Keithley 640 User manual

Instruction Manual
Model 640
Electrometer
Keithley Instruments, Inc.
Cleveland, Ohio, U.S.A.

MODEL640 ELECIROMETER
CONTENTS
1. GENERAL OESCRIPTION------------------------------------------------- 1
2. OPERATION----------------------------------------------------------- 5
3. CIRC”IT DESCRIPTION------------------------------------------------- 14
4. *CCESSORIES--------------------------------------------------------- 1,
5. SER”ICING----------------------------------------------------------- 20
6. CALIBRATION--------------------------------------------------------- 30
7. REPLACE&&E PARTS---------------------------------------------------
35
SCHEMATICS---------------------------------------------------------- 51
0472B

MODEL 640 ELECTRO”ETER
SPECIFICATIONS
AS A MICROVOLTMETER:
RANGE: 30 microvolts full scale to 30 volts in thir-
teen lx and 3x ranges.
ACCURACY: il% of full scale on 30-volt to 300-micro-
volt ranges, decreasing to 25% on 30-microvalt range.
ZERO DRIFT: seas than 35 6” in the first hour and in
each succeeding 24-hour period after l-hour warm-up.
Less than 35 wVv/‘C.
METERNOISE: Less than 0.4 microvolt rm‘m8(2 microvolts
p-p) with 1 megohm or less input resistance on most
sensitive range.
INPUl’ IMPEDANCE: Greeter than 1016 ohms shunted by
less than 2 picofarads. Input reaistsnce may also
be selected in four steps from 106 to 1012 ohms.
RISE TINS (IO%-90%, with up to 100 megohms source re-
sistance and no external capecitance): Less than 10
milliseconds on 1-mv and higher ranges, increasing
to 6 seconds on the 30-p’! range.
AS AN AMmiTER:
RANGE: 10-15 ampere fuL1 scale to 3 x 10-5 ampere in
twenty-two lx and 3x ranges using built-in high-
megoh,,, resistors and range switch.
ACCLmcY: 23% of full scale on 3 x 10-5 to 10-11 am-
pere ranges using the smallest recommended multi-
plier setting; 24% of full scale on 3 x 10-12 to
10-15 ampere ranges. Instrurwnt can be calibrated
to 22% accuracy below IO-9 ampere with external
voltage supply and built-in calibrating circuits.
METER NOISE: mess ,rhan 2 x 10-l’ ampere rms (lo-16
-15 ampere range when overdamped
:;;“li-:&,::,‘:. (5 x lo-16 ampere p-p) when
critically damped. Less than 24 alpha pulses per
hour as observed on the 30-millivolt range.
DAMPING: Variable from critical damping to overdamp-
ing with 20 picofarads shunting the high-megohm ra-
sister.
CURRENTSTABILITY: Setter than 5 x 10-L’ ampere/day
after stabilization. Long-term drift is non-cumu-
lative.
MAX. EXTERNAL CAPACITANCE (Feedback c”rrent ranges):
500 pf.
RISE TIME: Seconds, from 10% to 90%.
Critically Overdamped;
Recommended Resistor Damped; up to
Full-Scale Value, no external maximum
Ranges ohms cepeeitenC* capecitance
10-15 to 3x10-11 1012 1.5 44
10-12 to 3x10-9 1010 0.2 0.5
10-11 to 3x10-7 108 0.05 0.05
10-9 to 3x10-5 106 0.01 0.01
AS A CO”LOMS”ETER/C”RRENT INTEGRATOR:
RANGE (recoannended): 2 x lo-14 coulombs full scale to
6 x lo-lo coulombs in ten 2x and 6x ranges.
ACCURACY: Integrating capacitance ia 20 picofarads
+0.25%.
METER NOISE: Less than 3 x Lo-16 coulomb rms (1.5 x
lo-L5 coulomb p-p) on lowest recommended range.
Less than 24 alpha pulses per hour a8 observed on
30-mFllivolt range.
AS AN AMPLIFIER:
RECORDER0"TPLPl':
scale input. +l vole at up to 1 ma for full-
output polarity is opposite input
DOlaritY.
Gain: 6.033 to 3.3 x 104.
Frequency Response (Within 3db): dc to 0.07 cps at
e 9ain of 3.3 x 104, rising to 35 cps at a gain of
10 or below.
NOiS*: Below 1 cps: same 88 meter noise for spec-
ified function. Above 1 cps: less than 2% of full
output p-p on the 30-v to lo-mv ranges, increasing
to 10% on the 1-w and lower ranges.
UNITY GAIN ODTPW: At dc, output ia equal to input
within .Ol% or 10 p’f, excLuaiva of zero drift, for
output C”rre*ts of LOO * or less.
ZERO cHEcx: Remote “zero” solenoid shorts input to
low through L kilohm in volt8 position, to feedback
in current or integrate position.
ISOLATION: Circuit ground to chassis ground: Greater
than LOgG shunted by 0.05 j.,f. Circuit ground may
be floated up to +LOOV with respect to main case.
Head case is circuit nround. On battery o,,eration,
instrument nay be cc.m;Letely isolated f;om’power
Line and ground.
POIARITY: Meter switch selects Left-zero (wsieive
OK W3.StiVa) or center-zero sce3les. Mete; switch
does not reverse polarity of output.
CONNECTORS: Input: Special type, metes with many
commercially available ion chambers and other ac-
cessories (adapter to “SF included). Low: Binding
pst. Recorder output: Amphenol SO-PCZP. “nity-
Gain Output and Case Ground: Binding posts.
PCUER:
Line operation: 105-125 or 210-250 volts (switch
selected), 50 or 60 cps, 20 watts.
Battery Operation: Rechargeable nickel-cadmium 6-
volt battery pack, S hours full charge to complete
discharge. For maximum battery life, battery oper-
ation recomuanded for no more than 6 consecutive
hours before recharge.
DIMENSIONS, WRIGHT:
Power Chassis: 7” high x S-314” wide x LO” deep;
net weight, 14 lbs.
Amplifier Head: 6” high x 5” wide x 6” deep; net
weight, 6 Lbs.
ACCESSORIES SUPPLIED: Connecting Cable: 5’ long, con-
nects head to main chassis. UHF Adapter: adapts
input to UtlF connw.tor. Shield Cap. Mating output
COll”*CtO=. InternaLly mounted nickel-cadmium bat-
tery pack and charging circuit.
LL’LR

