR
2-12. The Model 845AR may be used to monitor small
voltage differences in bridge circuits, potentiometers,
and other measuring apparatus. In most of these appli-
cations the circuits are adjusted for zero deflection or
anull on the Model 845AR. Equipment connections for
various types of null detector configurations are illus-
trated by Figure 2-2 through 2-4. To operate the Model
845AR as a Null Detector, perform the preliminary
operations according to paragraph 2-5 and proceed as
follows:
a. Select the desired equipment application as illus-
trated by Figure 2-2 through 2-4 and make the
appropriate equipment connections.
b. 'Place the Model 845AR controls as follows:
POWER ON
OPR/ZERO OPR
RANGE as desired
c. Adjust the circuit being measured for zero or a null
deflection on the Model 845AR meter.
DC SOURCE
FLUKE 845AR
Figure 2-2. BRIDGE DETECTOR -FLOATING SUPPLY
Figure 2-3. BRIDGE DETECTOR -HIGH RESISTANCE
2-2

845AR
Figure 2-4. BRIDGE DETECTOR -FLOATING
NULL DETECTOR
2-13. MEASURING VOLTAGES WITH A
STANDARD CELL
2-14. The Model 845AR may be used with avoltage
divider and astandard cell to calculate unknown volt-
ages with ahigh degree of accuracy. Connect the equip-
ment as illustrated in Figure 2-5. Perform the pre-
liminary operation as outlined in paragraph 2-5 and
proceed as follows:
a. Place the Model 845AR controls as follows:
POWER ON
OPR/ZERO OPR
RANGE as desired
b. Adjust the voltage divider for zero or null deflection
on the Model 845AR meter while placing the RANGE
switch to successively more sensitive ranges.
Figure 2-5. STANDARD CELL VOLTAG EMEASUREMENTS
c. Calculate the unknown voltage by dividing the
standard cell voltage by the final division ratio of
the divider.
2-15. USE OF ISOLATED OUTPUT
2-16. DC ISOLATION AMPLIFIER
2-17. The Model 845AR may be used as adc isolation
amplifier having avoltage gain of up to 120 db, de-
pending on the settings of the RANGE switch and the
OUTPUT LEVEL control. To compute the maximum
voltage gain on any range of the Model 845AR, use the
following formula:
1volt (maximum
Voltage gain in db =20 log ^isolated output)
Range (in volts)
2-18. RECORDER OUTPUT
2-19. The Model 845AR ISOLATED OUTPUT may be
used to provide an output voltage, adjustable from zero
to one volt for afull-scale meter deflection for use with
arecorder. Since the output is isolated from the input,
floating measurements can be made without the use of a
floating recorder. To use the adjustable recorder out-
put, proceed as follows:
a. Connect the recorder to the ISOLATED OUTPUT
terminals.
The lower ISOLATED OUTPUT terminal is
connected to chassis ground. If aground
reference is undesirable, remove the
jumper wire above R202 on the power supply
circuit board. Refer to Figure 2-6 for
jumper wire location.
b. Turn the recorder on.
c. Proceed as outlined in paragraph 2-9 or 2-11, as
desired.
d. Adjust the ISOLATED OUTPUT LEVEL control
for the desired output to the recorder. This control
has alog taper so that smooth control is possible
at both high and low settings.
"Hotel
The ISOLATED OUTPUT current capability
is 100 microamperes with a5kilohm source
impedance.
2-20. OPERATING NOTES
2-21. SPURIOUS VOLTAGES AND CURRENTS
2-22. Voltage measurements at the microvolt level
involve the persistant problems of thermoelectric ef-
fects. These effects may be compensated for by tempo-
rarily disconnecting the voltage from the circuit under
measurement and noting the meter deflection of the
2-3

