INSTRUCTION MANUAL
MODEL 660
GUARDED DC
DIFFERENTIAL
VOLTMETER
WARRANTY
We warrant each of our products to be free
from defects in material and workmanship. Our
obligation under this warranty is to repair or
replace any instrument or part thereof (except
tubes and batteries) which, within a year after
shipment, proves defective upon examination.
To exercise this warranty, contact your Keithley
field engineering representative. You will be
given assistance and shipping instructions.
REPAIRS AND RECALIBRATION
Keithley Instruments and its internat.ional dis-
tributors maintain complete repair facilities.
To insure prompt repair or recalibration service,
please contact your Keithley field representative
before returning the instrument.
Estimates for repairs, normal recalibrations, and
calibrations traceable to the National Bureau of
Standards are available upon request.
MODEL 660 DIFFERERTIAL VOLTMETER
TABLE OF CONTENTS
CONTENTS
Section
GENERAL DESCRIPTION. . . . .
1.
l-l.
Description. . . . . .
1-2. Operating Modes. . . .
l-3.
Applications . . . . .
1-4. Accessories. . . . . .
l-5. Specifications . . . .
l-6.
Equipment Shipped. . .
2. OPERATION. . . . . . . . . .
2-l. Front Panel Controls
and Terminals . . . .
2-2. Rear Controls and
Terminals . . . . . ,
2-3.
Preliminary Procedures
2-4. Operating Procedures .
2-5.
Model 6601A High
Voltage Divider . . .
2-6.
Recorder Output. . . .
2-7. Measuring Resistances.
2-8. Effects Due to Kelvin-
Varley Divider Gut-
put Resistance. . . .
2-9. loading and Off-Null
Resistance. . . . . .
2-10. Thermal EMF Precau-
tions . . . . . . . .
2-11. AC Effects on Measure-
ment. . . . . . . . .
2-12. Rack Mounting. . . . .
2-13. Placing in Rack. , . .
2-14. 234-Volt Operation . .
Page
1
5
5
5
5
6
8
8
LO
11
12
12
12
12
14
14
Section
Page
3-3. Kelvin-Varley Divider. . 16
3-4. Null Detector. . . . . . 16
3-5. Guarding . . . . . . . . 17
4. MAINTENANCE. . . . . . . . . . 19
4-l.
General......... 19
4-2. Parts Replacement. . . . 19
4-3. Troubleshooting. . . . . 19
4-4. Adjustment of
km
Balancing Controls. . . 22
4-5. Zener Current Test . . . 23
4-6. Calibration Procedures , 24
4-7. Kelvin-Varley Divider
Verification. . . . . . 24
4-8. Range Calibration. . . . 25
4-9. Reference Voltage Sup-
ply Stability Test. . . 29
REPLACEABLE PARTS. . . . . . . 37
5.
5-1. Replaceable Parts List . 37
5-2.
How to Order Parts . . . 37
Model 660 Replaceable
Parts List. . . . . . . 38
Model 6601A Replaceable
Parts List. . . . . . . 44
Model 660 Schematic
Diagram 15199B. . . . . 47
Model 6601A Schematic
Diagram 16321B. . . . . 48
* Change Notice
3. CIRCDIT DESCRIPTION. . . . .
3-l. General, . . . . . . .
3-2. Reference Voltage
Supply. . . . . . . .
0664R
15
15
15
Wellow Change Notice sheet is in-
cluded only for instrument modifications affecting the Instruction Manual.
i
GENERAL DESCRIPTION
MODRL 660 DIFFERENTIAL VOLTMETER
.
FIGURE 1.
ii
Keithley Instruments Model 660 Guarded DC Differential Voltmeter.
0564R
MODEL 660 DIFFERENTIAL VOLTMETER
GENERAL DESCRIPTION
SECTION 1.
l-l. DESCRIPTION.