SECTION 1. GENERAL DESCRIPTION
GENERAL DESCRIPTION
l-l. GENERAL. The Model 640 Vibrating Capacitor
Electrometer is an ultra-stable, solid-state microvolt
electrometer.
a. As a Microvoltmeter. When used as a microvolt-
meter, the Model 640 has an input resistance greeter
than 1016 ohms with thirteen ranges from 30 micravolts
full scale to 30 volts.
b. As a Picoammeter. When used with the built-in,
high-megohm SHUNT RESISTORS, the instrument has twenty
two ranges from lo-l5 ampere full scale to 3 x 10m5
ampere.
C. As a Coulombmeter. By switching en accurate
guarded capacitor in rhe feedback loop the instrument
is useful as a coulombmeter or current integrating
amplifier. In the CURRENT INTEGRATE mode the instru-
ment has ten ranges Pram 2 x lo-14 coulomb full scale
to 6 x l’~?-~~ coulomb.
d. As an Amplifier. The analog OUTPUT permits we
of the instrument as e very stable, variable gain
amplifier.
1-2. FEATURES.
a. Excellent Stability. A stability specified at
better than 5 x lo-11 ampere/day is useful for mass
spectrometer, resistivity, end ion chamber meesure-
menes.
b. Remote Inwt Head. A compact Remote Input pre-
amplifier permits convenient set up of en experiment.
C. High Input Impedance. Guarding plus the use of
sapphire insulation provides an input resistance
greater then 1016
ohms shunted by less than 2 pico-
farads.
d. Battery or Line Operation. A choice af beteery
or line operation permits complete isolation (in
battery mode) from power line when required.
e. Built-In Shunt’Resistors. Four high-megahm
shunt resistors ten be switch selected (Input Head)
for shunt or feedback current measurements.
1170

GENERAL DESCRIPTION
MODEL 640 ELECTROMETER
TABLE l-l.
Front Panel Controls.
CO”trOl
POWERSwitch (5301)
FINCTION Switch (S402)
RANGE Switch (S403)
METER Switch (S404)
ZERO Controls
KEDIUM (S407)
FINE (R431)
ZERO CHECK Switch (S401)
Functional Description Paragraph
Controls the power to the instrument. 2-3, a
Selects the mode of operation. 2-3, b
Selects the meter sensitivity. 2-3, c
Selects meter polarity, center scale, and meter off. 2-3, d
Adjusts meter zero.
2-3, e
Adjusts meter zero (fine control). 2-3, e
Permits a meter zero check. 2-3, f
!
TABLE 1-2.
Input Head Controls and Terminals.
Control
Functional Description Paragraph
SHUNT RESISTOR Switch (5102) Selects P shunt or feedback resistc.r frw 106 to
1Ol2 ohms. 2-2, a
ZERO CHECK Switch (SlOl) Permits a meter zero check. 2-2, b
FEEDBACK Terminal (5103) Useful for unity gain or guarded wsauremsnts. 2-2, d
Input Receptacle (5105) Provides connection to Input High 2-2, c
I DAMPING Control (RlOS) Adjusts damping for CURRENT INTEGRATE function. 2-2, e
2
1170

MODEL640 ELECTROELETER GENERAL DESCRIPTION
POWER
Switch
(S301)
ZERO
Switch
(S407)
FUNCTION
Switch
(5402) METER
Switch
(S404)
RANGE ZERO
Switch CHECK
(S403) (S401)
FIGURE 2. Front Panel Controls.
SHUNT RESISTOR
FEEDBACK
(5103)
Input
(JlO5) ZERO
CHECK
(SlOl)
FIGURE 3. Input Head Controls.
1170 3

GENERAL DESCRIPTION
MODEL 640 ELECTROMETER
TABLE l-3.
Rear Panel Controls and Terminals.
CO”tr01 Functional Description Paragraph
REMOTEHEAD Receptacle (5405)
OUTPUT Receptacle (5404)
GND Terminal (5406)
LO Terminal (3402)
FEEDBACKTerminal (5403)
COARSEZERO Switch (S405)
LINE VOLTAGE Switch (S302)
Line Power Fuse (f301)
Battery Power Fuse (F302)
lV-1MA Switch (S406)
IHA CAL Control (R423)
Provides connection to Input Head.
Provides an analog output.
Connection to Main Chassis ground.
Provides connection to Input LO.
Useful for unity gain or guarded measurementa.
Adjusts meter zero (coarse control).
Sets instrument far either 117 or 234 V power.
Protects line power circuit.
Protects battery power circuit.
Sets OUTPUT for either 1V or 1W..
Adjusts OIJTPLWcurrent for .95-1.05 MA.
2-4, a
2-4, b
2-4, c
2-4, d
2-4, e
2-4, f
2-4, g
2-4, h
2-4, i
2-4, j
2-4, k
FIGURE 4. Rear Panel Controls and Terminals.
4 1170