Figure 2-6. CHASSIS GROUND -JUMPER WIRE LOCATION
Model 845AR on the desired range. This reading must
then be subtracted from all subsequent voltage measure-
ments. Athorough understanding of these effects can
lead to reducing or eliminating them completely.
2-23. THERMOELECTRIC VOLTAGES
2- 24. If acircuit is composed of two dissimilar metals,
anet voltage will result if the two dissimilar junctions
are maintained at different temperatures. These ther-
moelectric voltages, also known as thermals, thermo-
couple voltages, or Seebeek voltages, can be reduced
by using metals having low thermoelectric potentials,
and keeping ail junctions at the same temperature. The
terminals of the Model 84SAB are made of pure copper,
gold-flashed to prevent tarnish. For lowest thermal
voltages, all connections to the Model 845AR should foe
made with, pure copper wire. Silver plated copper or
solder coated copper also produce satisfactory results.
Tinned copper is less satisfactory than silver -plated or
copper coated copper, Nickel and nickel-based alloys
are not suitable for connections to the instrument.
Excellent results can be obtained using ordinary TV
twin lead, or even lamp cord if high insulation re-
sistance is not required. If shielding is necessary, use
a length of flat braid over the cable.
2-25. HIGH SOURCE IMPEDANCE
2-26. Due to the very high input resistance and ex-
treme sensitivity of the Model 845AR, it. is charge,
sensitive. Thus, aperson's body potential, an electro-
static voltage, can cause charge redistribution at the
input to the instrument and result in meter needle de-
flection as ahand approaches the input terminals.
Careful shielding will eliminate this problem. Also,
due to charges that may be deposited on the- input termi-
nals when the OPR-ZERO switch is set to ZERO, an
appreciable transient will result when the switch is set
to OPR if nothing is connected to the input terminals.
Turning the switch back and forth will dissipate this
charge, eliminating the problem. With a high source
impedances, the response of the instrument is unavoid-
ably slow due to the low pass filter used to suppress
superimposed noise. However, the design of the low
pass filter is such that common mode rejection is ex-
tremely high while the response time for the normally
encountered low source impedances is very fast,
2-4
2-27. OVERLOAD VOLTAGES
2-28. The instrument, is designed to withstand up to
1100 volts dc or 1100 volts peak ac continuously applied
between any two of the three input terminals or between
cabinet ground and any of the three input terminals
regardless of the setting of the RANGE or OPR-ZERO
switch. However, repeated or continuous overloads
above 200 volts in the ranges below 3millivolts will
result in dissipation in protective, low-pass-filter
resistor RUG. This will result in thermal voltages
which may take several minutes to subside after the
overload is removed.
2™ 29. GUARDING
2™ 30. The instrument has an inner chassis connected
to the GUARD terminal on the front panel. Ordinarily,
this GUARD terminal is strapped to the COMMON termi-
nal,, When connected in this way tSie inner chassis
serves as ashield. This greatly improves the leakage
resistance to ground and the common mode rejection.
However, since the inner chassis is available at the
GUARD terminal, it may be driven at the same voltage
as the COMMON terminal. This further increases the
leakage resistance and common mode rejection by about
ten times. The voltage used to drive the GUARD termi-
nal should be obtained from aseparate source or by
means of avoltage divider connected directly across
the source so that the leakage currents do not cause
voltage drops across impedances in the circuit under
measurement.
2-31. CREASING INPUT RESISTANCE
2-82, in the 1microvolt to imillivolt ranges, aJS‘
megohm resistor is connected directly across the input
of the instrument. The input resistance may be in-
creased on these ranges by disconnecting the ><3 meg-
ohm resistor where it attaches to the RANGE switch.
However, the input resistance will no longer be well
defined. Typical input resistances with the 3$ megohm
resistor removed are as follows:
t
Range Input Resistance
.1 uv 300 megohms
3uv 1,000 megohms
.10 uv 3, 000 megohms
iv to Imv 10,000 megohms

845AR
3-1. INTRODUCTION
3-2. The Model 845AR High Impedance Voltmeter-
Null Detector theory of operation is contained in this
section of the manual. Ablock diagram is illustrated
in Figure 3-1, and afunctional schematic diagram is
located at the end of Section V. The block diagram and
functional schematic diagram are to be used as an aid
in understanding circuit theory, and in troubleshooting.
3-3. BLOCK DIAGRAM ANALYSIS
3-4. The Model 845AR is aphoto-chopper stabilized
amplifier with the overall gain of the amplifier being
precisely controlled by negative feedback. The instru-
ment' smain circuits are an input range divider, a
photocell modulator, an ac amplifier, asynchronous
demodulator, adc amplifier, ameter, an isolation
converter, a neon drive, an 84 Hz multivibrator, a
supply rectifier, and arectifier filter.
3-5. The input range divider provides afixed input
impedance to signals of less than 1millivolt while al-
lowing reduction of input signals above 1millivolt.
Photochoppers modulate the input signal to the ac am-
plifier at 84 Hz. The drive signal for the photo modu-
lator is provided by the neon drive which is composed
of neon lamps driven alternately at 84 Hz by the 84 Hz
Figure 3-1. MODEL 845AR BLOCK DIAGRAM
3-1

