Fluke 845A User manual

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-yearfromdateof 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.
If 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. Alt 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 any way, 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 enhance your
use of this instrument. Please address your requests or correspondence to: JOHN FLUKE MFG. CO., INC., P.O. BOX
C9090, EVERETT, WASHINGTON 98206, 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 FSuke Miq. Co., 8nc., P.O. Box C909O, Everett, Washington 98206
Rev. 6/81

845A
iii

845A
SECTION I
1-1. INTRODUCTION
1-2. The Fluke Model 845 High Impedance Voltmeter-
Null Detector allows measurement of dc voltages from
one microvolt to 1000 volts dc in^l9 ranges. When used
as a null detector on the 100 millivolt range and below,
the input impedance is 10 megohms. Alinear recorder
output allows the instrument to be used for production
testing and also as adc amplifier with amaximum gain
of 120 db.
1-3. The instrument consists of the line powered
Model 845A, and the line-battery-powered Model 845AB.
The instruments are half-rack in size, and are equipped
with resilient feet and atilt-up bail for field or bench
use. Asingle instrument may be mounted in astandard
19 inch rack by means of metal handle rack adapter kit
881A-102, Two instruments may be mounted side-by-
side in astandard 19 inch rack by means of metal handle
rack adapter kit 881A-103.
1-4. ELECTRICAL SPECIFICATIONS
INPUT VOLTAGE RANGE
1microvolt to 1000 volt dc end scale in nineteen ranges,
using XI and X3 progression.
INPUT RESISTANCE
100 megohms on 300 millivolt range and above,
10 megohms on 100 millivolt range and below.
ACCURACY
±{2% end scale +0. 1microvolt).
MAXIMUM METER NOISE (Input Shorted)
RANGE NOISE (peak-to-peak)
1microvolt 0. 20 microvolt
3microvolt 0. 25 microvolt
10 microvolt -1000 volt 0. 30 microvolt
METER RESPONSE TME (to 90% of reading)
RANGE TIME
1microvolt 5seconds
3microvolt 3seconds
10 microvolt -1000 volt 1-1/2 seconds
INPUT ISOLATION
Better than 1012 ohms at less than 50% relative humidity
and 25° Cregardless of line, chassis, or recorder
grounding. Better than lOlO ohms up to 80% relative
humidity and 35° C. With driven guard, isolation im-
proves by at least one order of magnitude up to 10l3
ohms. Any input terminal may be floated 1100 volts
off chassis ground.
DC COMMON MODE REJECTION
Better than 160 db, input short-circuited,
80% relative humidity.
Better than 140 db, open-circuited,
50% relative 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.
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 ac voltage not to
exceed 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. Response
time is approximately half that of the meter, therefore
noise may exceed meter noise by 6 db,
STABILITY OF ZERO
Better than 0. 15 microvolt/hour.
Better than 0. 3microvolt/day.
TEMPERATURE COEFFICIENT OF ZERO
Less than 0. 1microvolt/® Cfrom 15° Cto 35° C.
Less than 0. 2microvolt/® Cfrom 0°C to 50® C.
ZERO CONTROL RANGE
±5 microvolt minimum.
OVERLOAD PROTECTION
Up to 1100 volts dc may be applied on any range. Typical
recovery time is 4seconds.
1-1

845A
INPUT POWER
Model 845A
115/230 volts ac ±10%, 50 to 440 Hz, approximately
3watts.
Model 845AB
Rechargeable battery or 115/230 volts ac ±10%, 50 to
440 Hz, approximately 6watts during recharge (40
hours operation on full charge, batteries trickle-
charged while instrument operates from line power).
1-5. ENVIRONMENTAL SPECIFICATIONS
OPERATING TEMPERATURE RANGE
Within all specifications from 15° Cto 35° C.
Within all specifications from 0°C to 50° Cexcept:
Maximum noise and meter response time -
derated by afactor of two.
DC Common Mode Rejection -
derated by 20 db.
STORAGE TEMPERATURE RANGE
Model 845A -40° Cto +70° C.
Model 845AB -40° Cto +60° C.
RELATIVE HUMIDITY RANGE
0to 80%.
SHOCK
Meets hammer blow requirements of MIL-T-945Aand
MIL-S-901B.
VIBRATION
Meets 10 Hz to 55 Hz tests of MIL-T-945A,
1-6 MECHANICAL SPECIFICATIONS
MOUNTING
Resilient feet provide for bench and portable use. For
side-by-side EIA rack mounting of two units, add Adapter
Kit 881A-103 (includes handle-brackets and key plate).
For EIA rack mounting of asingle unit, add Adapter Kit
881A-102.
WEIGHT
Model 845A 9pounds.
Model 845AB 10-1/4 pounds.
SIZE
8inches high by 8-1/2 inches wide by 9inches deep.
1-2
Figure 1-1 .MODEL 845 OUTLINE DRAWING

