Keithley 160B User manual

Instruction Manual
Model 160B
Digital Multimeter
Contains Operating and Servicing Information
for Model 160B Digital Multimeter and
Model 1602B Digital Output
01975, Fourth Printing, December 1977
Keithley Instruments, Inc.
Instrument Division
Cleveland, Ohio, U.S.A.

INSTRUCTION MANUAL
MODEL 1606
DIGITAL MULTIMETER
(AND MODEL 16028 DIGITAL OUTPUT)
@COPYRIGHT 1975, KEITHLEY INSTRUMENTS, INC.
FOURTH PRINTING, DECEMBER 1977, CLEVELAND, OHIO, U.S.A.

CONTENTS
CONTENTS
:,OOEL ibOJ
SECTION
SPECIFICATIONS. .
I. GENERAL INFORMATION. ......
I-l, ,NTROD"CT,ON ........
i-2. FEATURES
. .
l-3. WARRANTY lNFORHATlON. ............
l-4. CHANGE NOTlCE ................
l-5. OPTIONAL MODEL 16028 D,G,TAL o"TP" T. .....
1-6. OPTIONAL MODEL 1688~ RECHARGEABLE BATTERY PACK
2. INITIAL PREPARATlON ................
2-l. GENERAL ...................
2-2. INSPECTlON. .................
Z-1. PREPARATION FOR "SE
3. OPERATlNG INSTRUCTIONS . .
3-l. GENERAL
3-2. HOW TO SELECT POWER. .
3-3. HOW TO MAKE INPUT CONNECTION!
3-4. HOW TO SELECT FUNCTlON
3-5. HOW TO MEASURE VOLTAGE
3-b. HOW TO MEASURE RESISTANCE.
3-7. HOW TO MEASURE CURRENT .
3-8. NOISE CONSIOERATIONS
3-9. THER"AL EHFS . .
3-10. MAGNETIC FlELDS .
3-11. AC ELECTRlC FIELDS .
3-12. SHIELDING. .
-
.
.
*
.
I
. .
.
3-13. HOW TO "SE THE ANALOG OUTPUT
3-14. HOW TO USE MODEL 1608 OFF-GROUND . .
3-15. HOW TO USE MODEL 16028 DlGlTAL OUTPUT.
3-16. HOW TO RACK MOUNT THE MODEL 1608
4 THEORY OF OPERATION. . . . . . . . . : :
4-l. GENERAL .
4-2. ANALOG AMPLIFIER ClRC",TRY : : : : : :
4-3. ANALOG-TO-DIGITAL CONVERTER. .
4-4. POWER SUPPLY . . . .
4-S. MODEL 16028 DIGITAL OVTPUT . : :
5. ACCESSORIES. .
MODEL 1601 AC DC PROBE
MODEL 1688A RECHARGEABLE BATTERY bAiK: :
MODEL 1683 UNIVERSAL TEST LEA0 KIT
MODEL 1600 HIGH VOLTAGE PROBE.
MODEL 1682 RF PROBE... .
MODEL 1609 CALlBRATlON COVER .
.
MODEL 1681 CLIP-ON TEST LEAD SET
6.. "AINTENANCE.
.
6-1. GENERAL.
6-2. REQUIRED TEST EQUIPHENT. t
6-3. PERFORMANCE VERIFICATION .
b-4. ADJUSTMENT/CALIBRATION PROCEDURE
7. REPLACEABLE PARTS. ...........
7-l. GENERAL. .............
7-2. ORDERING INFORMATION .......
.
.
.
.
.
., .
7-3. SCHEMATlCS
ii
. .
.
. .
.
. .
.
PAGE
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26
,'P
32
32
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37
45
45
15
45