The Keithley Model 660 Guarded DC Differential Voltmeter is a convenient,
8.
self-contained potentiometric system that accurately measures dc voltages. It
has 20.02% limit of error from 100 millivolts to 500 volts full scale, 0.005%
repeatability, and a reference supply stable to 0.005% indefinitely without
manual standardization.
resolution is within 2 microvolts.
Features for convenient use include:
b.
automatically lighted decimal points;
input polarity switch; floating operation up to 500 volts off ground; fast
overload recovery; better than 45 decibels of 60-cps rejection.
l-2. OPERATING MODES.
a conventional vacuum tube voltmeter. As a potentiometer, it can measure
from 100 millivolts to 500 volts full scale with 20.02% limit of error and
from 100 microvolts to 100 millivolts full scale within 20 microvolts. As
a VTVM, the Model 660 can measure from 100 microvolts to 500 volts full
scale with an accuracy of *3% of full scale.
tances from 10 megohms to 50,000 megohms within *5%.
Maximum null sensitivity is 2100 microvolt6 full scale;
The Model 660 can be used as a potentiometer or as
GENERAL DESCRIPTION
five in-line readout dials with
10 to 25-millivolt recorder output;
It can also measure resis-
l-3. APPLICATIONS.
The Model 660 is used for measurements over a wide range. Typical ap-
a.
plications in the microvolt region are the matching of semi-conductors and
the monitoring of noise, transients and drift.
tions include power supply and meter calibration, and tube potential measure-
ments.
Overall stability of 0.01% indefinitely makes the Model 660 useful in
b.
measurements of extended duration.
stability, long-term drift runs, and monitoring during environmental and
reliability tests are possible uses.
The null-detector output permits use with potentiometric recorders and
c.
digital voltmeters equipped with automatic print-out. The Model 660 is useful in quality control, product development, inspection and production.
Floating operation to 500 volts is provided for measurements such as
d.
plate potential differences of balanced amplifiers.
l-4.
range of the Model 660 to 5000 volts.
its input resistance is 10 megohms. The overall limit of error of the Model
660 with the Model 6601A is 20.03%.
for the Divider,
ACCESSORIES.
a. Model 6601A High Voltage Divider is a 1OO:l divider which extends the
Measurements of long-term power supply
The divider accuracy is tO.Ol% and
Paragraph 2-5 gives operating instructions
High level voltage applica-
0664R
GENERAL DESCRIPTION
Model 4000 Rack Mounting Kit, containing two brackets and a top cover,
b.
adapts the Model 660 for standard 19-inch rack mounting.
the Model 660 is 19 inches wide x 13-l/2 inches deep x 5 inches high.
to paragraph 2-12 for installing instructions.
l-5. SPECIFICATIONS.
AS AN ULTRA-STABLE POTENTIOMETER:
MODEL 660 DIFFERENTIAL VOLTMETER
For rack mounting,
Refer
LIMIT OF ERROR:
after 15-minute warm-up.
er,
REPEATABILITY: Within 0.005%.
COMBINED STABILITY OF KELVIN-VARLEY DIVIDER AND REFERENCE VOLTAGE SUPPLY:
?O.Ol% indefinitely,
exceed 0.002% per OC.
MAKIm NULL SENSITIVITY:
resolution.
INPUT RESISTANCE: Infinite at null, from 0 to 500 volts.
POLARITY:
FLOATING OPERATION:
(chassis ground).
RESISTANCE - LOW TO GROUND: lo8 ohms shunted by 0.05 microfarad.
RESOLUTION CHART:
Input Voltage
Range
(volts)
Positive or negative, selectable by switch.
