Keithley 630 Instruction Manual
INSTRUCTION MANUAL
MODEL 630
POTENTIOMETRIC ELECTROMETER
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 which,
within a year after shipment, proves defective
upon examination. We will pay domestic
surface freight costs.
To exercise this warranty, call your local
field representative or the Cleveland factory,
DDD 216-248-0400. You will be given assist-
ance and shipping instructions.
REPAIRS AND RECALIBRATION
Keithley Instruments maintains a complete repair service and standards laboratory in Cleveland, and has an authorized field repair facility
in Los Angeles and in all countries outside the
United States having Keithley field representatives.
To insure prompt repair or recalibration service, please contact your local field representative or the plant directly before returning the
instrument.
Estimates for repairs, normal recalibrations,
and calibrations traceable to the National Bu-
reau of Standards are available upon request.
MODEL 630 POTENTIOMETRIC ELECTROMETER
CONTENTS
TABLE OF CONTENTS
section
Page
1.
2.
3.
,4.
GENERAL DESCRIPTION . . . . . .
1-L.
Description . . . . . .
1-2.
Operating Modes . . . .
1-3.
Applications. . . . . .
l-4.
Specifications. . . . .
1-5.
Accessories . . . . . .
1-6.
Equipment Shipped . . .
OPERATION . . . . . . . . . . .
2-l.
2-2.
2-3.
2-4.
2-5.
2-6.
2-7.
2-8.
2-9.
Front Panel Controls
and Terminals . . . .
Rear Controls and
Terminals . . . . . . .
Preliminary Procedures. .
Operating Procedures. . ,
Recorder Output . . . . ,
Effective Input Resis-
tance . . . . . . . . .
Null Sensitivity and
Source Resistance . . .
AC Effects on Measurement
Current Measurements. . .
CIRCUIT DESCRIPTION . . . . . .
3-1.
General . . . . . . . .
3-2.
Reference Voltage Supply.
3-3.
Kelvin-Varley Divider . .
3-4.
Null Detector . .
3-5.
Oscillator. . . . .
3-6.
Guarding . . . . .
SERVICING . . , . . .
4-l.
General . . . . . .
4-2.
Servicing Schedule.
4-3.
Parts Replacement .
4-4.
Troubleshooting . .
4-5.
Adjusting Grid Bias
Cathode Follower.
. . . . .
14
. . . . .
15
. . . . .
16
. . . . .
17
. . . . .
17
. . . . .
17
. . . . .
17
. . . . .
17
of
. . . . .
19
4-b.
Zener Diode Replacement . . . 20
4-7.
Meter Adjustment . . . . . . 20
4-a.
Adjusting Quadrature Control. 20
.
.
1
.
.
1
.
.
1
.
.
2
.
.
2
.
.
3
.
.
3
.
.
5
. . 5
6.
ACCESSORIES . . . . . . . . . , . 35
. . 5
. . 6
. . 6
. . 9
6-1.
Model 6301 Guarded Probe. . 35
6-2.
Rack Mounting . . . . . . . 35
6-3.
Placing in Rack . . . . , . 36
6-4.
Model 6013 pH Electrode
Adapter . , . . . . . . . 37
. . 9
. . 11
. . 11
. . 11
. . 13
. . 13
. . 13
. . 14
7.
REPLACEABLE PARTS , . . . . . , . 39
7-l. Replaceable Parts List. . . 39
7-2.
How to Order Parts. . . . . 39
Model 630 Replaceable
Parts List. . . . . . . . 40
Model 6302 Replaceable
Parts List. . . . . . . 48
Model 630 Schematic Diagram
1826lE . . . . . . . . . 51
Green Repair and
Calibration Form . . . . . 53
Section Page
5.
CALIBRATION . . . . . . . ., . . 21
5-l.
General . . . . . . . . . . 21
5-2.
Calibration Schedule. . . . 22
5-3.
Kelvin-Varley Divider . . .
Verification. . , . . . . 22
5-4.