ILLUSTRATIONS
rzg. NO. IIcLe
1 “fme”sio”al Data. --___--____-____-_____________________
2 Tilt Bail Positions. ------------------------
3 *pant panel. -------------------------------------------
4 *ottcm View Showing Line Cord. -------------------------
5 Rear View Showing Line Switches. -------__--_____--_____
6 Mode11688A Rechargeable Pack. ------------------
Batrery
7 Exploded View of Model 160B/1688A.----------------------
8 Installatio" of Pack. ------------------------
Battery
9 Battery Test Location. ------------_----___-------------
10 Front Panel Controls. -------------------_--------------
11 Imcarion of Protection Fuse. _---____-_________-________
12 Voleage Measurements "sing Model 1601 AC/DC Probe. -----
13 Current Measurements Using Made1 1651 Shunt. ----------
14 Floating operation. --_---__-__-_______________________
15 Model 16028 ,,+&tal O”f,,“t. _____-____-___-____-________
16
Exploded View of Model 16011602B. __-_-----__--____--__
17 lnstallati,,,, of &,&I 1602~. _---_______-____-__________
1.8 Rear Panel Digital Connector. -------------------
output
19 Timing Diagram for Model 1602~. ------------------------
20 Rack M,,unti,,g of Model
1.3,~. __--_______--______________
21 Overall Block Diagram. _---____-___-____--_____________
22 DC Voltage Operation. -------------_----__-------------
23 Resistance operation. __-____-_________--______________
24 current Operaelan. -------------------------------------
25 Absolute Value Deeector. ---__--_-________-_____________
26 LSI Block Diagram. ----_---___----___--_________________
27 A/D Converter Diagram. _--___-_________--_______________
28 Identification af Se~,ent.s and Mulriplex lines. --------
29 Tap and Bottom Cover Assembly. -------------------------
30 Locarion of Calibration Ad,ustmenfs. ------------------
31 Location of Test mints.
32 Location of Chassis Connections. ----------------------
33 Location of Fuses. -------------------------------------
34 case Outline - Integrated CirCUitS. --------------------
35 case Outline - Integrated circuits. _---_______________-
36 Case Outline - Integraeed Circuits. _--____--_____--__-
37 case Outline - Inregrated circuits. --------------------
38 case ourline - Transistors.
39 Case Outline - Thick Film Networks. _---__-_____-_______
40 Case Outline - Thick Film Networks. --------------------
41 Case O"tline - Custom LSI. ---_----_---_-_-------------
42 Component Layout* pc-349. --_-_--______________________
43 component Layout, pc-352. ----_---_-----____--_______I_
rage
iv
vi
1
3
4
4
;
9
11
13
13
14
15
16
17
18
*"
22
23
25
25
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27
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29
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44
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0976

SPECIFICATIONS MODEL 1608
I
10.7
(27: 5
i
I
DIMENSIONS IN INCHES (MM)
I
FIGURE1.
Dimensional Data.
1174

SPECIFICATIONS
Calibrated at 25” *ST
war,mte* to, 1 year
AS A DC VOLTMETER
AS AN OHMMETER

Liz2 m
NORMALBENCH POSITION LEVEL POSITION - TILTEAIL AT FRONT
J
&
45" POSITION
-
LEVEL POSITION - TILTBAIL%

MODEL 160B
SECTION 1. GENERAL INFORMATION
l-l. INTROD”CTION. The Model 1608 is a wide-range
digital multimeter capable of measuring voltage, cur-
rent, and resistance. me Model 1608 offers SeYen
ranges of voltage from lrn" to 1000 volts, nine ranges
of current from 1O"A to 1ooomA, and ten ranges Of re-
sistance from 1n to lOOOM12. The Model 1608 display
is 3 digifs plus 100% overrange.
1-2. FEATURES.
a. Voltage sensitivity to 1 micravole per digit.
b. current .se"Giti"ity to O.OlnA per digit.
C. Resistance sensitivity to 0.001n per digit.
d. Floating capabilify LO _+1*00 "OlC.5 above case.
e. Built-in analog output.
f. Optional Made1 1602R Digital Output
s. Optional Model1688ARrchargeable "attery Pack.