20.02% of input voltage or 20 microvolts, whichever is great-
after l-hour warm-up. Temperature coefficient does not
100 microvolts full scale with 2.0-microvolt
May be operated with up to 500 volts between LO and GND
Maximum Usable
Maximum Dial
Resolution
(millivolts)
Full-Scale Null
Sensitivity
(millivolts)
Maximum Meter
Resolution
(microvolts)
10 10
1 1
0.1
0.01 0.1
5
- 500
- 50
- 5
- 0.5
500 volts
50 volts
5 volts
0.5 volt
50
0.5
0
AS A SENSITIVE VTVM:
Voltage Ranges (oositive or negative)
0.1
100
10
2
2
@out Resistance
10 megohms
10 megohms
10 megohms
10 megohms
0664R
MODEL 660 DIFFEXENTIAL VOLTMETER
GENERAL DESCRIPTION
Null Rannes
100 volts
10 volts
1 volt
100 millivolts
10 millivolts
1 millivolt
0.1 millivolt
NOTE:
Input resistance on O.l-millivolt range
Input Resistance (Slewind
10 megohms
10 megohms
10 megohms
1 megohm
1 megohm
1 megohm
100 kilohms
can be increased; see paragraph 2-8.
VTVM ACCURACY: +3% of full scale on all ranges.
DRIFT: Less than 10 microvolts, non-cumulative, after 15-minute warm-up.
60 CPS REJECTION:
Greater than 45 db.
GENERAL CHARACTERISTICS:
STABILITY OF REFERENCE VOLTAGE SUPPLY: +0.005% indefinitely, after l-hour
wan-up.
STABILITY OF ZENER REFERENCE:
20 ppm per year and five ppm per 'C.
LINE TRANSIENT REGULATION:
Less than 5 ppm for 10% variation from 117 volts.
ACCURACY OF DIVIDER: 0.005%.
CHARACTERISTICS OF NULL DETECTOR OUTPUT:
Output:
Output Resistance:
Adjustable 10 to 25 millivolts dc full scale, all ranges.
300 ohms maximum.
Noise: 2 microvolts peak-to-peak (referred to input).
Note:
Recorder used must have input fully isolated - 10"
minimum leakage.
CONNECTORS:
Input: Binding posts on front panel.
Output: Banana jacks on rear.
TUBE COMPLEMENT:
One OG3, five 12AX7, one 12BH7, two 12B4A, one 6CMS.
POWER: 105-125 volts or 210-250 volts, 50-60 cps, 75 watts.
DIMENSIONS (Bench):
5-l/2 inches high x 17-l/2 inches wide x 13-l/2 inches
deep.
NET WEIGHT: 24 pounds.
ohm
l-6. EQUIPMENT SHIPPED.
The Model 660 Guarded DC Differential Voltmeter is
factory-calibrated and is shipped with all components in place. All units are
shipped for bench use.
to paragraph 2-12 for assembly instructions.
Model 4000 Kit may be ordered for rack mounting; refer
The shipping carton also contains
the Instruction Manual.
0664R
3
GENERAL DESCRIPTION
MODEL 660 DZFFF.RENTIAL VOLTMETER
FIGURE 2.
Model 660 Front Panel Controls and Terminals.
Designations refer to Parts List.
._ . ..-. IT -.,,
--
-m-
m-
-T-
-.
.
:
FIGURE 3. Model 660 Rear Controls and Teminals.
4
0564R
MODEL 660 DIFFERENTIAL VOLTMETER
OPERATION
SECTION 2.
2-1. FRONT PANEL CONTROLS AND TERMINALS. (See Figure 2.)
a. POLARITY Switch.
the input polarity.
voltage supply so both positive and negative voltages may be measured; it does
not reverse the meter polarity.
b. NULL Switch.
decade ranges from 0.1 millivolt to 100 volts full scale. When the Switch is
in the VTVM position,
voltmeter for the four ranges of the RANGE Switch.
c. RANGE Switch. The RANGE Switch adjusts the sensitivity of the VTVM in
four steps: 0.5, 5, 50 and 500 volts full scale, It also determines the
voltage across the Kelvin-Varley divider and the position of the decimal point
light - which also serves as a pilot light - between the five Reference
Voltage Dials.
Reference Voltage Dials.
d.
panel set the reference voltage when the Model 660 is used as a potentiometer.