845AR
multivibrator. Eighty four Hz is used to provide the
Model 845 with an operating frequency asynchronous
with the power line frequency and its harmonics. The
84 Hz multivibrator also drives the following circuits;
(1) the supply rectifiers which provide operating volt-
ages for the amplifiers, (2) the synchronous demodu-
lator which demodulates the amplified d€" signal, (3) the
isolation converter which produces the meter and iso-
lated recorder output. The entire amplifier and the
secondaries of both transformers are surrounded by a
guard shield which permits the use of external guard
voltages.
3-6. The ac amplifier is ahigh impedance amplifier
whose gain is controlled by the resistance selected by
the RANGE control. The amplified .^ signal is then
detected by the synchronous demodulator.
3-7. The synchronous demodulator is driven by the
84 Hz reference signal and detects the amplified .dC“A.<L
signal. The detected dcfsugnal is then amplified by a
dc amplifier whose gain is controlled by fixed feed-
back. The output signal of the dc amplifier is applied
to the isolation converter which drives the isolated re-
corder output, and the meter which indicate the polarity
and magnitude of the measured voltage. This same dcvrj
iC.
signal is also fed back to the input of the ac amplifier
to control overall amplifier gain. The feedback ratio
is determined by the setting of the RANGE control and
allows overall amplifier gain to be precisely controlled.
3-8. CIRCUIT DESCRIPTION
3-9. POWER SUPPLY
3-10. Input power transformer T201 receives 115 volts
ac, or 230 volts ac if the instrument is so wired, through
the power switch, SI. The primary winding of T201
is constructed in such amanner as to utilize either 115
volts ac input, windings parallel, or 230 volts ac, wind-
ings in series. Fuse, FI, protects the Model 845AR
circuitry from overloads.
3-11. The secondary voltage of T201 is rectified by
bridge rectifier CR201 through CR204. The bridge
rectifier output voltage is filtered by C201 and regu-
lated by zener CR207. This regulated output voltage is
used as the operating voltage for the 84 Hz multivibrator.
3-12. The 84 Hz multivibrator is used to provide syn-
chronous drive voltages and dc operating voltages for
the Model 845AR amplifier circuits free from any power
line frequency variations and harmonics. The multi-
vibrator is atransformer-coupled free running multi-
vibrator composed of transistors Q201 and Q202, trans-
former T202, and frequency determining components
C203 and R206 through R208. Variable resistor R206
is used to adjust the frequency of the multivibrator to
84 Hz. The voltage at the secondary of T202 is recti-
fied by CR104 and CR105 to produce the positive and
negative 15 volt dc operating voltages for the amplifier
circuits. The same winding furnishes the synchronous
demodulator and isolation converter drive signals and
is tapped at ahigher voltage level to drive the neon
lamps DS101 and DS102. These neon lamps provide the
drive signal for the photocell modulators V101 and
V102.
3-13. INPUT DIVIDER
3-14. The basic full-scale sensitivity of the Model
845AR is limited to amaximum of 1millivolt. There-
fore, input signals above this value must be reduced.
The input divider consists of R101 through R109 and
RANGE switch S101A. On ranges being amultiple of 1,
input voltages above 1millivolt are divided down to 1
millivolt or less, upon selection of the proper range.
On ranges being a multiple of 3, input voltages above 1
millivolt are divided down to 300 microvolts or less,
upon selection of the proper range. On ranges of 1
millivolt and below, a 10 megohm resistor, R104, is
connected across the input to provide afixed value of
input impedance.
3-15. AC AMPLIFIER
3-16. The input signal from the input divider is filtered
by athree stage, low-pass filter composed of R110,
C101, Rill, C102, R112, and C103. This filter re-
duces any ac voltage having afrequency above 1Hz.
The filtered dc voltage is then square-wave modulated
by photocell modulators V101 and V102, which are
driven by DS101 and DS102. The resulting square-wave
signal is coupled through C104 and amplified by Q101,
Q102, and Q103 which form athree stage amplifier
having ahigh input impedance. The gain of the ac am-
plifier ig controlled by the common emitter resistance
selected by the RANGE switch S101B. Maximum gain
is used on the 1, 3, 10, and 30 microvolt ranges and is
gradually reduced by the selection of R124 through R126
as the range is increased. The output of Q103 is ca-
pacitively coupled to atwo stage current amplifier
composed of Q104 and Q105. The current amplifiers
have aconstant gain controlled by fixed negative feed-
back through R130 and Clll.
3-18. SYNCHRONOUS DEMODULATOR
3-19. Tiie synchronous demodulator detects the magni-
tude and phase of the amplified signal. The 84 Hz drive
signal is applied to the base of transistor Q106 which
references the synchronous demodulator to the same
phase as the photocell modulator. The demodulated
signal is filtered by R134 and C114 before being applied
to the dc amplifier.
3-20. DC AMPLIFIER
3-21. The dc amplifier amplifies the detected dc signal
from the synchronous demodulator. Transistors Q107
through Q112 comprise atwo-stage differential amplifier
with acomplementary emitter-follower output. Negative
feedback through R149 and Cl 16 is applied to the base of
Q108 and controls the dc amplifier gain. The output
from the common emitter of Qlll and Q112 is one volt
dc for afull range input on any range, which drives the
isolation converter. Overall negative feedback through
the resistive network of R138 through R142 and R114 is
controlled by the position of the RANGE switch S101C.
This negative feedback allows precise control of the
overall gain of the Model 845AR amplifiers.
3-2

845AR
3-22. ISOLATION CONVERTER
3-23. The isolation converter drives the recorder cut-
put and meter while providing isolation from the Model
845 amplifier circuitry. The output signal from the am-
plifier is applied to the transistors Q113 and Q114. An
84 Hz reference drive signal is applied to the bases of
transistors Q113 and Q114 which causes modulation of
the dc input signal to occur. The resulting modulated
signal is coupled to the secondary of T203 where tran-
sistors Q203 and Q204 demodulate secondary signals
occuring at their 84 Hz base signal rate. Capacitor
C204 charges to the peak of the demodulated signal and
discharges through the OUTPUT LEVEL control Rl,
R211 through R213, and the meter Ml. The meter Ml
indicates the polarity and magnitude of the input voltage.
Capacitor C3 and resistor R2 filter the resulting dc out-
put voltage for the recorder output.