HANDLE AND
TILT BAIL
I
LINE
COW
STORAGE
~
S S
BAT TEST A
BAT TEST B
I
CONDENSEDOPERATING INSTRUCTIONS S = SCREW
LOOSEN FOR
DISASSEMBLY
2 1174

MODEL 1608 INITIAL PREPARATION
5112 J113 !
ANALOGOUTPUT 520I
SWITCHES SHOWNFOR
OPERATION FROM105-125V LINE
DiGi
OUTF
J302
:TAL
'UT
3

L‘IG”RE 7. Exploded view Of Model 1600,16881\
4 0976

I
I BATTERi FUSES
I F203, F204
MODEL 1608 CHASSIS \\
SPACER
(4 PLACES)
CAUTION
DISCONNECT LINE CORDBEFORE OPENING
THE TOP AND BOTTDMCOVER,
LINE VOLTAGE ON THE CHASSIS
REPRESENTSA SHOCKHAZARD.
I FIGURE8.
Installation "f Battery Pack.
0976 5

INITIAL PREPARATION
SECTION 2. INITIAL PREPARATION
2-l. GENERAL. This section provides information
needed for incoming inspecrian and preparation for
use.
2-2. INSPECTION. The Model 1603 was carefully in-
spected both mechanically and electrically before
shipment. "pm receiving the instrument, check for
any obvious damage which may have occurred during
f*El"Sit. Report any damages to tne shipping agent.
TO verify the electrical specifications, follow the
procedures given in Section 6.
2-3. PREPAR**ION FOR "SE. The Model 160~ is shipped
ready-to-use. The instrument can be powered from
line voltage or from rechargeable nickel-cadmium bat-
teries (when the oprional Model 1688A Rechargeable
Battery Pack is installed).
a. How to operate From IdlIe Power. The Model 16OB
provides a three-wire cord which mates with third-wire
;rounded receptacles (see Figure 4 for ~1% configura--
tion,. the permanently installed line cord is stored
by wrapping the card around the base of the i"str"-
ment as shown in Figure 4.
1. wow to Set Line Switches. The Made1 1608 has
two rear panel Line Switches which enable selection
of line voltages over ranges from x0-llO", 105125v,
195-235", or 210-250". TO operate from line power.
determine the appropriate line voltage range to be
used and set the Line Swirches as fallows:
TABLE 2-1.
Summary of Line Switch Settings. I
I
I
‘Jo-110” I
1
LOW. 117
105-125”NORM,117
I
195-235” LO", 234
210-250" NORM. 234
For example, if the line voltage ea be used is
approximately 115 volts, select NORMand 117 which
permits B range of operarion from 105 to 125v.
hfter line Switches are set, connect the line cord
and set the Power Switch to LINE.
2. ,.ine Fuse Requirements. The Model 1608 uses
~VO line fuses to protect the line-operated power
SWPlY. ne fuse types are l/8 ampere, MB, slow-
blow. Replace with Keithley Part No. F"-20.
3. How to Replace Fuses in Model 1688A. The
!Jodel, 1688H uses 1 ampere fuses to protect the
power supply regulators in case of malfunction.
Fuses F203 and F204 are plug-in types and are lo-
cated on pc board PC-349 as shown in Figure 8.
Keplace with 1 ampere, 3AB, slo-blo, Keithley Part
NO. FU-28.
MODEL 160B
b. Row to operate From Battery Power. TO operate
the Model 16OB from batteries, the Model 1688A Re-
chargeable battery Pack must be installed. The Model
1688A can be either field or factory installed. (The
batteries in the Model 1688A may need recharging be-
fare their first usage to power the Model 1600.)
NOTE
The Model 1688A Rechargeable Battery Pack can
be insralled within the Model 1608 chassis at
any time. However, if the Model 16028 Digital
Output is already installed, the Model 1688A
cannot be used simultaneously.
1. HOWto Install Model 168% Rechargeable Bat-
tery Pack. The batteries furnished with the Model
1688A come already installed in the battery pack.
the battery pack includes 7 rechargeable "C" cells
(1.2", 3 AMP HR) and 1 rechargeable pack (16.8V).
If baeeeries need to be replaced or re-installed,
be certain to observe the proper polarity of indi-
vidual cells as shown in Figure 6. To install the
Model 1688A Battery Pack, turn the instrument over
so that the bottom cover faces up. Loosen four
slotted screws on the batrom cover as shown in
Figure 4. (A chisel-blade screwdriver is requires'
to loosen the slotted screws.) Turn over the in-
strument with tap cover facing up, taking care to
hold the top and bottom covers together. Carefully
remove the top cover to gain access fo the printed
circuit board. (Two "ires that connect to the
Analog Oufput must be disconnected at the PC board
before the top cover can be set aside.) Check to
see that the four insulating spacers are in yasi-
tion on the printed circuit board. Place the Model
1688A Battery Pack in position an the spacers with
the cable oriented as sham in Figure 8. Plug the
4-wire Conneceor (5205) into the mating receptacle
(P205) taking care to orient the connector as
shown in Figure 8. After the Battery Pack is in-
stalled. replace the top cover. Turn over the in-
strumen; wiih bottom co&r facing up and tighten
down the four slotted-head screws.
TABLE 2-2.
Sumnary of nateerie Used in Model 1688A
Rechargeable "C" cell,
1.2". 2 AMP-FIR
Rechargeable battery
pack, 16.8V, .225
Am-m
1 TM-32
0976
6