Input Divider Calibration . 23
5-S. Range Calibration , . . . . 23
5-6.
Reference Voltage Supply
Stability Test , . . . . 26
5-7.
Oscillator Adjustment . . .
27
*Change Notice . . . . . . . . . . . Last
Page
*Yellow Change Notice sheet is included
only for instrument modifications affect-
ing the Instruction Manual.
0 168il
MODEL 630 POTENTIOMETRIC ELECTROMETER
GENERAL DESCRIPTION
SECTION 1.
GENERAL DESCRIPTION
l-l.
DESCRIPTION
a.
The Keithley Model 630 Potentiometric Electrometer is a convenient, self-contained
potentiometer, It measures from 300 millivolts to 500 volts dc with 0.01% limit of error
from source resistances as high as lOlo ohms,
and below 300 millivolts within 30 microvolts.
The upit need not be constantly calibrated or manually restandardized for a full year;
the limit of error is specified for one year.
in ut resistance of 1013 ohms,
F"'
Using the guarded, vibrating-reed null detector,
the Model 630 has a minimum (slewing)
on the l-millivolt to l-volt ranges, increasing to over
10 5 ohms at 1% off null and to infinite resistance at null.
With this extremely high input resistance , .the Model 630 draws an absolute minimum of current to avoid loading and
polarizing the source.
c.
Features for convenient use include:
5 in-line readout dials with automatically
lighted decimal points; one-volt,
one-milliampere output for full-scale deflection on all
ranges except the l-millivolt; input polarity switch;
floating operation up to 500 volts
off chassis ground; better than 60 db line-frequency rejection on the l-millivolt range.
Also, full guarding minimizes leakage problems.
l-2. OPERATING MODES. The Model 630 can be used as a five-dial potentiometer or as a
vibrating-reed electrometer.
As a potentiometer,
it can measure from 300 millivolts to
500 volts full scale with *O.Ol% limit of error and from 1 millivolt full scale to 300
millivolts within 30 microvolts.
As a vibrating-reed electrometer, the Model 630 can
measure from 1 millivolt full scale to 500 volts within 23% full scale.
FIGURE 1.
Keithley Instruments Model 630 Potentiometric Electrometer.
1265R
1
GENERAL DESCRIPTION
MODEL 630 PO'TENTIOMETRIC ELECTROMETER
l-3.
APPLICATIONS.
a.
Due to its very high input resistance,
the Model 630 is very useful in making accurate dc measurements from high resistance sources frequently encountered in electrochemical
and physical-chemical research.
It is especially useful for measuring potentials from
piezo-electric crystals, electro-chemical cells, grids and plates of tubes, biological
cells and pH electrodes.
The Model 630 is also ideally suited for making Hall-effect and
fuel-cell studies,
silicon resistivity and capacitor charge measurements, and for mrasur-
ing gate potentials of field effect transistors.
b.
The null-detector output permits use with potentiometric recorders and digital volt-
meters equipped with automatic print-out.
The Model 630 is
useful
in quality control, pro-
duct development,
inspection and production.
l-4. SPECIFICATIONS.
AS A POTENTIOMETRIC ELECTROMETER:
LIMIT OF ERROR:
?O.Ol% of reading or 30 microvolts, whichever is greater, after 30-min-
ute warm-up.
Accuracy is exclusive of null-detector drift.
LONG-TERM STABILITY:
Will operate within stated limit of error for one year.
TEMPERATURE COEFFICIENT:
Does not exceed 0.001% per OC.
REPEATABILITY:
Within 0.0025% or 30 microvolts, whichever is greater.
MAXIMUM NULL SENSITIVITY:
1 millivolt full scale with 30-microvolt resolution.
INPUT RESISTANCE:
Infinite at null, from 0 to 500 volts.
10 15
ohms at.l% off null frop
0 to 500 volts with DETECTOR INPUT Switch in OPEN position.
FLOATING OPERATION:
500 volts maximum off chassis ground.
INPUT ISOLATION:
Circuit ground to chassis ground: 108 ohms shunted by 0.05 microfarad.