MODEL640 ELECTROMETER
SECTION 2. OPERATION
GENERAL DESCRIPTION
2-1.
INPUT CONSIDERATIONS.
a. Input Head Connections.
1. Remote Cable. A shielded coaxial cable (5
feet long) is supplied to permit remote location of
the Input Head from the Main Chassis. A” accessory
Model 6401 Cable (25 feet long) also can be used
without degradation of specifications.
2. Mounting. The Input Head Chassis can be cus-
tom mounted as described in paragraph 2-12.
3. Input Assembly. This assembly (5105) consists
of B” insulated input High terminal (center post)
and a machined housing which is input Low. High in-
put resistance (over 10~6 ohms shunted by less than
2 picofarads) is maintained by “se of sapphire in-
sulation.
a.) Custom Connections. The input housing has
been designed to easily adapt for use with ion
chambers and other applications where high input
impedance and low capacitance is required. Dimen-
sions of the input housing are given in Figure 5.
b.) UHF Adapter. The adapter supplied with the
Model 640 is useful when quick connections must
be made using standard UHF cables, However, this
adapter is limited to measurements above lo-l3
ampere or source resistances below lOI4 ohms.
c.) GRg74 (General Radio) Adapter. This ac-
cessory adapter is available for use with G&374
Series coaxial accessories. The limitations of
this adapter are similar to those for the UHF
adapter.
b. ~nsulatio”. Use high resistance, low-loss mate-
riels such as sapphire, teflon, polyethylene or poly-
styrene for insulation of the input circuit.
FIGURE 5. Dime”sio”s of Input Housing.
1170
NOTE
The input terminal and sapphire insulator
should be protected from contamination so
that the insulation will not be degraded.
Clean, dry connections and cables are very
important to maintain the value of all in-
sulation materials. Even the best insula-
tion can be compromised by dust, dirt,
solder, flux, films of oil or water vapor.
A good cleaning agent is methyl alcohol,
which dissolves most common dire without
chemically attacking the insulation.
c. Noise Consideration. The limit of resolution
in voltage and current meaeurements is determined
largely by the noise generated in the source. Stray
low-level noise is present in some form in “early sll
electrical circuits. The instrument does “ot dtsttn-
guish between stray and signal voltages since it meas-
ures the “et voltage. When using the microvolt ranges
consider the presence of low-level electrical phenom-
ena such as thermocouples (thermoelectric effect),
flexing of coaxial cables (trioelectric effect),
apparent residual charges on capacitors (die-lectric
absorption), and battery action of two terminals
(gslvanic action).
1. Thermal EMFS. Thermoelectric potentials
(thermal emfs) are generated by thermal gradients
between two junctions of dissimilar metals. These
can often be large compared to the signal to be
measured.
To minimize thg drift caused by thermal
emfs, “se pure copper leads wherever possible in
the source circuit. Drift ca” rlso be mL”immLzed by
m*ine*ining c0**t**t ju*cei0* temperatures especially
by using a large heat sink,“ear the connections.
The Keithley accessory Model 1483 Low Thermal Con-
nection Kit contains all necessary materials for
making very low thermal copper crimp connections for
minimizing thermal effects.
2. AC Electric Fields. The presence of electric
fields generated by power lines or other source8
can have a” effect on instrument operation. AC
voltages which are very large with respect to the
full-scale range sensitivity could drive the ac
amplifier into saturation, thus producing a” erron-
eous dc output. Proper shielding as described in
paragraph 2-1, d can minimize noise pick-up when
the Fastrument Fa in the presence of large ac fields
or when very sensitive me.ssuremBnta are being made.
3. Magnetic Fields. The presence of strong mag-
netic fields can be a potential source of BC noise.
Magnetic flux lines which cut a conductor can pro-
duce large ac noise especially at power line fre-
quencies. The voltage induced due to magnetic flux
is proportional to the area enclosed by the circuit
as well as the rate of change of magnetic flux. Par
5