The instrument must he operated in the BAT
mode in order t” obtain a valid battery con-
dition at test points A and 8. This3 Will
enstire that tile batteries are supplying
power to the instrument. If the voltages
are measured when the Model 1608 is opera-
ted in the LINE mode a different reading
may be observed since the batteries are not
connected and rherefore do not supply power
to the instrumenr.
0976

OPERATING INSTR”CTIONS
SECTION 3. OPERATING INSTRUCTIONS
3-l. GENERAL. This sec~ian provides information
needed to operate the Model 160B for measU*eme"f Of
voltage, cuirent, and resistance.
3-2. HOWTO SELECT POWER. The Model 160B may be
powered from line voltage or rechargeable nickel-cad-
mium batteries ("hen the Model 1688A is installed).
The Model XOB has a built-in line-voltage power sup-
ply and line cord. If the accessory Model 1604 Re-
chargeable Battery Set is ordered and installed, then
the user has the oprion of selecting line or bateery
operation via the front panel rotary power switch.
NOTE
The accessory Model 1688A Rechargeable Bat-
tery See may be ordered at ebe time of pur-
chase of the Model 160B or may be purchased
and field installed at a later time if so
desired. me Model 1688A features plug-in
Wi*i"g. As a result, no modifications need
to be made to the Model 160B chassis.
a. "m, to Operate from Line Power. The Model 160B
can be powered from Line voltage over four ranges
from a minimum of 90" to a maximum of 25OV. see
Table 2-1.
1. Set rear panel Line Switches to appropriate
positions as show" in Figure 5.
3. Set front panel Power Switch to LINE.
b. "cm to Operate from Battery Power.
I . Install Mode11688ARechargeable Battery Pack.
2. Check battery voltages at Teat Points A and
B to ensure thaL batteries are charged sufficiently.
1. If necessary, plug line cord info ac power
and set Power Switch to CHG to bring battery "alt-
age up to useable levels.
4. When bartery level is sufficient, disconnect
line cord and set Power Swifch LO BAT.
MODEL 160B
3-3. HOWTO MAKE INPUT CONNECTIONS. The Model 1608
has two front panel terminals identified as "HI"
(red) and '3.0" (black). These terminals accomodate
banana plugs. alligator clips, spade lugs, bare
wirea, and other similar input connections. Leads
may be fabricated using a good quality capper wire
terminated by single banana plugs such as Keichley
Part No. K-5 or dual banana plug such as Keithley
Part NO. m-7. Ready-made test leads are also avail-
able from Keithley. Accessory Model 1681 Clip-o"
Test Lead Set includes two 40 inch long leads ter-
minated by a banana plug and spring-loaded clip which
easily attaches co wires and terminals on pc boards,
etc. Model 1683 Universal Test Lead Kit features in-
terchangeable probe tips for various applications.
'I%e Kit includes regular probes, alligator clips,
banana plugs. spade lugs, and phone tips. All-cow==
leads and teminarions are best for measurements on
the lm" and ln ranges.
3-4. HO" TO SELECT FUNCTION. Function is selected
by means of a single front panel Range Switch. llie
Range dial is marked in "en&wring" units for meas-
"remenf Of voltage (rn", V), C"r*e"t ("‘4, LL4, n!A), and
resistance (‘2, kn, M0). For each function, rotating
the knob clockwise causes the Model 160B to switch
to a less sensitive range.
a. DC “catage. The voltage sectors of the Range
Switch are designated in millivolts ("IV) and volts
I", for full ranees from lm" to 1000". A full stop
it'the 1000 "&range prevents inadverranr switch-
ing to the nanoampere ranges by clockwise rotation.
b, Resistance. The resistance sectors of the
Lange Switch are designared in ohms (n), kilohms
(kn), and megohms (MO) for full ranges from 1 ohm
to 1000 megohms.
C. CU**e"t. The current sectors of the Range
Switch are designafed in nanoamperes ("A), micro-
amperes (,,A), and milliamperes (mA) for full ranges
franI 10 nanoamperes to 1000 milliamperes.
Condition of Instrument
switch Line Connected Line connected
POSition 1688A not installed 16888 installed
OFF OFF OFF
LINE ON ON
CHG OFF ON
BAT OFF ON
Line not connected
1688~ installed
8 0976