AS A VIBRATING REED ELECTROMETER:
VOLTAGE RANGES:
O-5 volt full scale to 500 volts in four decade ranges.
NULL RANGES:
1 millivolt full scale to 100 volts in six decade ranges.
INPUT RESISTANCE:
1013 ohms or 1010 ohms selectable by switch, L-millivolt to l-volt
ranges; 1010 ohms, 5 to 500-volt ranges.
ELECTROMETER ACCURACY:
f3% of full scale on all ranges, exclusive or noise and drift.
ZERO DRIFT:
Less than 2 millivolts per 24 hours after Z-hour warm-up.
Long term drift is
non-cumulative.
RISE TIME (10% to 90%): Less than 2.5 seconds on any range with 1010 ohm source resistance.
LINE-FREQUENCY REJECTION:
Greater than 60 db on the l-millivolt range, decreasing to 35
db on the 500-volt range.
2
1067R
MODEL 630 POTENTIOMETRIC ELECTROMETER
GENERAL DESCRIPTION
GENERAL CHARACTERISTICS:
LINE STABILITY:
Better than 5 ppm for 10% change in line voltage.
RECORDER OUTPUT:
output:
1 volt, 1 milliampere for full-scale meter deflection on all ranges except on
l-millivolt range, where it is 10% less.
Noise:
30 microvolts peak-to-peak referred to input up to 1 cps.
Note: Recorder used must have fully isolated input, lOlo ohms minimum to ground.
POLARITY: Positive or negative, selectable by switch.
CONNECTORS: Input:
Special Triaxial. output: Banana jacks.
POWER:
105-125 or 210-250 volts (switch selected); 50, 60 or 400 cps; 50 watts.
DIMENSIONS, WEIGHT:
5-l/2 inches high x 17-l/2 inches wide x 13-l/2 inches deep; net
weight, 24 pounds.
ACCESSORIES SUPPLIED:
Model 6302 Input Cable consisting of a mating plug and 3-foot
triaxial cable.
l-5.
ACCESSORIES.
a.
Model 6301 Guarded Probe with 3-foot connecting cable allows measurements to be made
more conveniently.
Using the Probe does
not
affect any Model 630 specification.
b.
Model 4000 Rack Mounting Kit, containing two brackets and a top cover, converts the
Model 630 to fit standard 19-inch racks. Rack mounted,
the Model 630 is 5-l/4 inch&s
high x 19 inches wide x 13-l/2 inches deep. Section 6 has assembly instructions.
C. Model 6013 pH Electrode Adapter,which has a Z-foot cable and triaxial connector,
accepts Leeds & Northrop and Beckman pH electrode connectors.
The Adapter allows accurate
and convenient,pH potential measurements with the Model 630.
l-6.
EQUIPMENT SHIPPED. The Model 630 Potentiometric Electrometer is factory-calibrated
and is shipped with all components in place. All units are shipped for bench use. Model
4000 Kit may be ordered for rack mounting;
refer to Section 6 for assembly instructions.
The shipping carton also contains the Instruction Manual and the Model 6302 Mating
Connec-
tor with 3-foot triaxial cable.
016GR
3
OPERATION
?4ODEL 630 POTENTIOMETRIC ELECTROMETER
-
Reference Voleage Dials
s303
5304
s305
S-306 It381
DETECTOR INPUT
Swirch (5102)
Receptacle
5103 5104
Switch
CO"tL-01
Switch
SwFtch
.JlOl
(S101) (R122)
(5104)
(5301)
FIGUKlc L.
I'W~~L OJU rronr: .raneL wncrols and Terminals.
The designations refer to the.
Replaceable Parts List and schematic diagram.
1265
FIGURE 3. Model 630 Rear Panel
Controls
and Tarminzlr.
4
MODEL 630 POTENTIOMETRIC ELECTROMETER
OPERATION
SECTION 2.
2-1. FRONT PANEL CONTROLS AND TERMINALS.
a.
polarity.
POLARITY Switch.