The POLARITY Switch turns the instrument on and selects
The Switch reverses the polarity of the internal reference
The NULL Switch sets the null detector sensitivity for seven
the Model 660 operates as a conventional vacuum tube
Five in-line dials at the top of the front
OPERATION
e. METER ZERO Control.
zero. The control is needed on only the 0.1 and l.O-millivolt null ranges; on
the other ranges, the pointer will normally be on zero without adjustment.
The control has a range of approximately +50 microvolts.
Input Terminals.
f.
is to be connected to the low impedance terminal of the unknown voltage and
the red HI terminal is to be connected to the high impedance termiual. A
second set of terminals marked Lo and GND is provided for grounding the LC
input terminal to the chassis when desired.
together internally.
2-2. REAR CONTROLS AND TJBMLNALS. (See Figure 3.)
a. Fuse.
fuse. For 210-250 volt operation,
Power Cord.
b.
unless otherwise specified on the rear panel. A three-wire power cord is used.
c. NULL DETECTOR OUTPUT. Two terminals, marked + and -, supply a dc signal
from the null detector.
d.
OUTPUT ADJUST. '
justs the null detector output between 10 and 25 millivolts full scale.
For 105-125 volt operation,
The Model 660 is for a 105-125 volt, 50-60 cps line source,
The METER ZERO Control adjusts the meter pointer to
At the lower left front panel, the black LO terminal
The LO terminals are connected
the Model 660 uses a l-ampere 3 AG
the Model 660 uses a 0.5-ampere 3 AG fuse.
A screwdriver control next to the OUTPUT terminals ad-
2-3.
0664R
PRELIMINARY PROCEDURES.
a. Connect the Model 660 power cord to a 105-125 volt, 50-60 cps line
5
OPERATION
MODEL 660 DIFFBRENTIAL VOLT?fETER
source.
Set the controls as follows:
RANGE
500
NULL VTVM
POLARITY
Reference Voltage Dials
+
Zero
The decimal light between the third and fourth dials will light.' Allow the
instrument to warn up for 15 minutes to meet the specified accuracy on all ranges,
With the input terminals open, set the NDLL Switch to 0.1 MV and zero the
b.
meter with the METER ZERO Control.
no adjustment should be required in eight hours after a 15-minute warm-up.
The stability of the Model 660 is such that
Then
return the NULL Switch to VTVM.
2-4.
OPERATING PROCEDURES.
a. The Model 660 is used first as a VTVM to determine the approximate value of
the unknown voltage.
It is then used in the potentimetric mode to determine the
voltage to ti.O2%.
VTVM Operating Procedures.
b.
Eleven full-scale ranges are available for VTVM operation. When the
1.
NIJLL Switch
ranges.
is
at VTVM, the RANGE Switch determines one of four full-scale
By putting the five Reference Voltage Dials at zero, the Model 660
can then operate as a VTVM on the seven null ranges,
Connect the unknown voltage to the input terminals, using the LO term-
2.
inal for the low impedance terminal of the unknown. Refer to Figure 4.
LH $C
,
I
Unknavn
Voltage =
0000
Off-Grou,,d’=r
Potential -
B
FIGIJRE 4. Input Connections to Model 660.
The two diagrams show the input
circuit for measuring at ground and for floating.
In A, the unknown voltage has one terminal at ground.
The shorting link
is between the LO and GND terminals of the Model 660.
In B, the unknown voltage has both terminals off ground potential. Note
this floating or off-ground potential mst be less than 500 volts. Also
note the shorting link is not used.
6
0664R
MODEL 660 DIFFRRENTIAL VOLTMRTER
Switch the RANGE Switch to the most sensitive range lor an on-scale
3.
'~meter deflection.
c. Potentiometric Operating Procedures.
NOTE
Avoid large overload voltages on the null detector. No permanent
damage will occur even with 500-volt overloads, but some open circuit offset will be caused in the null detector.
to the polarization of the input filter capacitors, will disappear
after about an hour.
Leave the RANGE Switch at the last setting used in the VTVM operation.
1.
If the VTVM reading is negative, reverse the POLARITY Switch.
Set the first two Reference Voltage Dials to the first two digits of
2.
the unknown voltage found in the VTVM operation.