AMessage From
The Static Sensitive (S.S.) devices are identified in
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 for handling them.
3. Using the procedures, and packaging and
bench techniques that are recommended.
the Fluke technical manual parts list with the symbol
The following practices should be followed to minimize damage to S.S. devices.

5. USE ANTI-STATIC CONTAINERS FOR
HANDLING AND TRANSPORT
6. DO NOT SLIDE S.S. DEVICES OVER
ANY SURFACE
7. AVOID PLASTIC. VINYL AND STYROFOAM®
IN WORK AREA
PORTIONS REPRINTED
WITH PERMISSION FROM TEKTRONIX, INC.
AND GENERAL DYNAMICS. POMONA DIV.
®Dow Chemical
8.
WHEN REMOVING PLUG-IN ASSEMBLIES,
HANDLE ONLY BY NON-CONDUCTIVE
EDGES AND N^ER TOUCH OPEN EDGE
CONNECTOR EXCEPT AT STATIC-FREE
WORK STATION, PLACING SHORTING
STRIPS ON EDGE CONNECTOR USUALLY
PROVIDES COMPLETE PROTECTION TO
INSTALLED SS DEVICES.
9. HANDLE S.S. DEVICES ONLY AT A
STATIC-FREE WORK STATION
10. ONLY ANTI-STATIC TYPE SOLDER-
SUCKERS SHOULD BE USED
11. ONLY GROUNDED TIP SOLDERING
IRONS SHOULD BE USED,
Anti-static bags, for storing S.S. devices or pcbs
with these devices on them, can be ordered from the
John Fluke Mfg. Co., Inc.. See section 5in any Fluke
technical manual for ordering instructions. Use the
following part numbers when ordering these special
bags. John Fluke Part No. Description
680892
680934
680942
680983
681023
5" X8" Bag
8" X10" Bag
8" X12" Bag
12" X16" Bag
18" X18" Bag
Pink Poly Sheet Wrist Strap
30" X60" X60 Mil P/N TL6-60
P/N RC-AS-1200 $7.00
$20.00
Page 2of 2J0089C-07U8412/SE EN Litho in U.S.A.

845A
ON
2-1. INTRODUCTION
2-2. This section of the manual contains information
necessary to operate the Model 845 High Impedance
Voltmeter-Null Detector. Should any difficulties be
encountered during operation of your instrument, feel
free to contact your nearest John Fluke Sales Repre-
sentative or write directly to the John Fluke Mfg. Co.
Inc.
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-
atii^ descriptions are given in the following pars^raphs.
2-5. PRELIMINARY OPERATION
2-6. Connect tlie Model 845 line plug to a115 volt ac
power outlet or to 230 volts ac, if the instrument is so
wired. Kyour instrument is aModel 845A, proceed
with steps aand b, if your instrument is aModel 845AB,
check the batteries as outlined in paragraph 2-7. Upon
completion of asatisfactory battery check, proceed with
steps cand d.
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.
a. Place the Model 845A controls as follows:
POWER ON
RANGE 10 MICROVOLTS
OPR ZERO
b. Adjust the ZERO control for an initial zero meter
deflection. Place the RANGE switch to the 1MI-
CROVOLT RANGE and re-zero with the ZERO
control.
c. Place the Model 84SAB controls as follows:
POWER LINE OPR or BAT OPR
RANGE 10 MICROVOLTS
OPR ZERO
d. Adjust the ZE.RO control for an initial zero meter
deflection. Place the RANGE switch to the 1MI-
CROVOLT RANGE and re-zero with the ZERO
control.
2-7, MODEL 845AB BATTERY CHECK
2-8. The Model 845AB batteries must be in the proper
charge state for the instrument to operate properly in
the BAT OPR mode. To check the batteries proceed
as follows:
a. Place the POWER switch to BAT CHK position.
b. The Model 845AB meter needle should deflect with-
in the BATTERY OK region. If the meter needle
does not stay within the BATTERY OK region for
at least 10 seconds, charge the batteries as indi-
cated in paragraph 2-9. Kthe batteries are ade-
quately charged, refer to paragraph 2-5.
2-9. MODEL 845AB BATTERY CHARGING
2-10. If the Model 845AB is left in the BAT OPR mode
for an extended period of time, the batteries will be-
come discharged. If the batteries are fully discharged,
the instrument will not operate properly in the LINE
OPR mode until the battery voltage can be brought up to
3/4 of full voltage. This will require about 15 minutes
of charging. To charge the Model 845AB batteries pro-
ceed as follows:
a. Connect the line plug to a115 volt ac power outlet
or to 230 volts ac, if the instrument is so wired,
b. Place the POWER switch to BAT-CHG LINE OPR.
After 16 hours the batteries will be fully charged
and capable of operating the Instrument for at least
40 hours.
2-1