The Switch reverses the polarity of the internal rc,Ccrcncc voltage supply so
The POLARITY Switch turns the instrument on and sclccts
OPERATION
(See Figure 2).
the
input
both positive and negative voltages may be measured; it does not reverse the meter polar-
ity.
DETECTOR INPUT Switch.
b.
A 3-position toggle switch allows selection of
the
Model
630 input resistance and also allows checking the meter zero.
1) Setting the Switch to 10 10 OIIMS sets the input resistance at 1010 ohms for all
ranges.
Setting the Switch to OPh'N sets the input resistance at 1013 ohms for the l-milli-
2)
volt through l-volt ranges.
The input resistance remains
at
1010 ohms for the ranges
above 1 volt.
3) Setting the Switch to ZERO CK internally disconnects the input from the vibrating
capacitor and shunts the capacitor to the amplifier low. This allows meter zeroing on
any range without removing the input signal.
'c . NULL Switch.
ranges from 1.0 millivolt full scale to 100 volts.
The NULL Switch sets the null detector sensitivity for six decade
When the Switch is in the VTVM position, the Model 630 operates as a vibrating-reed electrometer for the four ranges of the
RANGE Switch.
RANGE Switch.
d.
0.5, 5, 50 and 500 volts full scale.
Varley divider and the position of the decimal point light
The RANGE Switch adjusts the sensitivity of the VTVM in four steps:
It also determines the voltage acrws the Kelvin-
- which also serves as a pilot
light - between the five Reference Voltage Dials.
Reference Voltage Dials. Five in-line dials at the top of the front panel set the
e.
reference voltage when the Model 630 is used as a potentiometer.
have decade steps as indicated on the dial.
METER ZERO Control. The METER ZERO Control adjusts the meter needle to zero. The
f.
The last dial is continuously adjustable.
The first four switches
Control allows precise meter zeroing on all ranges when the DETECTOR INPUT Switch is set
to ZERO CK.
Input Terminals. The Model 630 uses a special triaxial receptacle for its INPUT
g.
Receptacle.
Model 6302 Input Cable.
The Control has a. range of approximately 120 millivolts.
The Receptacle is similar to a bnc connector, but requires using the furnished
Using a standard bnc conmctor could damage the INPUT Receptacle.
A set of binding posts marked LO and GND is provided for grounding the LO input terminal
to the chassis when desired.
The LO terminal and the shell of the triaxial INPUT Recep-
tacle are internally connected.
2-2.
REAR CONTROLS AND TERMlNALS.
a.
Fuse.
For LOS-125 volt operation, the Model 630 uses a l-ampere 3 AG fuse.
210-250 volt operation, the Model 630 uses a 0.5-ampere 3 AG fuse.
0166R
(See Figure 3)
For
5
OPERATION
MODEL 630 POTmTIOMETRIC ELECTROMETER
b.
Power Cord.
The Model 630 is designed for a 105-125 volt, 60-cps line source, un-
less otherwise specified on the rear panel.
The 3-wire power cord with the NEMA approved
3-prong plug provides a ground connection for the cabinet. A 3:2 prong adapter is provided.
C.
NULL DETECTOR OUTPUT.
Two terminals, marked + and -,
supply a dc signal from the
null detector.
d.
COARSE ZERO Switch.
A screwdriver adjustment is provided for bucking out the con-
tact potential of the vibrating-reed capacitor.
The Switch has a i-100 millivolt span in
ten ZO-millivolt steps.
e.
117-234 Switch.
The screwdriver-operated slide switch sets the Model 630 for 117
or 234volt ac power lines.
2-3.
PRELIMINARY PROCEDURES.
a.
Check the 117-234 Switch and the Fuse for the proper ac line voltage. Connect the
power cord.
b.
Set the Model 630 front panel controls as follows:
DETECTOR INPUT Switch
ZERO CK
RANGE Switch
500
NULL Switch VTVM
POLARITY Switch
-I.