Set the NULL Switch to the initial null setting shown in Table 1.
3.
Adjust the Voltage Reference Dials progressively for zero meter deflection
while increasing the null detector's sensitivity with the NULL Switch.
Deflections to the right indicate the voltage being measured is more
positive than the Reference Voltage Dial setting.
The offset, due
OPERATION
RANGE Initial NULL
Setting Setting
I
500 volts 100 volts
50 volts
~ 5 volts 1 volt
0.5 volt 100 millivolts
/
TABLE 1. Recommended Null Sensitivities and Settings.
The most accurate resistors in the Kelvin-Varley divider are in
the first two Reference Voltage Dials. Therefore,
most accurate readings, use the first tm dials whenever possible.
The value of the unknown voltage is read directly from the Reference
4.
Voltage Dials.
a) The Dial reading will be within the specified limit of error
if the NULL Switch is at the most sensitive setting (Table 1) for the
range used and if the meter is as close to null as possible.
need not be actually at null.
b) For Dial settings below 499910, only the five Dials need be
read to be within specifications. However,
an approximation of a sixth digit.
10 volts
NOTE
the meter may be read as
Most Sensitive
NULL Setting
10 millivolts
1.0 millivolt
0.1 millivolt
0.l millivolt
to obtain the
The meter
For Dial settings above 499910, the first Dial is not used and
c)
the voltage is read directly from the remaining four Dials.
0664R
7
OPERATION
MODEL 660 DIFFERENTIAL VOLTMETER
d) Use the meter asa null indicator when balancing voltages. For
Dial..settings above 499910, the meter approximates a fifth dial reading.
However, the loading effect of the Kelvin-Varley divider on the meter
causes some quantative inaccuracies when the meter is off null.
paragraph 2-8).
(See
2-5. MODEL 6601A
a. The Model 6601.A extends the range
scale.
The ratio of the Divider is lOO:l, ?O.Ol%; the input resistance is
10 megohms.
HIGH
VOLTAGE DIVIDER.
of
the Model 660 to 5000 volts full
The limit of error when the Model 660 is used with the Model
6601A is 20.03%.
NOTE
Maximum input into the Model 66Ol.A is 5000 volts.
Set the Model 660 RANGE Switch to 50 volts. The meter will read to 5000
b.
volts full scale.
Connect the HI and LO Divider terminals to the HI and Lx)
Voltmeter terminals, respectively. Connect the unknown voltage to the Divider
INPUT, and use the Model 660 as outlined in paragraph 2-4.
FIGURE 5.
Model 6601A High Voltage Divider.
2-6. RECORDER ODTPwr.
a. Reconsaended recorders for use with the Model 660 are the F.L. Moseley Auto-
graf 680 series recorder and the Minneapolis Honeywell recorder (lOmv-0-101~ scale,
50 kilohms input
resistance).
Any recorder used must be able to float 500 volts
off ground and its input nest be fully isolated (1010 ohm minimum leakage resistance to ground).
Before attaching the recorder, set all Reference Voltage Dials to zero.
b.
Disconnect the unknown voltage and short both Model 660 input terminals.
Set
the NULL Switch to 10 MV. Connect the recorder to the OUTPUT terminals on the
Model 660 rear panel.
0664R
MODEL 660 DIFFRRRRTIAL VOLTMETER
Set the Reference Voltage Dials to 10 millivolts to apply an accurate
c.
OPERATION
ID-millivolt potential to the null detector on the lo-millivolt null range.
This will provide a full-scale recorder output which can be matched to the
recorder’s range between 10 and 25 millivolts by adjusting the OUTPUT ADJUST
Control.
To obtain accurate results and/or to prevent damage to the instruments,
d.
the recorder must be able to float off-ground with the Model 660. Leakage
and pickup between the two instruments should also be minimized.
Make sure neither recorder terminal is grounded. Use a 3-wire
1.
grounded power line for the recorder.
If a Z-wire line is used, connect
the recorder chassis and the Model 660 chassis with a separate lead.