POWER
S203
-ZERO
R114
- RANGE
SlOl
FIGURE 10. Front Panel Controls.
3-5. HO” TO ME‘“LS”RE“OLTIICE. The Model 1608 meas-
ures dc “aleage in seven ranges: Im”, lOnl”, loom”,
lV, lOV, LOO”, and 1ooov. Maximum dc input is 1200”
dc + peak ac.
a. How to Select Ra”E,e. Function and range is
selected by rotating the Range Switch to the appro-
priate position. Three positions are direct-reading
in millivolts CmV); four positions are direct-read-
ing in voles (VI. Decimal point is selected by the
Kan,qe Switch. Polarity is automatically displayed.
If the input signal exceeds twice full range, the
display blanks (3 least significant digits) to in-
dicate an “overran& condition.
“01
Range
SeCti”
1000 v
,
100 v
10 v
1 v
100 mv
10 mv
1 In”
TABLE 3-2.
1tage Ranges Displ;
Max. Display
1999 *
199.9
19.99
1.999
199.9
19.99
1.999
d
*Maximum allowable input is 1200” dc plus
peak ac even though display can be read
beyond 1.200”.
1174
b. How to Determine Accuracy. The xodel 16011 nc-
curacy is iO.I% of readina f 1 dirit. For enarno1e.
a display reading of 1.005 volt dc will have a” ,a,,-
certainty of 10.1% f 1 digit or r.002 volts. The in-
put resisrance in the dc m”de is 10 o,c~“,,cs. !,eai-
urements from relatively high source resistnnccs
could cause an additional reading error. The dln”“,:i
of error due to loading can bc determined by rhc
following relatiansbip:
% error = 100 x RS i (R, + 10’)
where R, = Source resistance in ohms
For example, a so”rcc resistance “f 10,000 ohms Viii
result in a loading error of approximately 0.1% ai
reading.
The input current of the 1608 Cal” also cause
reading errors on the nlOSL sensitive vo1cage
ranges when high source resistances are pre-
sent. For example, an input CUrrent of lOpA
and a so”rce resistvnce of MI! produce a”
error voltage of loll”.
c. Maximum Allowable Input. The maximum L”P”L to
the Model 1608 is 1200” dc + peak ac. On lrn”, lOn,V,
and lOOn,” ranges, the maximum conti”uox,s input is
6OOV (12OOV momentary) dc + peak ac. ‘,‘hc ?,odel ,,SO”
can display dc voltages greater tba” r120O” but dnm-
age to the input is possible.
0

