Tektronix 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
TABLE OF CONTENTS
CONTENTS
section
GENERAL DESCRIPTION . . . . . .
1.
1-L.
1-2.
1-3.
l-4.
1-5.
1-6.
2.
OPERATION . . . . . . . . . . .
2-l.
Description . . . . . .
Operating Modes . . . .
Applications. . . . . .
Specifications. . . . .
Accessories . . . . . .
Equipment Shipped . . .
Front Panel Controls
and Terminals . . . .
2-2.
Rear Controls and
Terminals . . . . . . .
2-3.
2-4.
2-5.
2-6.
Preliminary Procedures. .
Operating Procedures. . ,
Recorder Output . . . . ,
Effective Input Resis-
tance . . . . . . . . .
2-7.
Null Sensitivity and
Source Resistance . . .
2-8.
2-9.
3.
CIRCUIT DESCRIPTION . . . . . .
3-1.
3-2.
3-3.
3-4.
3-5.
3-6.
AC Effects on Measurement
Current Measurements. . .
General . . . . . . . .
Reference Voltage Supply.
Kelvin-Varley Divider . .
Null Detector . .
Oscillator. . . . .
Guarding . . . . .
.
.
.
.
.
.
.
.
. . 5
. . 5
. . 6
. . 6
. . 9
. . 9
. . 11
. . 11
. . 11
. . 13
. . 13
. . 13
. . 14
. . . . .
. . . . .
. . . . .
Page
.
.
.
.
.
.
.
.
14
15
16
1
1
1
2
2
3
3
5
Section Page
CALIBRATION . . . . . . . ., . . 21
5.
5-l.
5-2.
5-3.
General . . . . . . . . . . 21
Calibration Schedule. . . . 22
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.
6.
ACCESSORIES . . . . . . . . . , . 35
6-1.
6-2.
6-3.
6-4.
Oscillator Adjustment . . .
27
Model 6301 Guarded Probe. . 35
Rack Mounting . . . . . . . 35
Placing in Rack . . . . , . 36
Model 6013 pH Electrode
Adapter . , . . . . . . . 37
REPLACEABLE PARTS , . . . . . , . 39
7.
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
,4.
SERVICING . . , . . .
4-l.
4-2.
4-3.
4-4.
4-5.
General . . . . . .
Servicing Schedule.
Parts Replacement .
Troubleshooting . .
Adjusting Grid Bias
Cathode Follower.
4-b.
4-7.
4-a.
0 168il
Zener Diode Replacement . . . 20
Meter Adjustment . . . . . . 20
Adjusting Quadrature Control. 20
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
of
. . . . .
17
17
17
17
17
19
*Change Notice . . . . . . . . . . . Last
Page
*Yellow Change Notice sheet is included
only for instrument modifications affect-
ing the Instruction Manual.
MODEL 630 POTENTIOMETRIC ELECTROMETER
GENERAL DESCRIPTION
SECTION 1.
l-l.
potentiometer, It measures from 300 millivolts to 500 volts dc with 0.01% limit of error
from source resistances as high as lOlo ohms,
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,
10 5 ohms at 1% off null and to infinite resistance at null.
put resistance , .the Model 630 draws an absolute minimum of current to avoid loading and
polarizing the source.
lighted decimal points; one-volt,
ranges except the l-millivolt; input polarity switch;
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.
500 volts full scale with *O.Ol% limit of error and from 1 millivolt full scale to 300
millivolts within 30 microvolts.
measure from 1 millivolt full scale to 500 volts within 23% full scale.
DESCRIPTION
The Keithley Model 630 Potentiometric Electrometer is a convenient, self-contained
a.
Using the guarded, vibrating-reed null detector,
F"'
Features for convenient use include:
c.
on the l-millivolt to l-volt ranges, increasing to over
one-milliampere output for full-scale deflection on all
As a potentiometer,
As a vibrating-reed electrometer, the Model 630 can
GENERAL DESCRIPTION
and below 300 millivolts within 30 microvolts.
the Model 630 has a minimum (slewing)
With this extremely high in-
5 in-line readout dials with automatically
floating operation up to 500 volts
it can measure from 300 millivolts to
1265R
FIGURE 1.
Keithley Instruments Model 630 Potentiometric Electrometer.
1
GENERAL DESCRIPTION
MODEL 630 PO'TENTIOMETRIC ELECTROMETER
l-3.
APPLICATIONS.
Due to its very high input resistance,
a.
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.
fuel-cell studies,
silicon resistivity and capacitor charge measurements, and for mrasur-
The Model 630 is also ideally suited for making Hall-effect and
ing gate potentials of field effect transistors.