Reference Voltage Dials
ZWXJ
The decimal light between the third and fourth Dials will light. Allow the instrument
to'warm up 30 minutes to meet the specified accuracy on all ranges.
c.
Set the NULL Switch to 1.0 MV.
Zero the meter with the METER ZERO Control.
Normally, using the COARSE ZERO Control on the rear panel is not necessary. Return the
NULL Switch to VTVM.
Check meter zero periodically.
Null detector drift can be up to 2 millivolts
per day,
which will cause inaccuracies in continuous readings.
Setting the
DETECTOR INPUT Switch to ZERO CK disconnects the input signal source from the
instrument, avoiding polarization.
It is not necessary to disconnect the input.
2-4. OPERATING PROCEDURES,
a.
The Model 630 is used first as a voltmeter to determine the approximate value of
the unknown voltage.
It is then used in the potentiometric mode to determine the voltage
to 10.01%.
6
Vibrating capacitors are extremely sensitive to shock and vibration. when
making a measurement,place the Model 630 on a vibration free, rigid structure
or on some material which will minimize vibration.
Also,
tightly tie down
the input cable and any attached circuitry; any cable flexure will generate
electrostatic charges which the Model 630 will detect.
L265R
MODEL 630 POTENTIOMETRIC ELECTROMETER
OPERATION
FIGURE 4.
Input Connections to Model 630. 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 Posts of the Model 630.
In B, the unknown voltage has both terminals off ground potential. The floating or
off-ground potential must be less than 500 volts. Also note the shorting line is pe&
used.
b.
The Model 630's high input resistance allows circuit measurements without causing
circuit loading. (See paragraph 2-6.) On the l-millivolt to L-volt null ranges, the input resistance can be set to 1013 er 1010 ohms with the DETECTOR INPUT Switch. Use the
OPEN (1013 ohm input) setting for high source resistances; use the LOLO OHMS setting to
avoid pickup problems or to increase response speed.
resistance is 1010 ohms for either setting.
On ranges above 1 volt, the input
NOTE
When making measurements on devices with high contact resistances (such as
silicon), completely isolate the power supply from ground to avoid measuring
the drop acress the contact resistance.
c. Voltmeter Operating Procedures.
1.
Ten full-scale ranges are available for VTVM operation. When the NULL Switch is
at VTVM, the RANGE Switch determines one of four full-scale ranges.
By putting the five
Reference Voltage Dials at zero,
the Model 630 can then operate as a voltmeter on the
six null ranges.
2.
Connect the unknown voltage to the INPUT Receptacle, using the Model 6302 Input
Cable or the Model 6301 Guarded Probe.
When using the guarded probe, use a separate
lead to the LO Terminal.
3.
Switch the RANGE Switch to the most sensitive range for an on-scale meter deflec-
tion.
7
OPERATION
MODEL 630 POTENTIOMETRIC ELECTROMETER
d.
Potentiometric Operating Procedures.
NOTE
Avoid large overload voltages on the null detector. No damage occurs even
with 500~volt overloads, but some open circuit offset will be caused in the
null detector.
The offset will disappear after about 5 minutes.
1.
Leave the RANGE Switch at the last setting used in the voltmeter operation. If
the voltmeter reading is negative, reverse the POLARITY Switch position.
2.
Set the first two Reference Voltage Dials to the first two digits of the unknown
voltage found in the voltmeter operation.
NOTE
When switching between the 50-volt and SOO-volt ranges and when changing the
POLARITY Switch, the meter will sometimes temporarily read off-scale.
This is
due to transients introduced when the circuit is interrupted and it does not
indicate a faulty instrument.
3.
Set the NULL Switch to the initial
null setting shown in Table 1. Adjust
the Voltage Reference Dials progressive-
ly for zero meter deflection while in-
creasing the null detector's sensitivity
with the NULL Switch. Deflections to the
right indicate the voltage being measured
is more positive than the Reference Vol-
I
RANGE
Initial
Most Sensitive
Switch
NULL Switch NULL Switch
Setting Setting Setting
tage Dial setting.