OPERATING INSTR”CTIONS MODEL L6OB
IMPORTANT
The Model 1608 provides ac rejection (NMRR)
of greater than 60 dB on the highesf range.
However, a large ac signal superimposed on
a dc level could cause damage if the input
CO the Model 160" exceeds 1200 volts dc +
peak UC.
d. How to Zero the Display. To accurately zero
the instrument, place a low-thermal short (such as a
piece Of clean copper wire) across the input rermin-
als, set Lhe Range Switch to the lm" range, and ad-
just the front panel Zero conerol for a flashing "i"
sign at the display. The display should then read
t.OOO, with some flicker of the least significant
digit due to noise and A-D converter uncertainfy.
Once the zero has been set accurately, there should
be little reason to rezero the instrumenf again soon
when making measurements on the 10~1" through 1000"
ranges. The time stability of rhe zero setring is
excellent, and in applications requiring continuous
operation of the Model 160B, the zero setting will
hold for months. The front panel Zero control is
convenienf when making measurements on fhe 11"" Range.
A test set-up and cables when connected to the 160B
may cause offset voltages because of thermoelectric
effects (see section 3-9). These offset voltages
might be only a few microvolts or several tens of
microvolts. In such situations, the Zero control
can be used to buck out any initial offsefs. I.""
thermal cabling and connections should be used when-
ever possible. The instrument zero can also be check
ed, and adjusted, by shorring the input on any range
(voltage, current, or resistance). me zero c**t*01
will have varying effects on the differenr ranges
depending on the sensitivity of the input amplifier,
but the zero control can be used in this manner.
Again, it should be emphasized, that for most meas-
"remene~ with the Model 160B, the Zero control can
be set once and then left untouched. (See also ?ec-
tion 3-6c "LO" Ohms Zeroing").
NOTE
With the input open 0" the lm" Range, the
Model 160~3display may show a reading as
high as i.100. This 19 equ*va1ent to an
offset currene of lo.opA. If the display
shows a reading grearer than t.100, then
the internal potentiometer, R121, should
be readjusted. Typically, ehe offset cur-
rent will be less than 5p~.
e. 80" to "se Model 1601 AC/DC Probe. TO make ac
measuremenfs, connect the Model 1601 accessory K/DC
Probe to the Model 1608 inpur terminals. Set the
Range to desired voltage range. Set switch on probe
to AC. "se the probe tip plus ground clip to make
connections to circuit under eesc. see section 5 for
complete specifications.
1-6. HOWTO MEASURERESISTANCE. The Model 1608
measures resistance in 10 ranges: lsr, 1on, 10011,
Ikn, IOk&?, lOOk0, IM,,, lOMn, lOOK,, and 100011!2.
a. HO" to Select Range. Function and range is
selected by rotafinf: the Ranae Swifch LO rhe BPPTO-
priate position. Three posi;ions are direct-reading
in ohms (n): three positions are direct-reading in
kilohms (kn); four positions are direct-reading in
megohms (MQ) Decimal point is selected by Range
Switch. If the input signal exceeds twice full range,
the display blanks to indicate an "overra"& condl-
ti**.
TABLE 3-3.
Resistance current Ranges
Range Max. Impressed
setting LIiF3play units current
1. n 1.999 ii I InA
10 n 19.99 n lti
1 MI1 1.999 Mn 100 “A
10 Mn 19.99 MO 10 "A
100 Mr! 199.9 Mn I"‘4
1000 M* 1999 MO 0.1 "A
b. How to Determine Accuracy. The accuracy of cbe
Model 1608 is specified in terms of a nercen~ of
reading and a p&cent of range. For example, a dis-
play of 1.000 kilohID (kn.1 will haw an uncertainty
of ~0.004kQ.
C. HO” to Measure “LOW” ohms. when making meas-
urements on the ln, lOn, and 1OOn ranges, rhe Model
160B is to be zeroed on the IQ range. The zeroing
is accomplished by shorting the test leads together.
and adjusting the display for a flashing + and -
.ooon. By zeroing the instrument in this manner,
lead resistance of 200 mill.iohms (minimum) can be
compensated for. The instrument must be rezeroed on
the lil range each time a different set of iese leads
is used. Before making meaS"reme"tS right *t the in-
put rerminals (no test leads used), rhe instrumene
should be zeroed on the iii Range after placing a
short, such as a piece of copper wire, across the
binding posts. Copper leads and clips are best for
making measurements on the ln Range, because the
voltage drop across a In resistor is only Im".
CAUTION
Care should be taken when making resistance
measurements in circuits which may have volt-
ages on capacitors, etc. or "here line volt-
age is present. Although the Model 1608 is
fully protected against accidental voltages
up to 250" rms in resistance function, if
higher voltages are applied, damage may occur.
f. How to "se Model 1682 RF Probe. To make rf ac
measurements, connect the Model 1682 Probe to the
Model 160B input terminals. Set the Range to desired
voltage range. Maximum allowable input is 30" *Ins
ac, 200” dc.
10 1174