The null-detector output permits use with potentiometric recorders and digital volt-
b.
meters equipped with automatic print-out.
duct development,
inspection and production.
The Model 630 is
useful
in quality control, pro-
l-4. SPECIFICATIONS.
AS A POTENTIOMETRIC ELECTROMETER:
LIMIT OF ERROR:
ute warm-up.
LONG-TERM STABILITY:
TEMPERATURE COEFFICIENT:
?O.Ol% of reading or 30 microvolts, whichever is greater, after 30-min-
Accuracy is exclusive of null-detector drift.
Will operate within stated limit of error for one year.
Does not exceed 0.001% per OC.
REPEATABILITY:
MAXIMUM NULL SENSITIVITY:
INPUT RESISTANCE:
Within 0.0025% or 30 microvolts, whichever is greater.
1 millivolt full scale with 30-microvolt resolution.
Infinite at null, from 0 to 500 volts.
0 to 500 volts with DETECTOR INPUT Switch in OPEN position.
FLOATING OPERATION:
INPUT ISOLATION:
500 volts maximum off chassis ground.
Circuit ground to chassis ground: 108 ohms shunted by 0.05 microfarad.
AS A VIBRATING REED ELECTROMETER:
VOLTAGE RANGES:
NULL RANGES:
INPUT RESISTANCE:
O-5 volt full scale to 500 volts in four decade ranges.
1 millivolt full scale to 100 volts in six decade ranges.
1013 ohms or 1010 ohms selectable by switch, L-millivolt to l-volt
ranges; 1010 ohms, 5 to 500-volt ranges.
ELECTROMETER ACCURACY:
ZERO DRIFT:
Less than 2 millivolts per 24 hours after Z-hour warm-up.
f3% of full scale on all ranges, exclusive or noise and drift.
non-cumulative.
10 15
ohms at.l% off null frop
Long term drift is
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 CHARACTERISTICS:
GENERAL DESCRIPTION
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:
POWER:
105-125 or 210-250 volts (switch selected); 50, 60 or 400 cps; 50 watts.
DIMENSIONS, WEIGHT:
Special Triaxial. output: Banana jacks.
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.
more conveniently.
ACCESSORIES.
Model 6301 Guarded Probe with 3-foot connecting cable allows measurements to be made
a.
Using the Probe does
not
affect any Model 630 specification.
Model 4000 Rack Mounting Kit, containing two brackets and a top cover, converts the
b.
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;
The shipping carton also contains the Instruction Manual and the Model 6302 Mating
refer to Section 6 for assembly instructions.
Connec-
tor with 3-foot triaxial cable.
016GR
3
OPERATION
s303
?4ODEL 630 POTENTIOMETRIC ELECTROMETER
Reference Voleage Dials
5304
s305
S-306 It381
-
DETECTOR INPUT
Swirch (5102)
Receptacle
FIGUKlc L.
.JlOl
5103 5104
Switch
(S101) (R122)
I'W~~L OJU rronr: .raneL wncrols and Terminals.
CO"tL-01
Replaceable Parts List and schematic diagram.
Switch
(5104)
SwFtch
(5301)
The designations refer to the.
4
FIGURE 3. Model 630 Rear Panel
Controls
and Tarminzlr.
1265
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 posi-
tion, 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.
a.
REAR CONTROLS AND TERMlNALS.
Fuse.
For LOS-125 volt operation, the Model 630 uses a l-ampere 3 AG fuse.
(See Figure 3)
210-250 volt operation, the Model 630 uses a 0.5-ampere 3 AG fuse.
0166R
For
5
OPERATION
MODEL 630 POTmTIOMETRIC ELECTROMETER
Power Cord.
b.
less otherwise specified on the rear panel.
3-prong plug provides a ground connection for the cabinet. A 3:2 prong adapter is provided.
NULL DETECTOR OUTPUT.
C.
null detector.
COARSE ZERO Switch.
d.
tact potential of the vibrating-reed capacitor.
ten ZO-millivolt steps.
117-234 Switch.
e.
or 234volt ac power lines.
2-3.
power cord.
PRELIMINARY PROCEDURES.
Check the 117-234 Switch and the Fuse for the proper ac line voltage. Connect the
a.
Set the Model 630 front panel controls as follows:
b.
DETECTOR INPUT Switch
RANGE Switch
NULL Switch VTVM
Reference Voltage Dials
The Model 630 is designed for a 105-125 volt, 60-cps line source, un-
The 3-wire power cord with the NEMA approved
A screwdriver adjustment is provided for bucking out the con-
The screwdriver-operated slide switch sets the Model 630 for 117
POLARITY Switch
Two terminals, marked + and -,
The Switch has a i-100 millivolt span in
ZERO CK
500
-I.