OPERATION
MODEL 640 ELECTROMETER
example, the motion of a 3-inch diameter loop in the
earth’s magnetic field will induce a signal of sev-
eral tenths of a microvolt. One way to minimize
magnetic pickup is to arranSe all wiring so that the
loop area enclosed is as small as possible (such as
twisting input leads). A second way to minimize
magnetic pickup is to use shielding aa described in
paragraph 2-1, d.
d. Shielding.
1. Electric Fields. Shielding is usually nec-
essary “he” the instrument is in the presence of
very large ac fields or “hen very sensitive measure-
ments are being made. The shields of the measure-
ment circuit and leads should be connected together
to Sround at only one point. This provides a “tree”
configuration, which minimizes ground loops.
2. Magnetic Fields. Magnetic shielding is useful
where very large magnetic fields are present. Shield-
ing, which is available in the form of plates, foil
or cables, can be used to shield the measuring cir-
cuit, the lead wires, or the instrument itself.
e. Moisture. The Model 640 Inp;t Head is shipped
with a dessicant bag sealed inside. This bag soaks
up the moisture inside the Input Head to insure opti-
mum operation. The dessicant bag, however, will event-
ually become saturated. At this point the Model 640
offset will increase beyond the specified amount.
When this happens take off the bottom cover of the In-
put Head to remove the desaicant bag. Reactivate it
according to the instructions on the bag.
2-2. INPLT HEAD CONTROLSAND TESMINALS. The Input
Head is shown in Figures 1 and 3. The operation of
each control or terminal is described aa follows:
a. SHUNT RESISTOR Switch (SlOZ1. This switch se-
lecte 5 positions corresponding to the shunt resistor
(acro88
ment. input of feedback) require% by the me~8ure.
The switch positions are 10 , lOa, lOlo, 10lz
and “OPEN”. The “OPEN” position has no resistor
connected.
by 1000 ohms.
c, Input ReceDeecle (JlOSl. This receptacle pro-
vides input connection to the Model 640 Input High
and Input Low.
d. FEEDBACK Terminal (51031. This terminal is
used for unity gain or guarded measurements. _
e. DAMPING Control (RlOSl. (Not Shown). This
control permits adjustment of the damping for CURRENT
INTEGRATE operation. When the control is set fully
clockwise to “MAX” damping,the rise time is approxi-
mately 44 seconds with a 1012 shunt resistor. When
the control ts set fully counter-clockwise to “MIN”
damping,the rise time corresponds to the critically
damped or CURRENTFAST condition 88 given in the
specifications.
2-3. FRONT PANEL CONTROLS. The front panel controls
are shown in Figures 1 and 2. The operation of each
control is described 88 follows:
8. POWERSwitch (5301). This switch has four
positions designated AC, OFF, BATTERY, and BATT TEST.
1. AC Position. This position permits normal
aperacion of the instrument when the power cord is
connected to line power. (The battery charging
circuit operates in this position.)
2. OFF Position. This position disables both
AC
and BATTERY power to the electrometer circuits ex-
cept for the battery charging circuit which operates
in this position.
3. BATTERY Position. This position permits
normal operation of the insteument when the internal
battery pack is satisfactorily charged.
4. BAIT TESTYPosition. This position permits a
check of the battery voltage as indicated by the
meter.
b. FVNCTION Swtich (S4021. This switch has three
positiona designated VOLTAGE, CURRENT FAST, and
C”RRENT INTEGRATE.
1. VOLTAGE Position. This position co,,,,ects the
electrometer .8 8 very sensitive, high impedance
voltmeter with the SHLMT RESISTORS connected in
shunt across the input.
2. CDRRENT FAST. This position cannects the
electrometer as a feedback pieoammeter which neu-
tralizes the effect of input capacitance and in-
cream response speed. The SHUNT RESISTORS are
connected in the feedback loop of the amplifier.
3. CDRRENT INTEGRATE. This Dosition connects
the 20 picofarad warded caPa&or in the feedback
loop of the ampli ier.
c. RANGE Switch (54031. This switch has thirteen
positions corresponding to full scale voltage sensi-
tivity from 30 microv01ea to 30 volts.
d. METER Swftch (S4041. This switch has 4 poai-
tions designatad OFF, +, -, and CENTER ZERO.
6
1170