-FUSE SHOWNFUSE SHOWN
TWICE ACTUAL SI
Range Max.
setting IDisplay Units s,,une R*
I,0 nA 19.99 Il.4 1060
100 nA 199.9 IL4 10%
1 LB.4 1.999 II‘4 1040
LO ,lh 19.99 irA 10311
100 UA 199.9 UA 10%
1. InA 1.999 n!A 10 il
10 rnA 19.99 In.4 I. 0
100 lx.4 199.9 mA 0.1 !1
1000 mi\ 1.999 ti 0.1 G

OPERATING INSTR”CTIONS MODEL 160B
d. HOWto "se the Model 1651 Current Shunt (for
meaS"remenfS to SOA). current measuremenf capability
Of the Model 160B may be extended to 50 amperes
through ctle use Of accessory Model 1651 SO-Ampere
shunt. The Model 1651 permits 4-terminal connections
to minimize meaeuremene error.ciue to lead resistance.
To use the Model 1651, connect the voltage sensing
leads eo the Model 160B input terminals. Connect
separate current leads (not furnished) beuveen xhe
source and the large hex-head bolts on the Model 1651.
The current leads should be rated for currents up to
50 amperes. me Shunt resistance is 0.001 ohm, vhich
produces a sensitivity of lmV/Amp. As examples, 10
amps current results in a lOIn" drop on 1Oln" RANGE,
and 30 amps yields a voltage drop Of 3Oln" which can
be read on the loom" RANGE. Power diss*paCed in the
shunt is 2.5 watts at 50 amperes.
3-8. NOISE CONSIDERATIONS. The 1imic of resolution
in voltage and current measurements is determined
largely by rhe noise generated in the source. stray
low-level noise is present in some form in nearly al1
electrical circuits. The instrument does not distin-
guish between stray and signal voltages since it meas-
ures the net "olfage. when using the lnl" and 1omv
ranges, consider the presence of low-level e1ectri-
cal phenomena such as thermocouples (thermoelectric
effect), flexing of coaxial cables (triboelectric
effect), apparent residual charges on capacitors
(dielectric absorption), and battery action Of t"O
terminals (galvanic action).
3-9. THERMAL EMFS. Thermal emfs (thermoelectric
potentials) are generated by thermal differences be-
tween two junctions of dissimilar metals. To mini-
mize the drift caused by thermal emfs, "se copper
leads to connect the circuie to the instrument. The
front panel ZERO control can be used to buck out a
cmstant thermal offset "aleage if necessary. The
Keithley accessory Model 1483 La" Thermal. Connecrio"
Kit contains all necessary materials for making very
low thermal copper crimp connections far minimizing
thermal effects.
3-10. MAGNETIC FIELDS. The presence of strong mag-
netic fields can be a potential sw.,rce of ac noise.
Magnetic flux lines which CUt a conductor can produce
large a-c noise especially at power line frequencies.
The voltage induced due to magnetic flux is propor-
tional fO the area enclosed by the circuit as well
as the rate of change of magnetic flux. For example,
the motion of a 3-inch diameter loop in the earth's
magnetic field Will induce a signal of several tenths
of a microvolt. one way to minimize magnetic pickup
is to arrange all wiring so that rhe loop area en-
closed is as small as possible (such as twisting in-
pur leads). A second way co minimize magnetic pickup
is to use shielding as described in Section 3-12.