Minimize all sources of leakage between the output terminals, the
2.
recorder and ground. Use polystyrene or Teflon-insulated wire where
possible.
If the connecting wires are shielded,
connect
the shield to
the LO terminal.
Avoid long leads between the Model 660 and the recorder.
3.
If difficulty is encountered in off-ground measurements, such as
4.
unstable readings, connect a lo-microfarad capacitor between the LO and
GND terminals on the Model 660 front panel.
NOTE
Do not short either Model 660 output terminal to the case;
this may damage the Kelvin-Varley divider.
e. If there is substantial recorder
jitter
on the O.l-millivolt null
range, place a filter between the Model 660 and the recorder. Refer to Figure
6 for this connection.
Rote the filter must also be insulated from ground.
Terminals
FIGURE 6.
Recorder Filter.
A filter between the Model 660 and the
recorder may be necessary when using the O.l-millivolt null detector range.
0664R
9
OPERATION
MODEL 660 DIFFEREXTIAL VOLTMETER
2-7. MJ?ASURING RRSISTANCES.
Procedure&: The Model 660 can be used to rapidly measure resistances
frzm 10 megohms to 50,000 megohms with an accuracy of 25%.
tance,
connect the resistor across the Model 660 RI and LO terminals.
To measure resis-
Use a
short isolated lead to the HI terminal to prevent measuring leakage between
the leads.
Set the RANGE Switch to 500.
Then determine the value of the
resistor as follows:
For resistances between 10 megohms and 100 megohms, set the NULL Switch
1.
to 10 volts; adjust the Reference Voltage Dials to obtain a full-scale meter
deflection.
Subtract 10.000 from the dial setting to obtain the value of
the resistor in megohms.
For resistances between 100 megohms and 1,000 megohms, set the NULL
2.
Switch to 1 volt; adjust the Reference Voltage Dials to obtain a full-scale
meter deflection. Subtract 1.0000 from the dial setting and multiply the
difference by ten to obtain the value of the resistor in megohms.
For resistances between 1,000 megohms and 50,000 megohms, set the NULL
3.
Switch to 1 volt; adjust the Reference Voltage Dials to obtain a convenient
deflection on the meter. Calculate the value of the resistor using,
Q = 10 Rd
megohms
v
Where R, is the unknown resistance;
Ed is the Reference Voltage Dial setting in volts;
V is the null detector meter reading in volts.
FIGURE 7. Simplified Model 660 Circuit for Measuring Resistances. Rr is
the unknown resistance. R,, is the input resistance of the null detector;
V is the null detector; Ed is the buckout voltage.
Theory: The above method for determining the value of an unknown resistor
b.
is based upon the equation for the circuit (see Figure 7).
If an unknown resis-
tance is across the Model 660 input terminals, then
=i C&+&-J
Ed
Es.
2
10 0664R
MODEL 660 DIFFRRRRTIAL VOLTMETER OPERATION
where Ed is the Reference Voltage Dial setting in volts;
i is the current in the circuit;
R, is the unknown resistance;
Rn is the input resistance of the null detector meter in ohms.
The current can be written i = V/R.,.,,, where V is the null detector meter
reading in volts.
If measurements are made on the 1 to loo-volt null ranges, the input resist-
ance, Rn, is 10 megohms. Equation 3 becomes
Equation 2 now becomes
Eq. 3
Eq. 4
2-a.
last Reference Voltage Dial may appear to be inaccurate, The apparent error
is due to a voltage drop across the Kelvin-Varley divider. This effect involves only the null detector sensitivity and not the accuracy of the dial
setting. When the Model 660 is as near to null as possible, the Reference
Voltage Dial setting is correct within the instrument's specifications.
There is no error
millivolt off-null setting of the reference voltage will not produce a full-
scale deflection on the meter. The output resistance of the Kelvin-Varley
divider is significant compared to the shunt resistance across the null
detector meter, and the IR drop across the divider will cause the meter to
be in error from 1% to 36%.
setting of 100, 10 and 1 millivolt off-null respectively, can show up as
an error of up to 6% for Reference Voltage Dial settings near 25000. The
effect cannot be observed on the other null ranges of the Model 660.