ZWXJ
supply a dc signal from the
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.
Set the NULL Switch to 1.0 MV.
c.
mally, using the COARSE ZERO Control on the rear panel is not necessary. Return the
NULL Switch to VTVM.
Check meter zero periodically.
per day,
DETECTOR INPUT Switch to ZERO CK disconnects the input signal source from the
instrument, avoiding polarization.
2-4. OPERATING PROCEDURES,
The Model 630 is used first as a voltmeter to determine the approximate value of
a.
the unknown voltage.
to 10.01%.
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.
the input cable and any attached circuitry; any cable flexure will generate
electrostatic charges which the Model 630 will detect.
which will cause inaccuracies in continuous readings.
It is then used in the potentiometric mode to determine the voltage
Zero the meter with the METER ZERO Control.
Null detector drift can be up to 2 millivolts
Setting the
It is not necessary to disconnect the input.
Also,
tightly tie down
Nor-
6
L265R
MODEL 630 POTENTIOMETRIC ELECTROMETER
OPERATION
FIGURE 4.
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.
The Model 630's high input resistance allows circuit measurements without causing
b.
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.
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.
at VTVM, the RANGE Switch determines one of four full-scale ranges.
Reference Voltage Dials at zero,
six null ranges.
Input Connections to Model 630. The two diagrams show the input circuit for
On ranges above 1 volt, the input
NOTE
Ten full-scale ranges are available for VTVM operation. When the NULL Switch is
By putting the five
the Model 630 can then operate as a voltmeter on the
Connect the unknown voltage to the INPUT Receptacle, using the Model 6302 Input
2.
Cable or the Model 6301 Guarded Probe.
lead to the LO Terminal.
Switch the RANGE Switch to the most sensitive range for an on-scale meter deflec-
3.
tion.
When using the guarded probe, use a separate
7
OPERATION
Potentiometric Operating Procedures.
d.
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.
Leave the RANGE Switch at the last setting used in the voltmeter operation. If
1.
the voltmeter reading is negative, reverse the POLARITY Switch position.
Set the first two Reference Voltage Dials to the first two digits of the unknown
2.
voltage found in the voltmeter operation.
When switching between the 50-volt and SOO-volt ranges and when changing the
POLARITY Switch, the meter will sometimes temporarily read off-scale.
due to transients introduced when the circuit is interrupted and it does not
indicate a faulty instrument.
The offset will disappear after about 5 minutes.
NOTE
MODEL 630 POTENTIOMETRIC ELECTROMETER
This is
Set the NULL Switch to the initial
3.
null setting shown in Table 1. 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 Vol-
tage Dial setting.
NOTE
The most accurate resistors in the Kelvin-Varley divider are in the first
two Reference Voltage Dials. Therefore,
dings, use the first two dials as much as possible.
The value of the unknown voltage is read directly from the Reference Voltage Dials.
4.
The dial reading will be within the specified limit of error if the NULL Switch
a)
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,
read to be within specifications (*O.Ol% of reading or 30 microvolts).
meter may be read as an approximation of a sixth digit,
RANGE
Switch
Setting Setting Setting
I
I
500 50 0.5 5v v v v 100 100 10 1.0 Mv v v v
I
TABLE 1. Recommended Model 630 Null Sensitivities and Settings.
to obtain the most accurate rea-
Initial
NULL Switch NULL Switch
only the five Dials need to be
Most Sensitive
1.0 1.0 10 1.0 Mv Mv Mv Mv
However, the
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,
8
the meter approximates a fifth dial reading.
0166R
MODEL 630 POTENTIOMETRIC ELECTROMETER OPERATION
2-5. RECOFDER OUTPUT.
Recommended recorder for use with the Model 630 is the Keithley Model 370. Any re-
a.
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).
before attaching the recorder, set all Reference Voltage Dials to zero. Disconnect
b.
The Model 370 meets these requirements.
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.
Set the Reference Voltage Dials to 10 millivolts to apply an accurate lo-millivolt
c.
potential to the null detector on the lo-millivolt null range.
scale recorder output of 1 volt at 1 milliampere.
corder may load the Model 630 output. On this range,
On the l-millivolt null range, the re-
output resistance is 100 ohms.
This will provide a full-
To obtain accurate results and/or
d.
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.
The Model 630 input resistance varies. It can be set either 1013 or 1010 ohms for
a.
the l-volt and lower ranges, and it is 1010 ohms for ranges above 1 volt.
are not the Model 630's effective input resistances.
higher due to the potentiometric principle of operation.