SOQEL 640 ELECTROMETER OPERATION
1. OFF Position.
This position disables the
meter movement to protect against averlaads. This
position has no effect on the OUTPUT voltage when
using a recorder or other instrument.
2. “i” and ‘I-” Positions. These p0sitiDne select
the polarity af the meter only. The OUTPUT voltage
is not affected by these pasitions.
3. CENTER ZERO. This position sets the meter
circuit so that zero is indicated at center scale
(mid-scale). The deflection of the meter corres-
ponds to one-half RANGE setting, The OUTPUT “olt-
age is nat affected by this position.
e. ZERO Switch. This switch is a dual-concentric
control.
1. MEDIUM Control (S407). This control is the
cuter knob with eleven pasitions which adjust the
meter-zero.
2. FINE Confrol (R431). This control is the
inner knob which permits fine meter-zero adjustment.
f. ZERO CHECK (S401). This switch is e normally--
open contact-type switch permitting meter,-zero check.
The ZERO CHECK switch shunts the input HI to input LO
(in voltage function) by 1000 ohms.
2-4. REAR PANEL CONTROLSAND TERMINALS. The rear
panel controls and terminals sre shown in Figure 4.
The operation of each control or terminal is described
as follows.
a. REMOTEHEAD Receptacle (5405). This receptacle
is a 24-pin cOnnector (Amphenol 57-40240) which mates
with the interconnecting cable between the Main Chas-
is and Input Head (Remote Head). Two “echenical re-
taining clips see provided on the receptacle to Secure
the mating plug (P405).
b. OUTPUT Receptacle (5404). This connector pro-
vides en analog output far recording or monitaring
pUrpOSeS. The output is 11 volt at up to 1 “A for
full scale input. The output polarity is ~posite
the input palarity. The front panel METER switch has
n0 effect on the polarity of the analog output.
c. GND Terminal (54061. This terminal is connected
to Main Chassis ground and the outside shell of con-
neceor 5405. With no cOnnection between GND and LO
(shorting link removed), the INPUT LO to Main Chassis
ground isolation is greaterthan 109 : shunted by .05
microfarad,
d. LO Terminal (5402). This terminal is connected
to INPUT LO on INPUT HEAD.
e. FEEDBACK Terminal (5403). This terminal is
used for unity gain or guarded measurements. The
terminal (5403) on the Main Chassis is connected to
5103 an the INPuT HEAD by way of the remote cable.
f. COARSEZERO Switch (5405). This switch hes ten
positions far adjusting the meter-zero circuit. This
switch should only be used when the FINE and MEDIUM
ZERO Controls do not provide sufficient range of
CO”tl-01.
g. LINE VOLTAGE Switch (S302). This switch sete
the instrument for either 117 or 234 volt rms line-
power. The line-power fuse (F301) should be checked
far proper line voltage rating.
h. Line Power Fuse (F301). This fuse pr~tecee the
power supply circuits when 117-234V line power is
used.
Fuse Racing
117 ” l/4 smp, 3AG
234 V l/B nmp, 3AG
1. Battery Power Fuse (F302). This fuse protects
the power supply circuite when battery power is used.
Fuse rating: 314 amp, 3AG.
j. lv-IMA Switch (5406). This switch sete the
OUTPUT for either 1 volt 0’ 1 mA.
k. 1MA GAL Control (R423). This control permits
adjustment of the OUTPUT (with lV-1MA Switch set co
1MA) over the range 0.95 to 1.05 mA.
2-5. OPERATING CONSIDERATIONS.
a. Mode of Opereeion.
1. AC Line-Power. The Model 640 can be operated
using ac line-power at 117V or 234V, 50 .x 60 Hz.
To operete,set LINE VOLTAGE Switch (5302) to 117
or 234, check for proper rated fuse (F301), and
connect the line cord. Set the POWERSwitch (S301)
to “AC” operation.
2. Battery Power. The Model 640 can be operated
using battery paver supplied by e rechargeable 6-
volt nickel-cadmium battery peck.
a.) To check the battery charge, set the POWER
Switch to “&ATT TEST” position. The meter should
indicate +6V or greeter if charge is sstisfactary.
b.) To recharge the battery pack, connect the
p,yF;z cord to ec power. Set the POWERSwitch to
. (The bettery will automatically recharge
when the POWERSwitch is in either “AC” or “OFF”
positians). Battei-y charging-time is approxi-
mately 16 hours for full charge after 8 hours of
continuous we.
3. AC Co Battery Switching. The Model 640 ten
be modified so that it will sutomsticallv switch
fro” “AC” operatia t0 “BATTERY” ~peraeibn if the
line power fails. An explanation of this modifica-
tion is given in paragraph 3-4 in the Circuit Des-
cription section.
b. Warm-UC. If the instrument is to be used for
very sensitive measurements,allaw the instrument to
stabilize far an haur or more. The POWERSwitch can
be see et either ‘AC” or “BATTERY”.
1170 7

OPERATION
C. Meter zero.
The meter zero circuit utilizes
three conerols PINE, MEDIUM, and COARSE.
1. After warm-up, set the METER Switch to CENTER
ZERO.
2. Adjust the MEDIUM ZERO Control for center-
zero meter position. (The rear panel COARSEZERO
Switch can be used tf meter reads off scale).
3. Increase sensitivity using the RANGE Switch
and adjust the FINE ZERO Control for center-zero
meter indication.
2-h. VOLTAGE FUNCTION.
a. General. When the FUNCTION Switch is set to the
VOLTAGE poaition,the Model 640 operates as a high in-
put-impedance electrometer.
b. Input Impedance.16
The input resistance (HI to
LO) is greater than 10 ohms shunted by less than 2
picofarads. This specification is valid 2 for the
SHUNT RESISTOR Switch sat to “OPEN” with no depreda-
tion of the input HI to input M insulation. ?‘he in-
put resistance can be lowered by se~~ti”gl~lWNUN~RE-
SISTOR values in four steps from 10 to 10 ,
c. Microvoltmeter Measurements.
1. Theory. The electrometer, when used as a
microvoltmeter, can be illustrated 88 shown in
Figure 6. In this configuration the instrument is
useful for making sensitive measurements from 30
microvolts full scale to 30 volts. The sensitivity
is adjusted by the RANGESwitch (5403) represented
by RA. The input voltage is represented by ei.
The
wltage eA is defined by the following expression,
MODEL 640 ELECTROMETER
eA = ei (j&)
where K is the amplifier loop gain.
Therefore iA = *A 2 ei where KA is selected by
XK
the RANGE Switch (S403).
2. Voltage “easurement.
a.) High Impedance. Although the electrometer
has a very high input impedance, the useability
of the Model 640 as a micravoltmeter is limited
by the thermal (Johnson) noise generated in the
m impedance. Refer to paragraph 2-10 for a
complete discussion of thermal notae.
b.) Low Impedance. The Model 640 can be used
on the more sensitive ranges by setting the SHUNT
RESISTOR Switch to 1012 ohms or lower. The laad-
ing effects should be considered when measuring
high source-impedance.
3. Current Measurement. The Model 640 can be
used for current measurements since the microvolt-
meter measures the volta e ac
k; li088 a known shunt
resistor selected for 10 , 10 , 1010, or 1012 ohms.
Current can be calculated by the ratio of voltage
reading to shunt resistance. “se this technique
where low noise is important,alehough faster re-
sponse is provided by setting the FUNCTION Switch
to CURRENTFAST as described in paragraph 2-7.
4. Unity Gain Meesurements. The Model 640 can
be used for measuring a potential from B very high
impedance source with .025% accuracy. Connect a
digital voltmeter (or differential) to FEEDBACK and
LO terminals as shown in Figure 7.
SOURCE
VOL’UGE
FEEDBACK- < FEEDBACK
(INPUT HEAD) (MAIN CHASSIS)
FIGURE 6. Voltage Function With Shunt Resistor KS

