Tektronix 621 Instruction Manual
INSTRUCTION MANUAL
MODEL 621
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 areavailable upon request.
MODEL ~~I.EL~CTROMETER
CONTEaTs
CONTENT8
section
I
II
III
A.
B*[lj
C.
II
Iv
A.
B.
C.
I
D.
(1)
(2)
E.
F.
G.
v
VI
A.
B.
C.
D.
u Page No.
Introduction
................. l-l
Specifications and Description
Circuit Description.
Voltmeter
Ammeter
...................
....................
.............
Shunt Resistor Method (NORMAL)
2
Feedback Method (FAST)
Ohmeter
1
Nomal Method. <
2
use of External Voltage supply
Operation
...... <.. ..........
...............
................... 4-1
............
Preparing the Instrument for Operation
Measuring Voltage.
MeasuringCurrent
Norm1 Method.~.
1)
Fast Method.
2)
.................
Measuring Resistance
Normal Method.
External Voltage Method.
Using External Ind.icators.
current source
Static Charge Measurements
Accessories.
Maintenance.
.................
.................
Trduble Shooting
Replaceable Parts List
Voltage Resistance Diagram
SchsmaticDiagrsm
.............. 4-l
...............
............... 4-1
...........
................ 4-2
...........
..........
.................
..........
...............
............
..........
...............
........ 2-1
3-l
3-1
3-2
........
3-2
3-2
3-3
3-3
........
3-3
.... 4-l
4-l
4-l
: .
4-2
4-2
4-3
;I{
2:::
6-l
23
6-g
1162
i
MODEL ~~~ELECTEOM~TEE
INTBODUC'IION
SECTION I - IETEOlXJCTION
The Keithley Model 621 Electrometer is an ultra-high -impedance voltmeter
with full-scale ranges of 0.10, 0.30, 1.0, 3.0, 10, 30, and 100 volts.
Accuracy Is within 2% of fW.l scale on all ranges.
The maximum input resistance is greater than 1014 ohms. in additipg, ~the
input resistance may be varied in decade steps from 106 ohm to 10 o&
by means of the shunt resistors built into the instrument. Tnus, the 621
is not only appropriate for measurement in high-impedance circuits, but
also can be used where a high input impedance would merely introduce unwanted pickup.
The 621 may be used as a direct-reading ammeter from low4 to lo-11
ampere
full scale. This seven decade range is covered in ovErlappi 3 3 x and 10 x
scales. Accuracy is within 23 ;J f$.j.lscale from 10
4% of full scale from 3 x 10
ampere.
to 10 ampere, and
Two current measuring methods are available to the user of the 621. They
are selected by a slide switch at the back of the instrument. Normally,
current is determined by measuring the voltage drop across a resistor shunted
from input to ground.
Alternately, negative feedback can be applied to the
input of the voltmeter through the current measuring resistor. This largely
eliminates the input drop and increases measuring speed, particularly on
the more sensitive ranges.
TIE 621 measures
105
to 10~
external supply would further extend the range to 10
ohms with a two-termina
input. A KXXI-volt
$4
ohms. Unlike conven-
tional ohmmeters, information is presented on the same linear scales used for
curren$.oand voltage readings.
Accuracy is within 3% of full scale up to
3 x 10 ohms, within 5% beyond.
As a dc preamplifier, the 621 has gains of 0.1, 0.3, 1, 3, 10, 30, and 100.
Gain accuracy is within 1% on all gain steps.
The continuing stability of
the gain is assured by a large feedback factor on all ranges.
The output is either 10 volts for driving high impedance devices such as
oscil.loacopes or pen recorder amplifiers,
recorders or similar devices.
A calibration potentiometer is provided with
or
1 ma for driving low impedance
the 1 ma position for calibrating recorders. A slide switch next to the
output connectorpermits selecting the desired output.
MODEL 621.~: Although this manual specifically describes the 621, the instruc-
tions also apply to the Model 621~ Electrometer. The principal difference between the two models is the 621~
is
designed for rack mounting.
(See Figure 1)
Both models have the same perforaance specifications, operating and maintenance
procedures, circuit and components.
Besides the cabinet dimensions, the models
differ in the placement of two switches (see section II) and the layout of CODponents within the chassis.
or62
l-l
INl'FODJCTION
FIGIJFG 1.
is
shown at left. The Model 621~
The Model 621 Electrometer
pictured below has the same spaclflc-
ations as the Model 621, but it is
designed for rack
mounting.
l-2
0762
MODEL 621 ELECTROMETER
SECTION II - SPECIFICATIONS AND DESCRIPTION
SPECIFICATIONS:
AS A VOLTMETER:
SPECIFICATION AND DESCRIPTION
RANGE:
0.1 volt full scale to 100 volts in seven lx and 3x ranges.