Its input resistance is considerably
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
These, however,
9
OPERATION
To find the loading effect the Model 630 will have on a circuit, use Equation 1 to
b.
MODEL 630 POTENTIOMETRIC ELECTROMETER
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.
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
The
For a ful.ler treatment of paragraphs 2-6 and 2-7,
Product Note,
0.6
zo.5
0
:,o.,
”
0.3
0.2
"The Effective Input Resistance of Potentiometric Voltmeters."
send for Keithley Instruments
0.L
18
FIGURE 6.
tance.
Model 630 Effective Input Res
The graph shows the effective input
resistance for the l-millivolt to l-volt
ranges when the DETECTOR INPUT Switch is
set to OPEN. Percent off null is 160 V/Ed
as defined in Equation 1.
10
~llrluw /. F,rrecr or xxlL^ce KeSlStnnCc on
Model 630 Null Sensitivity.
The graphs are
for the two null detector input resistance
settings.
in equation 2.
Ideally, V = Vs - Ed as defined
The graphs indicate the percent of the actual difference between the
source and reference voltages the meter
will indicate.
0266R
MODEL 630 POTENTIOMETRIC ELECTROMETER
OPERATION
2-7.
the Model 630 is less affected.
tiometric electrometer for accurate measurements from a high resistance source.
where V, is the source voltage in volts;
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).
resistance, the meter will indicate only a portion of the difference between the two
voltages.
ings for the Model ,630. The curves are the percent the meter reads the voltage difference. At loo%, V = V, - Ed.
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.
NULL SENSITIVITY AND SOURCE RESISTANCE.
Source resistance affects null sensitivity. Because of its high input resistance,
a.
The following paragraphs show the value of using a poten-
The equation below gives the relationship of the meter reading to source resistance:
b.
Equation 2
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
For increasing source
This discrepancy becomes more important the more off null a reading is.
Figure 7 contains graphs showing the effect source resistance has on off-null read-
c.
Note that as the Model 630 approaches null, the meter
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 re-
jection on its l-millivolt range.
sitivity and cause needle quiver or a zero shift.
the input. The component values depend on the source resistance.
might be a 109 resistor and a lOO-picofarad capacitor.
only about two seconds to the rise time.
Use a larger R or C if rise time is not important.
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,
trometer can be used to accurately measure currents as low as lo-13 amperes.
Greater line frequency pickup may reduce off-null sen-
To reduce this, use an R-C filter at
A good compromise
ohm source, this adds
With a
NOTE
Always use a polystyrene
the Model 630 Potentiometric Elec-
1010
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).
Current measuring procedure:
b.
Shunt the Model 630 input with a
seikcted resistor R. (See Table 2 for
resistor values).
Set the Reference Voltage Dial Set-
2.
ting to zero and the DETECTOR INPUT
Switch to OPEN.
Switch the Model 630 sensitivity
3.
to the lowest.setting to obtain an onscale reading.
4. The Meter reading is the voltage drop V across the shunt resistor R.
Use the Model 630 as a potentiometer and bring the meter to a null with the Re-
5.
ference Voltage Dials. Record the Reference Voltage Dial reading Ed.
Calculate the current I, using the equation,
6.
Current Range Shunt Resistor NULL Setting
in amperes in ohms
10-13
10-13
10-12
10-12
10-12
10-11
I
10-11 10-11 10-11
TABLE 2.
Use in Current Measurements.
Recommended Shunt Resistors for
10 10
1011
109
1010
1011
108
109 1010 1011
in millivolts
1
10
1
10
100
1
10
100
1000
I
I, =-
where I,
c.
the above procedure is followed.
of measurement may be *4% plus the accuracy of the shunt resistor.
calculate the current to compensate for the loading effect of the shunt resistor.
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.
It is recommended that shunt resistors no larger than 1011 ohms be used
a larger resistor the response is extremely slow.
enough
The accuracy of the measurement is i3% plus the accuracy of the shunt resistor, if
to
affect accuracy may not show up immediately due to the slow response.
However, when using a 1011 shunt resistor the accuracy
1
R
NOTE
(-
Ed v
Ed - V
An offset which 1s largFth
It is necessary to
.
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 supply'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
Simplified Model 630 Circuit Diagram.
3-2.
FIGURE 8.
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.
Regulator tube V3005 is used to keep the
The voltage then is applied
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%.
the 500-volt output of V3004 is sampled by a
The divider network ratio is ad-
Light modulator E302 compares the
sample voltage from the divider network to the voltage across zener diode, D302. Any difference 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.
1165
The amplifier output is applied to the grid of the series tube, ~3004, to
13
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