I
TABLE 1. Recommended Model 630 Null Sensitivities and Settings.
I
500 50 0.5 5v v v v 100 100 10 1.0 Mv v v v
1.0 1.0 10 1.0 Mv Mv Mv Mv
NOTE
The most accurate resistors in the Kelvin-Varley divider are in the first
two Reference Voltage Dials. Therefore,
to obtain the most accurate rea-
dings, use the first two dials as much as possible.
4.
The value of the unknown voltage is read directly from the Reference 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 indicates as close to null as possible.
b) When the first Reference Voltage Dial is used,
only the five Dials need to be
read to be within specifications (*O.Ol% of reading or 30 microvolts).
However, the
meter may be read as an approximation of a sixth digit,
c) When the first Reference Voltage Dial is not used, read the voltage directly
from the remaining four dials.
d) Use the meter as a null indicator when balancing voltages. When the first Ref-
erence Voltage Dial is not used,
the meter approximates a fifth dial reading.
8
0166R
MODEL 630 POTENTIOMETRIC ELECTROMETER OPERATION
2-5. RECOFDER OUTPUT.
a.
Recommended recorder for use with the Model 630 is the Keithley Model 370. Any re-
corder used must be able to float 500 volts off ground and its input must be fully isolated
(1010 ohm minimum leakage resistance to ground).
The Model 370 meets these requirements.
b.
before attaching the recorder, set all Reference Voltage Dials to zero. Disconnect
the unknown voltage and short the Model 630 high input to the low. Set the NULL Switch
to 10 MV.
Connect the recorder to the OUTPUT Terminals on the Model 630 rear panel. The
370 Recorder mates directly with the Model 630 using the 3701 Cable.
c.
Set the Reference Voltage Dials to 10 millivolts to apply an accurate lo-millivolt
potential to the null detector on the lo-millivolt null range.
This will provide a full-
scale recorder output of 1 volt at 1 milliampere.
On the l-millivolt null range, the re-
corder may load the Model 630 output. On this range,
output resistance is 100 ohms.
d.
To obtain accurate results and/or
to
prevent ,damage to the instruments, the recorder
must be able to float off-ground with the Model 630. Leakage and pickup between the two
instruments should also be minimized.
The Model 370 meets these requirements.
NOTE
On the l-millivolt range there is a 10% loading error if the recorder draws
1 ma of current. Thus the output for full scale deflection will be .90 volt,
1 ma.
1. Make sure neither recorder terminal is grounded. Use a 3-wire grounded power line
for the recorder.
If a 2-wire line is used,
connect the recorder chassis and the Model
630 chassis with a separate lead.
2. Minimize all sources of leakage between the output terminals, the recorder and
ground. Use polystyrene or Teflon-insulated cable where possible. If the connecting
wires are shielded, connect the shield to the LO Post.
3. Avoid long leads between the Model 630 and the recorder.
Do not short either Model 630 output terminal to the case; this may damage the
Kelvin-Varley divider.
2-6.
EFFECTIVE INPUT RESISTANCE.
a.
The Model 630 input resistance varies. It can be set either 1013 or 1010 ohms for
the l-volt and lower ranges, and it is 1010 ohms for ranges above 1 volt.
These, however,
are not the Model 630's effective input resistances.
Its input resistance is considerably
higher due to the potentiometric principle of operation.
When reference voltage (Refer-
ence Voltage Dial setting) is much greater than meter readings the value is
Ri* =
Rn( Ed )
Equation 1
v
Where Rin is the effective input resistance of the Model 630;
Ed is the setting of the Reference Voltage Dials in volts;
'Rn is the input resistance of the null detector in ohms;
V is the null detector meter reading in volts.
1067R
9
OPERATION
MODEL 630 POTENTIOMETRIC ELECTROMETER
b.
To find the loading effect the Model 630 will have on a circuit, use Equation 1 to
compute its effective input resistance. At null, where V = 0, the input resistance is
infinite. Off null, the Model 630 input resistance is usually much greater than the
source resistance, and loading will not be enough to affect the measurement accuracy.