I
METER
Indicates the voltage o|
input terminals
845AB POWER switch is
OFF: power removed from the
instalment.
LINE OPR: instrument operates on
line power.
BAT GHG LINE OPR: instrumenr
operates on line power while
batteries are being charged.
BAT CHK; meter indicates charge
state of batteries.
BAT OPR: instrument operates on
internal batteries.
ipplted to the
When the Model
placed to
BAT CHK position, the meter indi-
cates the charge state of the internal
batteries.
ZERO: opens input terminal
and shorts amplifier input,
allowing zeroing of instru-
ment.
OPR: instrument is ready for
operation as avoltmeter or null
detector.
Screwdriver adjustment used to set
the meter to zero.
INPUT AND COMMON
Provides connection to voltage
being measured.
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 may be applied to
the guard terminal, shunting
the leakage current from the
common terminal to ground.
For full-scale meter deflection,
this control provides adjustment
of the isolated output voltage
from 0to 1volt dc„
RANGE CONTROL
Provides selection of the desired
full-scale voltoge sensitivity.
gISOLATED OUTPUT TERMINALS
Provides connection to an external
g| recorder.
Figure 2-1. CONTROLS, TERMINALS, AND INDICATORS
2-2

845A
2-11.. MECHANICAL ZEROING
2-12. It may be necessary to adjust the mechanical
zero control of the Model 845 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 or to LINE OPR.
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 para-
graph 2-5.
d. Repeat steps athrough cuntil the meter is me-
chanically and electrically zeroed.
2-13, OPERATION AS AHIGH IMPEDANCE
VOLTMETER
2-14. To operate the Model 845 as aHigh Impedance
Voltmeter, perform the preliminary operations ac-
cording to paragraph 2-5 and proceed as follows:
a. Place the controls as follows:
POWER ON/LINE OPR or
BAT OPR
OPR OPR
RANGE 1000 VOLTS
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 845
INPUT terminal and connect the common point of
the voltage being measured to the Model 845 COM-
MON terminal.
c. Deflection of the meter indicates the polarity and
magnitude of the measured voltage. Increase the
sensitivity of the Model 845 for maximum on-scale
deflection.
2-15 OPERATION AS ANULL DETECTOR
2-16. The Model 845 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 845. Equipment connections for
various types of null detector configurations are illus-
trated by Figure 2-2 through 2-4. To operate the Model
845 as aNull Detector perform the preliminary oper-
ations 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 845 controls as follows:
POWER ON/LINE OPR or
BAT OPR
OPR OPR
RANGE as desired
c. Adjust the circuit being measured for zero or anull
deflection on the Model 845 meter.
Figure 2-2. BRIDGE DETECTOR-FLOATING SUPPLY
2-3