EFFECTS DUE TO KELVIN-VARLEY OUTPUT RESISTANCE.
a. When the Model 660 is used for nulling on the O.l-millivolt range, the
at
null.
The effect is most apparent on the O.l-millivolt null range, A O.l-
b.
On the 100, 10 and l-millivolt null ranges, a
c. The amount of deflection on the meter is equal to the ratio
R,
+ NW
51
where R,., is the shunt resistance across the meter (10 megohms for the 100 to
l-volt null ranges, 1 megohm for the 100 to l-millivolt null ranges,
and 100 kilohms for the O.l-millivolt null range);
Rkv is the output resistance of the Kelvin-Varley divider, which is
a maximum of 62.4 kilohms at settings of 24545 and 25455 and a
minimum of 100 ohms at settings of 49998 and 00002.
d. The input resistance on the O.l-millivolt range may be increased by
substituting a l-megohm, l/Z-watt resistor (Keithley Part No. R12-1M) for
the lOO-kilohm input resistor, Rl003, if the source resistance is not more than
0664R 11
OPERATION
MODEL 660 DIFFRRRNTIAL VOLTMETER
100 kilohms.
instrument open circuited only.
The higher input resistor will increase the input noise of the
The effect of line transients will also be
more apparent.
2-9. LOADING AND OFF-NULL RESISTANCE.
a. The input resistance of the voltmeter for the seven null ranges varies
from 10 megohms to 100 kilohms a8 given in the Specifications. This resist-
ance, however, is not the effective input resistance of the Model 660. Its
input resistance is considerably higher due to the potentiometric principle
of operation.
The Vehe is
given by
where Rin is the effective input resistance of the Model 660;
Rd is the voltage indicated on the Reference Voltage Dials;
Rn is the shunt resistance or input resistance of the null detector meter;
V is the null detector meter reading.
To find the loading effect the Model 660 will have on a circuit, equa-
b.
tion 5 may be used to compute the effective input resistance of the instrument.
At null, the input resistance is infinite.
Off null, the input resistance is
usually high compared to the internal resistance of the unknown voltage, and
the loading will not be enough to affect the accuracy of the measurement. For
example, the Model 660 input resistance is 1010 ohms if the Reference Voltage
Dials are set at 1.0000 volt on the l-millivolt null range for a reading off
null by 10% of full scale.
2-10.
THERMAL RMF PRRCAUTIONS. Observe standard thermocouple techniques to
reduce thermal emf errors for measurements using the most sensitive null ranges.
Since the Model 660 can read to 0.5 microvolt, thermal emf's can intro-
duce considerable error8 into the measurements. In general, use pure copper
lead8 throughout the system when measuring in the microvolt range. For extensive measurements in the microvolt region, request the article, DC Microvolt
Measurements, from Keithley Instruments, Inc.
2-11. AC EFFECTS ON MRASUFEMRNTS. To minimize errors from ac signals present
in the unknown voltage, the Model 660 employs a chopper-stabilized null detector
operating at a 42-cps chopping rate with a two-section R-C filter at the input.
Very large ac components on the measuring lines, however, may reduce off-null
sensitivity. Also, heavy 60-cps pickup will be observed as needle quiver. If
ac components affect measurements by the Model 660, additional filtering is required. For an ac signal of a single frequency, a twin T filter is effective.
For an ac variable frequency, an ordinary low-pass filter may be used.
2-12.
RACK MOUNTING. (See Figure 8.)
a. The tide1 660 is shipped for bench use with four feet and a tilt-bail.
The
Model
4000
Rack Mounting
Kit
converts the instrument to rack mounting
to the standard RIA (RRTMA) lV-inch width.
To convert the Model 660, remove the four screws at the bottom of each
b.
side of the instrument case.
Lift off the top cover assembly with the handles;
12
0664R
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