The
graph in Figure 6 shows the Model 630 effective input resistance for the l-millivolt to
l-volt null ranges with the DETECTOR INPUT Switch set to OPEN.
Note that the input resis-
tance for a reading 1.0% off null is 1015 ohms.
NOTE
For a ful.ler treatment of paragraphs 2-6 and 2-7,
send for Keithley Instruments
Product Note,
"The Effective Input Resistance of Potentiometric Voltmeters."
0.6
zo.5
0
:,o.,
”
0.3
0.2
0.L
18
FIGURE 6.
Model 630 Effective Input Res
~llrluw /. F,rrecr or xxlL^ce KeSlStnnCc on
tance.
The graph shows the effective input
Model 630 Null Sensitivity.
The graphs are
resistance for the l-millivolt to l-volt
for the two null detector input resistance
ranges when the DETECTOR INPUT Switch is
settings.
Ideally, V = Vs - Ed as defined
set to OPEN. Percent off null is 160 V/Ed
in equation 2.
The graphs indicate the per-
as defined in Equation 1.
cent of the actual difference between the
source and reference voltages the meter
will indicate.
10
0266R
MODEL 630 POTENTIOMETRIC ELECTROMETER
OPERATION
2-7.
NULL SENSITIVITY AND SOURCE RESISTANCE.
a.
Source resistance affects null sensitivity. Because of its high input resistance,
the Model 630 is less affected.
The following paragraphs show the value of using a poten-
tiometric electrometer for accurate measurements from a high resistance source.
b.
The equation below gives the relationship of the meter reading to source resistance:
Equation 2
where V, is the source voltage in volts;
R, is the source resistance in ohms;
V is the nuli detector meter reading in volts;
Ed is the setting of the Reference Voltage Dials in volts;
R,, is the input resistance of the null detector in ohms
With no source resistance, the null detector meter will read exactly the difference be-
tween the reference voltage and the source voltage (V = V, - Ed).
For increasing source
resistance, the meter will indicate only a portion of the difference between the two
voltages.
This discrepancy becomes more important the more off null a reading is.
c.
Figure 7 contains graphs showing the effect source resistance has on off-null readings for the Model ,630. The curves are the percent the meter reads the voltage difference. At loo%, V = V, - Ed.
Note that as the Model 630 approaches null, the meter
indication as an absolute value becomes insignif+cant regardless of source resistance.
For instance, if the Model 630 is 0.1 millivolt off null on the l-millivolt range and the
source resistance is such that the meter indicates only 10% of actual value, the metier
will be within -09 millivolt. At .Ol millivolt off null, the meter will be within 9
microvolts.
NOTE
When the Model 630 is as near to null as possible, the Reference Voltage Dial
setting is always correct within the instrument's specified limit of error.
2-8. AC EFFECTS ON MEASUREMENTS. The Model 630 has greater than 60 db line frequency rejection on its l-millivolt range.
Greater line frequency pickup may reduce off-null sen-
sitivity and cause needle quiver or a zero shift.
To reduce this, use an R-C filter at
the input. The component values depend on the source resistance.
might be a 109 resistor and a lOO-picofarad capacitor.
1010
A good compromise
With a
ohm source, this adds
only about two seconds to the rise time.
NOTE
Use a larger R or C if rise time is not important.
Always use a polystyrene
capacitor or one with as good an insulation resistance to avoid degenerating the
input resistance.
2-9. CURRENT MEASUREMENTS.
a. When used with a high-value precision resistor,
the Model 630 Potentiometric Elec-
trometer can be used to accurately measure currents as low as lo-13 amperes.
The choice
0266R
11
OPERATION MODEL 630 POTENTIOMETRIC ELECTROMETER
of the shunt resistor depends upon the
current to be measured and the setting of
the Model 630 (Refer to Table 2).
b.
Current measuring procedure:
Shunt the Model 630 input with a
seikcted resistor R. (See Table 2 for
resistor values).
2.