MODEL 640 ELECTROMETER OPERATION
-
FIGURE 7. “se Of FEEDBACK Connection.
2-7. CURRENTFAST FUNCTION.
a. General. When the FUNCTION Switch is set to the
CURRENT FAST position,the Model 640 operates es e feed-
back ammeter with feedback resistors selected by the
SHUNT RESISTOR Switch in four steps from 106 to 1012
ohms.
b. Feedback Aonoeter Measurements.
1. Theory. The Model 640,when used 8s e feedback
amm?ter,cen be illustrated 88 shown in Figure 8. In
this configuration the instrument is useful for mek-
ing sensitive meesurements from lo-l5 ampere full
scale. Response speed is greatly improved compared
to the VOLTAGE FUNCTION configuration since the
effect of input capacitance is largely neutralized.
The input voltage drop end effective ameter input
resistance is given for each RANGE setting es in
Table 2- 1.
INPUT LO
TABLE 2-1.
Inwt Resiacance in CURRENTFAST Function.
RANGE Current Input
Resistance Input
Voltage
1ov 1x10-11 108
1V 108 lmv
1OO~V
1OOmV ;;:;I$
:g: :: 108
108 1ovv
1OmV 4Jv
lmv 108 O.lvV
I
INPUT HI >-+
INPUT LO > I ( OUTPUT
t
5 RF RM a0
I
FEEDBACK> < LO
c FIGURE 8. Current Feet Function.
0472B

OPERATION MODEL 640 ELECTROMETER
2. Current Measurement.
e.) Rise Time. The actual rise time for a
particular meeeuremene depends on the shunt re-
sistor end residual capacitance ecroes the feed-
beck loop. The specified rise time (10 to 90%)
is given in the specifications for each resistor
value. These rise times era for a criticallv
stcondition where no external capacitance is
, Additional external capacitance ten be
cannected between the FEEDBACK terminal end u
ix
b.) Guarded Measurements. The Model 640 ten
be used for guarded resistance measurements using
the FEEDBACK Terminal and Input HI connections 89
shown in Figure 9. Since EB and RB develop e
known current IB,then the electrometer will in-
dicate the voltege develaped ecrose Rx (un!aowo
resistance).
Rx = Eo =
T %K RB
2-8. CURRRNTINTEGRATE FUNCTION.
a. General. When the FUNCTION Switch is eat ta the
CURRENTINTEGRATE position the Model 640 operates es
e feedback ammeter with damping.
b. Feedback Ammeter Measurements.
1. Theory. The Model 640 operation ten be illus-
trated es shown in Figure 10. In this configuration
the DAMPING Control is set te WAX” position so thet
a 20 pf cepecitence is connected in the feedback
loop (SHWT RESISTOR Switch set t0 “OPEN”). The
current measured is determined by the following
equation,
where I = cm-rent in amperes.
C = feedback capacitance (2 x 10-11).
AE = change in the meter resdtng during time
interval At.
At = time interval of me*aurement.
2. Variable Damping. When the DAMPING Control
(R108) is adjusted.counter-clockwise,the Model 640
can be used fot current meesuremente with veriable
damping.
2-9. SHUNT RESISTOR CALIBRATION.
8. General. The Model 640 ten be calibrated for
use as en smueter accurate to 20.25%.
& Theory. Calibration of rhe high value (lo”,
10 ) shunt resistors can be accomplished using a
current integrating technique. An accurately known
voleege eource can be connected in series with the
shunt resistor forming e current source where I =
V/R. With FUNCTION Switch set t0 CURRENT INTEGRATE
the meter reading EM is e function af cepacitance C
end the integral of the current.
c J
arAEM = $ AT =(!-) AT
Solving for R,
Where R - shunt resistance, ohms.
V = eource voltage.
C = integrsting capacitor (20 pf).
E-E0 - chenge in wltage indication.
T-T0 = time ineervel for voltage change.
Since the eccuracy of C is +.25% the overall eccurecy
af the calibration will depend on the accurecies of
the voltage swrce V, the meter accuracy EM, end the
time accuracy T. (To obtain the best possible eccur-
acy, maesure the enalog OUTPUT using a 0.01% digitel
voltmeter.) Refer to Figure 11 for circuit connec-
tions.
c. Calibretion Procedure.
1. Set the FUNCTION Switch to CURRENT?NTZGRATE.
2. Set the DAMPING to ‘WAX”.
3. Apply the voltage source between P106 end
input LO. (Remove the Input Heed bottom cover for
access).
4. Zero meter.
5. Select lOlo or 1012 SHUNT RESISTOR.
6. Measure time interval from zero to full scale
an the meter. Record time interval T-To.
7. Calculste the value of R using equation.
10 04728

MODEL 640 ELECTROMETER OPERATION
INPUT HI
INPUT LO
< OUTPUTOUTPUT
1
RI.! e. =
<
1
l/C
I
--c
FEEDBACK,
> ! I
LLOLO
FIGURE 10. Equivalent Current Integrator.
P106
p+k
E -
-
INPUT HI
RA, % *o
= l/C
J
E/R dt
FEEDSACK~
-< OUTPUT
t
‘< Lo
FIGURE 11. Current Integrate - Shunt Resistor Calibration.
04728 11