845
A
Figure 2-4. BRIDGE DETECTOR-FLOATING NULL DETECTOR
2-19. USE OF ISOLATED OUTPUT
2-20. DC ISOLATION AMPLIFIER
2-21, The Model 845 may be used as adc isolation
amplifier havii^ avdtage gain csf ip to 120 db, dependii^
on the settings of the RANGE switch and the OUTPUT
LEVEL control. To compute the maximum voltage gain
on any range of the Model 845, iise the following formula:
1volt (maximum
isolated output)
voltage gain to db .20 log ranGE (in volts)
2-22, RECORDER OUTPUT
2-23. The Model 845 ISOLATED OUTPUT may be used
to provide an output voltage, adjustable from zero to
one volt for afull-scale meter deflection for use with a
recorder. Since the output is isolated from the input,
floating measurments can be made without the use of a
floating recorder. To use the adjustable recorder out-
put, proceed as follows:
2-17. MEASURING VOLTAGES WITH ASTANDARD
CELL
2-18. The Model 845 may be used with avoltage di-
vider and astandard cell to calculate unknown voltages
with ahigh degree of accuracy. Connect the equipment
as illustrated in Figure 2-5. Perform the preliminary
operation as outlined in paragraph 2-5 and proceed as
follows:
a. Place the Model 845 controls as follows:
POWER ON/LME OPR or
BAT OPR
OPR OPR
RANGE as desired
b. Adjust the voltage divider for zero or null deflection
on the Model 845 meter while placing the RANGE
switch to successively more sensitive ranges.
c. Calculate the unknown voltage by dividing the stand-
ard cell voltage by the final division ratio of the
divider.
2-4
a. Connect the recorder to the ISOLATED OUTPUT
terminals.
The lower ISOLATED OUTPUT terminal
is connected to chassis ground. Kaground
reference is undesirable, disconnect the
green wire between the power supply cir-
cuit board and the grounding pin on the ac
line plug. Refer to F%ure 2-6 for wire
location.

845A
b. Turn the recorder on.
c. Proceed as outlined in paragraph 2-13 or 2~15, 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.
and forth will dissipate this charge, eliminating the
problem, Witii ahigh source impedance, the response
of the instiTiment is unavoidably 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 extremely high while the response
time for the normally encountered low source imped-
ances is very fast.
2-31. OVERLOAD VOLTAGES
The ISOLATED OUTPUT current capability
is 100 microan^eres with a5kilohm source
impedance.
2-24. OPERATING NOTES
2-25. SPURIOUS VOLTAGES AND CURRENTS
2-26, 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
Model 845 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.
2-27. THERMOELECTRIC VOLTAGES
2-28. Ha circuit 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 Seebeck voltages, can be reduced
by using metals having low thermoelectric potentials and
keeping all junctions at the same temperature. The
terminals of the Model 845 are made of pure copper,
gold-flashed to prevent tarnish. For lowest thermal
voltages, all connections to the Model 845 should be
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
solder coated copper .Nickel and nickel- based alloys
are not suitable for connections to the instrximent. Ex-
cellent results can be obtained using ordinary TV twin
lead, or even lamp cord if high insulation resistance
is not required, if shielding is necessary, use alength
of flat braid over the cable.
2-29. HIGH SOURCE IMPEDANCE
2-30. Due to the very high input resistance and ex-
treme sensitivity of the Model 845, it is charge sensi-
tive. Thus, aperson’s body potential, an electrostatic
voltage, can cause charge redistribution at the input to
the instrument and result in meter needle deflection as
a hand approaches the input terminals. Careful shielding
will eliminate this problem. Also, due to charges that
may be deposited on the input terminals when the OPR
switch is set to ZERO, an appreciable transient will
result when the switch is set to OPR if nothing is con-
nected to the input terminals. Turning the switch back
2-32. The instrument is designed to withstand up to
HOO 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 switch.
However, repeated or continuous overloads above 200
volts in the ranges below 3millivolts will result in ex-
cessive dissipation in the protective, low-pass -filter
resistor RllO. This will result in thermal voltages
which may take several minutes to subside after the
overload is removed.
2-33. GUARDING
2-34. The instrument has an inner chassis connected
to the GUARD terminal on the front panel. Ordinarily,
this GUARD terminal is striped to the COMMON termi-
nal. When connected in this way, the 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.
2-35. INCREASING INPUT RESISTANCE
2-36. In the 1microvolt to imillivolt ranges, a10
megohm resistor is connected directly across the input
of the instrument. The input resistance may be in-
creased on these ranges by disconnecting the 10 megohm
resistor where it attaches to the RANGE switch. How-
ever, the input resistance will no longer be well defined.
Typical input resistances with the 10 megohm resistor
removed, are as follows:
RANGE INPUT RESISTANCE
1microvolt
3microvolt
10 microvolt
30 microvolt to 1millivolt
300 megohms
1000 megohms
3000 megohms
10, 000 megohms
2-5