Set the Reference Voltage Dial Setting to zero and the DETECTOR INPUT
Switch to OPEN.
3.
Switch the Model 630 sensitivity
to the lowest.setting to obtain an onscale reading.
Current Range Shunt Resistor NULL Setting
in amperes in ohms
in millivolts
10-13
10 10
1
10-13
1011
10
10-12
109
1
10-12
1010
10
10-12
1011
100
10-11
108
1
I
10-11 10-11 10-11
109 1010 1011
10
100
1000
I
TABLE 2.
Recommended Shunt Resistors for
Use in Current Measurements.
4. The Meter reading is the voltage drop V across the shunt resistor R.
5.
Use the Model 630 as a potentiometer and bring the meter to a null with the Re-
ference Voltage Dials. Record the Reference Voltage Dial reading Ed.
6.
Calculate the current I, using the equation,
1
I, =-
Ed v
R
(-
Ed - V
where I,
is the current to be measured in amperes;
R is the shunt resistance in ohms;
V is the meter reading (from step 4 above) in volts:
Ed is the setting of the Reference Voltage Dials in volts.
NOTE
It is recommended that shunt resistors no larger than 1011 ohms be used
a larger resistor the response is extremely slow.
An offset which 1s largFth
.
enough
to
affect accuracy may not show up immediately due to the slow response.
c.
The accuracy of the measurement is i3% plus the accuracy of the shunt resistor, if
the above procedure is followed.
However, when using a 1011 shunt resistor the accuracy
of measurement may be *4% plus the accuracy of the shunt resistor.
It is necessary to
calculate the current to compensate for the loading effect of the shunt resistor.
12
0266R
MODEL 630 POTENTIOMETRIC ELECTROMETER
CIRCUIT DESCRIPTION
3-1. GENERAL.
SECTION 3.
The Model 630 Potentiometric Electrometer measures voltage by the potentio-
CIRCUIT DESCRIPTION
metric (null) method. The variable known voltage is an ultra-stable 500-volt reference
supply used in conjunction with a precision 5-dial Kelvin-Varley divider.
Electronic
referencing of the 500-volt output to a zener diode standard maintains the reference sup-
ply's stability and accuracy. This method eliminates repeated manual standardization.
The difference between the divider output and the unknown voltage is indicated by the
null detector,
directly from the in-line dials of the Kelvin-Varley divider.
a vibrating-reed electrometer.
At null the unknown voltage can be read
The input and null detector
are fully guarded to avoid leakage.
NOTE
_.__ -_ __ l_..l...-__ - 1_1o ^-...
GUARDED,
VIBRATING-REED
NULL DETECTOR
INPUT
__-__- __.^ _~ _^..... -.
KELVIN-VARLEY
Guard
..,..,.. ..^. .
5.DIAL
DIVIDER
REFERENCE
VOLTAGE
SUPPLY
500 v
SUPPLY
Zener
Reference
FIGURE 8.
3-2.
REFERENCE VOLTAGE SUPPLY.
Unregulated voltage from transformer T201 is rectified by a silicon half-wave
reziifier, D301, and is filtered by capacitors C302 and C303A.
to the regulator series pass tube, V3004.
screen of V3004 at a constant potential.
To obtain a stable, accurate voltage,
b.
divider network of wirewound resistors, R325 to R330.
justed with potentiometer R327 to better than 0.01%.
sample voltage from the divider network to the voltage across zener diode, D302. Any dif-
ference between the
amplifier, V3001.
two
voltages is chopped by E302 and amplified by a 2-stage ac-coupled
'L"ne amplified output of V3001 is converted to a dc signal by light
modulator ~301 and then is amplified by the two-stage differential dc amplifier, V3002
and V3003.
The amplifier output is applied to the grid of the series tube, ~3004, to
1165
Simplified Model 630 Circuit Diagram.
The voltage then is applied
Regulator tube V3005 is used to keep the
the 500-volt output of V3004 is sampled by a
The divider network ratio is ad-
Light modulator E302 compares the
13
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