OPERATION MODEL 640 ELECTROMETER
Z-10. ANALOG OUTPUTS.
a. OlPPPUT Terminal (54041. This terminal provides
en analog output for recording or monitoring purposes.
P
eo161”~“,~i?&,, With the l”-WA SWitch (S406) set is + 1 volt corresponding to a
full scale input. The-polarity of the output is
opposite the input signal.
Gain: 0.033 eo 3.3 x 104
Frequency Response (Within 3 db): dc to 0.07 cps
at a
of 10
3
ain of 3.3 x 10 , rising to 35 cps at e gain
or below.
Noise: Below lcps: seme es meter noise for spec-
ified function. Above 1 cp8: less than 2% of full
output p-p an the 30-v to lo-mv ranges, increasing
co 10% an the 1-w and lower ranges.
1MA Output. With the IV-1”A Switch set to
dtit~ the o”tp”t is approximately 1X4 for a full
scale input.
b. onio Gain output. When the FUNCTION Switch is
set to VOLTAGE the FEEDBACK terminal can be used for
measuring a potential from a very high impedance
SO”X.2. At dc, the output is equal to the input with-
in .01X or 10 M”, exclusive of zero drift, for output
current of 100 pA or less.
2-11. THERMALNOISE.
e. General. A common limitation of microvoltmeter
measurements from high so”rce impedances is the eherm-
al noise (Johnson noise) generated in the source.
b. Theory. Thermal noise in an w resistance
can be theoretically determined from the Johnson
noise equation as follows.
%m= JZXZ
where
Q-m =
rm8 voltage noise generated in the
resistence.
T = temperature, OK.
R = ideal resistance, ohms.
F = amplifier bandwidth, Hz.
K = soltzmenn CO*Ste”t (1.38 x 10-1ojaulea/4()
The peak-to-Peak noise is approximately five times the
rms value (from experimental measurements), therefore
the equation can be expressed as follows.
EPP = 5 x %ms
If the ambient temPerat”re is 300°K (room ambient)
then the peak-peak noise can be expressed as follows.
EPP = 6.45 x 1O-Lo s
C. Typical Example. The Peak-peak thermal noise
generated in an ideal so”rce resistance can be illus-
trated as follows.
Given: Amplifier BandwidthAF = 0.08 *
R - 1012 ohms.
RANGE see to 1 MY.
EPP (typically) = 6.45 x lo-lo
KPP = 180 vV Peak-Peak
*AF = kiK= 1
2% lo’& 2 x lo-‘f 2 .OB
2-12. MOUNTING DIMENSIONS.
8. casting Dimensions.
The overall dimensions of
the Input Head Casting are shown in Figures 12 and
13.
b. Input contact. The input COntaCt is spring
loaded with the dimensions from the base 8s shawn in
Figure 12.
c, “auntinn the Base Plate. The Base Plate can be
mounted on a machined surface for custom installation
of the Input Head. The Base Plate is fastened Co the
Input Head casting using four type 6-32 x l/4 BCTBWS.
The rubber feet are’attached to the base plate “sing
type 6-32 x l/2 Phillips Heed screws and mating #6
kep nuts. (The deesicant beg is also attached “sing
this hardware). In order to mount the Input Head
Casting to e surface Plate.clearance holes must be
drilled in the surface plate as shawn in Figure 14.
The Casting can be fastened to the surface using type
6-32 screwe. The rubber feet should be removed and
the four 8crews replaced. (Note that the holes drill-
ed should provide sufficient clearance for the 6-32
Phillips screw heeds.)
12 04728

MODEL 640 ELECTR‘WSTER
1
6.28
4
FIGURE 12. I”1 >ut Head Castins.
.I._.-._.-.-.
I
:-.i,-‘-‘-‘-‘-‘-.-’
1 i
i
I
i
I
L5.13 I
FIGURE13.
Base Plate Dimensions, FIGURE 14. Mounting Hole Locations
1170