ACCURACY: +2% of full scale on all ranges exclusive of drift.
ZERO DRIFT: Less than 3 millivolt per hour after Z-hour warm-up.
INPUT IMPEDANCE: Greater than 10"
ohms shunted by 30 picofarads.
Input resistance may
be selected in decade steps from 106 to lOlo ohms.
AS AN AMMETER:
RANGE: lo-11 ampere full scale to 10-4
ampere in fifteen lx and 3x ranges.
ACCURACY: i-3% of full scale on 10-4 to 10-9 ampere ran es using smallest available multi-
plier setting; +5% of full scale on 3 x 10-10 to 10-12 ampere ranges.
GRID CURRENT:
Less than 5 x lo,-14 ampere.
AS AN OHMMETER:
RANGE:
ACCURACY: *4% of full scale on 105 to 1010
setting;
105 ohms full scale to 1012 ohms in fifteen linear lx and 3x ranges.
ohm ranges using largest available multiplier
*6%of full scale on 3x 1010 to 1012 ohm ranges.
AS AN AMPLIFIER:
INPUT IMPEDANCE:
Greater than 1014 ohms shunted by 30 picofarads. Input resistance may
be*selected in decade steps from 106 to 1010 ohms.
OUTPUT:
VOLTAGE RECORDER OUTPUT:
ohms.
Gain:
Either voltage or+current recorder output.
-10 volts for full-scale input.
output polarity is opposite input polarity.
0.1, 0.3, etc., to 100.
Internal resistance is 9.1 kil-
Frequency Response (Within 3 db): dc to 200 cps.
Noise:
CURRENT RECORDER OUTPUT:
mess than 3% peak-to-peak of full scale.
fl milliampere for full-scale input (variable).
GENERAL:
POLARITY:
Meter switch selects left-zero (positive or negative) scales. Meter switch
does not reverse polarity of outputs.
CONNECTORS:
Input:
Teflon-insulated UHF type; ground binding post.
80-PCZF.
POWER:
DIMENSIONS, WEIGHT:
105-125 or 210-250 volts, 50-60 cps; 35 Watts.
9-l/2 inches high x 6-l/2 inches wide x 9-l/2 inches deep; net weight,
12 pounds.
ACCESSORIES SUPPLIED:
Mating input and output connectors.
0767R
output:
Amphenol
2-1
SPECIFICATIONS AND DESCRIPTION
MODEL 621 ELECTROMETER
DESCRIPTION:
The Keithley Model 621 is a line operated multipurpose LX measuring instru-
ment of extremely wide range. The measuring ranges are summarized below:
VOLTAGE:
100 millivolts
to 100 volts full scale. The input impedance is
greater than 1014 ohms shunted by approximately 30 micro-microfarads on the
VOLTS position of th
from
1010 ohms to
RANGE switch.
%
10 ohms in decade steps by rotating the RANGE switch in
The input resistance may be varied
the AMPERES range marking.
HIGH VOLTAGE WITH ACCESSORIES: The Model 6102~, 1O:l divider probe extends
the measuring range to 1000 volts.
The divider resistance is 1010 ohms and
its division accuracy is 1%.
The Model 6103~, 1OOO:l divider probe extends the measuring range to 30 MI.
Its input resistance is 1012 ohms and its division accuracy is 3%.
CURRENT:
10s4 amperes to lo-l1 amperes full scale. The current is measured
either by measuring the drop across a resistor shunted across the input, or
by placing the NORIUL-FAST switch on the back panel in the FAST position, and
applying negative feedback around the shunt resistor. This makes the input
drop negligible and improves speed of response considerably on the low current
ranges.
ems:
105 ohms to 1012 ohms full scale. The linear ohms scale is achieved
by measuring the unknown resistor with a known, constant current flowing
through it. The voltage drop across the
resistance.
Resistance from 105 ohms to lo1
s
T
is then proportionalto the
le
ohms full scale is measured
by a two terminal method.
a: AMPLIFIER:
The frequency response of the Model 621 as an amplifier is from
n: to 200 cycles on all ranges. The output is either 1O'volts or lmilliampere for full scale meter deflection.
ition the output is not grounded.
The output polarity is opposite the input
In the NORMAL micro-micrometer pos-
signal polarity.
For directions pertaining to the use of recorders see section IV-E.
Front panel controls and terminals are:
RANGE switch, located in the center of the front panel under the meter.
'Phiscontrol selects VOLTS, OHMS, or AMPERES. On the AMPERES position,
a shunt resistor whose value is the reciprocal of the designated range
may be used to decrease the input resistance as well as to measure current.
MULTIPLIER switch, located in the center directly under the meter, determines the voltage sensitivity of tne dc amplifier, and sets the voltage
range when the RANGE is set on VOLTS.