3-11. AC ELECTRIC FIELDS. The presence of electric
fields generated by power lines or other sources can
have an effect on instrument operation. AC voltages
which are very large with respect to the full-range
sensitivity could drive the input amplifier into sat-
urarion, thus producing an errOneo"S DC OUtpUt. At
line frequency or twice line frequency, the insLru-
ment is capable of rejecting AC voltages whose peak-
to-peak amplitude is equal to the full-range DC sell-
sftivity on the loom" through 1OOOVranges. This
level of interfering AC signal will produce no more
than 1 digit error. On the lm" and 1OmV Ranges, the
instrumenf can reject line frequency and twice line
frequency signals whose p-p amplitude is equal CD 10
eimes the full-range sensitivity. Far example, on
rhe ID," Range, a 1OmVp-p, 50 Hz signal will produce
no more than 1~" (1 digit) DC erroi-. Peak-to-peak AC
voltages greater than 1x the full-range sensitivity
o,, the 1OOm" through 1000" ranges, the 10X the full-
range sensitivity on the lln" and 1omv ranges will
cause clipping in the AC section of the Model 160B
input amplifier. For this reason, shielding is re-
com,,,ended when making sensieive DC voltage measure-
menrs, or when making measurements from high so"rce
impedances. (See Shielding, Section 3-12.) The in-
serunlent is also capable of rejecting frequencies
other than power line frequencies. There is one pro-
blem to consider; however, the input amplifier of the
Model l6OB uses a modulator to convert DC signals LO
AC signals before amplification. The drive frequency
of this modulator is 220 HZ nominally, f approx. 5%.
Interfering AC signals with frequencies equal to the
modulator drive frequency, or mu1tipl.e~ fhereof, will
appear 8.3 modulated DC, and produce large DC errors.
3-12. SHIELDING.
a. Electric Fields. Shielding is usually necea-
8arv when the insrrument is in the eresence of verv
l&e a-c fields or when very sensi;ive measuremen;s
are being made. The shields of rhe measurement cir-
cuit and leads should be connecee* together to ground
at only one point. This provides a "tree" configura-
tion, which minimizes ground loops.
b. Magnetic Fields. Magnetic shielding 19 useful
where very large magnetic fields are present. Shield-
ing, which is available in ehe form of plates, foil
or cables, can be used to shield the measuring cir-
cuit, the lead wires, or the instrument itself.
C. Other Considerations.
1. Voltmeter Measurements. Use shielded input
leads when source resiseances are greater than 1
kilohm or when long input cables are used.
2. Current Measurements. On the mA and PA cur-
rent ranges, no special shielding precautions need
be taken. However, an the 100 and 10 nanoampcre
ranges, shielded input leads are recommended.
3. llesiatance Measurements. Shielding of input
leads and source are recommended for measurements
on the 10 megohm through 1000 megohm ranges to pre-
vent errOneO"S readings.
12 1174

FIGURE 12. Voltage Measurements Using Model 1601 AC/DC Probe.
CURRENTTERMINALS
VOLTAGE TERMINALS
VOLTAGE LEADS FURNISHED
;*
1
1174