On OHM? or AMPERES, tie setting
of this knob multiplied by the OH&3 or AMtiE?iES setting gives the full
scale meter range.
ZERO control, located to the left of the MJLTIPLIER switch under the
meter, is
used to set the meter to zero.
2-2
0767K
MODEL 621 ELECTROMETER SPECIFICATIONS AND DESCRIFl'ION
METER switch, at the right under the meter, turns the instrument on and
determines meter polarity.
OPERATE - ZERO CHECK switch, located at the bottom right, selects normal
operation or zero check position.
terminals are shorted through 4.7 megohms,
In the ZERO CHECK position, the input
while the amplifier input is
shorted.
INPUT terminal is a teflon-insulated UHF type receptacle located in the
m left corner of the front panel.
The mating connector and an acces-
sory binding post which plugs into the center of the connector are sup-
plied with the 621.
A ground binding post labeled G is mounted on the
front panel above the input terminal.
Reafpanel controls and terminals are:
FUSE at the upper left. With 110 volts AC power, use 1.0 ampere SLOmfuse: with '220 volts power, use 0.5 amperes fuse.
FOWER INNT. Unless indicated, instrument is wired for ll'i' volts 50-60
cps.
For 230 volt operation, consult the circuit schematic diagram.
RECORDER CALIEPXCION. Used to calibrate 1 ma. recorders, so their scale
corresponds with the panel meter.
1MA - 1OV. In the 1MA position, OUTPUT will drive 1 milliampere recorders.
In the 1OV position, the output is 10 volts for full scale
panel meter deflection.
OUTFUT connector for external recorders is an Amphenol type 80-FC2F
receptacle.
The mating connector is an Amphenol type 80 MC2M.
NORMAL-FAST. This control is locked in NORMAL position. In FAST pos-
ition, current measurements are made with feed-back around the shunt
resistor.
COARSE ZERO. If the amplifier is quite badly unbalanced, the COARSE
ZERO switch is used to bring the front panel ZERO control in range.
ML)DEL 621~:
The specifications and controls described for the 621 in this
section are the same for the 621~ except for the location of two switches.
On the 621.13, the ZERO COARSE and FAST-JYQRMAL switches are located on the
front panel.
The function and operation of the switches are the same as
for the 621.
1063 R
2-3
SECTION III - CIRCUIT DISCUSSION
The basic element of the Model 621, is a highly accurate, stable dc volt;;;;$g$- gpi
scale sensitivity of 100 millivolts and an input Impedance
ohm shunted by 30 micro-microfarads. Amperes and ohms
are measured by the use of resistance standards. The various connections
necessary for amperes and ohms measurements will be discussed following the
detailed description of the amplifier.
A. VOL!UGTER
Refer to DR 13515-D at the ,rear of the manual.
The amplifier proper consists
of V-land V-2. V-3 is a cathode
follower which drives the amplifier
at the same instantaneous potential
as the Input signal. In other words
the neutral or ground terminal of
-4
lNP"T
0
the amplifier is not grounded to
the chassis but is attached either
directly or through divider R-201
through R-207 to the output cathode
follower. Figure 621-1 shows this
diagramatically. The amplifier A
is driven by the cathode follower.
If it is desired to have unity gain,
the amplifier is connected directly
OUTPUT DROUND
to the cathode. To increase the
voltage gain,, a fraction of the output voltage rather than all of it
FIGURE 621-i
is fed back.
The purpose of this arrangement is to allow the input to accept relatively
large input voltages without use of input dividers, which are neither stable
nor accurate at high impedance.
Consequently, the Model 621 will a~ce~~-t. 100
volts without the use of input dividers, preserving the high input impedance
and accuracy of the amplifier. Accessory probes are available for extending
the voltage range at reduced input resistance and accuracy.
Since the amplifier proper is driven by the cathode follower, the plus and
minus 220 volt supplies for the cathode follower are referred to input ground
while the +150 and +105 volt, and -150 volt supplies for the amplifier are
referred to amplifier ground which is "floating". In subsequent discussion,
reference will be made to the amplifier ground as "floating ground" and to
cathode follower ground as "output ground".
The amplifier input consists of a 5886 electrometer tube. The filament is
operated through a dropping resistor network from the regulated B plus supply.
The control grid of V-l, the electrometer tube, is protected by R-102, a 4.7
megohra resistor, by-passed for high frequencies by C-102. The input switch,
S-2 connects the grid of V-l to the Input terminal on the OPERATE position
and connects it to ground, through the 4.7 megohm protective resistor, on
zm?l cm.
The belance controls function by adjusting the dc voltages of the electrometer
tube screen and the grid.of V-2.
V-2 fo- an ordinary differential amplifier and the output is taken into V-3
the output cathode follower.
III - 1
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