CIRCUIT DESCRIPTION
SECTION 3. CIRCUIT DESCRIPTION
MODEL 640 ELECTROMETER
3-l. GENERAL. The Model 640 is composed of an Input
Head (Remote Preamplifier) end e Main Chassis (Am-
plifier end Power Supply).
8. High Impedance Microvoltmeter. When the FLNC-
TION Switch is set to VOLTAGE,the Model 640 operates
as e very sensitive, stable voltmeter with very high
input Lmpedance .
b. Vibrating Capacitor Electrometer. When the
FUNCTION Switch is set to either CURRENT position,the
Model 640 operates es a stable current enh charge
measuring instrument.
3-2. ELECTROMETERAMPLIFIER. The basic electrometer
amplifier utilizes e vibrating-capacitor input pre-
amplifier end variable-sensitivity emplifier. The
overall amplifier operates es a very sensitive dc
amplifier using e vibrating cepecitor es en input
signal modulator. The input signal is modulated,
amplified end demodulated in the preamplifier circuit.
The dc signal is filtered end amplified further
by the main dc amplifier. Feedback is used extensive-
ly to provide gain accuracy end stability, A block
diagram of the overall amplifier is shown in Figure 15.
3-3. INPm HEAD. (Remote Preamplifier). The Input
Heed contains the input modulator, high-gain ec em-
plifier, oscillator end demodulator. The Shunt Re-
sistors are connected across the overall amplifier-
feedbeck using Switch S102.
a. Vibrating Capacitor. A special capacitor is
used consisting of two stationary pletes and e vibrat-
ing membrane which is excited et e carrier frequency
of approximately 400 kHz. The glass membrane has de-
posited metal surfaces and is sealed in en evacuated
glass “bottle”. This unique capacitor provides very
high input-impedance end low drift. When driven et
the carrier frequency (under proper canditions),the
membrane resonates et approximately 6000 Hertz. Since
the carrier (drive) frequency is much higher then the
resonenC frequency, the terrier frequency end harmon-
ics does not appreciably effect the amplifier circuit.
b. Input circuit. The input High signal is spplied
to receuescle 5105 on the Inwe Heed. The inout LO
is isolated from Main Chassis ground. A 10 megohm
resistor (RIOS) prevents e rapid discharge of the
vibrating capacitor beck ineo the source circuit. The
modulated signal is applied to the first stage ac
amplifier through e guerded, three-terminal air cap-
acitor. (Cl05 which is 20 pF 5 0.25%).
c. Zero Check Circuit. The ZERO CHECK control is
a normally-open control which energizes solenoid RlOl.
When KlOl is energized,a cancect connects input High
to FEEDBACK through a 1000 ohm resistor. The input
source end vibrating cspecitor remain connected in
the circuit during zero check. A loading error will
result in the meter zero reading if the source resist-
ence (RS) is less then 100 K in accordance with the
following equation.
% Error = 100
RS’1’ where KS is expressed in kilohms.
r-
-INPUT HEAD -MAIN CHASSIS-INPUT HEAD -MAIN CHASSIS
FEEDBACKFEEDBACK
INPUTINPUT
FIGURE 15. Block Diagram of Model 640
14 1170

MODEL 640 El.ECTROMETER
CIRCUIT DESCKIPTION
d. AC Amplifier, The ac amplifier provides very
high gain through the use of a two-stage amplifier
and a phase splitter amplifier. A" FET (QlOl) pro-
vides a high input impedance. A" emitter-follower
stage (transistor Ql02) provfdes impedance matching
between QlOl and transistor Q106. Transistors Q103
9104, Q105 and QlOS are switches providing gain ad-
jwtment to prevent oscillation on the higher ranges.
Transistors Q107, Q109 and QllO form a second stage
ac amplifier. A phase splitter circuit is formed by
transistors Qlll and Q112. A tuned circuit composed
of integ. ckt. QAlOl, inductor LlOl and trimmer cap-
acitor Cl24 provide attenuation of carrier frequency
(6000 Hz) second harmonic noise. The ac signal is
synchronously demodulated by transistors 4506 and
Q507 (locatad in the oscillator circuit).
e. Oscillator Circuit. The high frequency drive
(400 kHz) signal is generated by a tuned circuit con-
sisting of transformer T501, capacitors c502, c503
and C504 and transistor Q501. Capacitor C502 adjusts
the 400 kHz carrier frequency. Pote"tiometer R506
adjusts the gain of the drive circuit. The drive o"t-
put is developed across transformer T502 and capacitor
C509 to excite vibrating capacitor plates (pins 1 and
4). The actual signal is a modulated "envelope" as
shown in Figure 28 in Section 6. FET Q503 and inte-
grated circuit QA501 form a wave-shaping circuit far
phase and syrranetry control. Integrated circuit QA502
is part of a phase control circuit for the demodulator
autput. Potentiometer R517 adjusts the phase of the
demodulator drive. Integrated circuit QA503 is part
of a swmletry control circuit. Potentiometee S.521
3-4. MAIN CHASSIS. The Main Chassis contains a dc
amplifier circuit, meter circuit, sensitivity switch-
ing circuit, power supply circuit, and battery charg-
ing circuit 88 shown on Schematic 213833.
a. DC Amplifier. A differential input stage is
formed by FET's Q401 and Q402 and transistors Q403
and Q404. Potentiometer R404 provides dc amplifier
balance. Capacitors C401, C402, and C403 provide
filtering of the demodulator ripple. FET Q405 and
transistors Q406 and Q409 (Darlingto" amplifier) pro-
vide additional gain for driving the meter circuit
and analog O"TPUT. Transistor Q4OS provides current
limiting when the output is overloaded. Transistor
4407 provides a constant current for biasing purposes.
b. Meter Circuit. The meter circuit consists of a
meter switch 5404, a 1-W meter movement (M401), and
various meter circuit adjustments. The Meter switch
has a" OFF position (which shorts o"t the meter move-
ment), "t" and 11-11
polarity positions (which connect
the meter for either positive 01: negative deflection),
and a ZERO CENTER position (which biases the meter
such that center scale represents zero). POtf?"tiO-
meter R421 is an internal adjustment of the ZERO
CENTER meter bias current. Pote"tiameter R455 is an
internal adjustment of the meter calibration.
c. Zero Controls. Switch,5405 is B COARSE ZERO
adjustment which can select up to 11 positions a" a
divider string (resistors R432 thtough R442). Switch
S407 is a~MSDILIM ZERO adjustment which can select up
to 11 positions a" a divider strinn (resistors R443
through R452). Potentiometer R431-is a FINE ZERO
adjustment.
adjusts the hemodulator a" and off times (syrmnetry).
Transistor 9505 controls the switching of demodulat-
ing transistors Q506 and Q507.
1170

CIRCUIT DESCRIPTION
MODEL 640 ELECTROMETER
d. Sensitivity Switching. SANGE Switch 5403 has
13 positions which connect resistors R457 through
R469. The range resistors determine the voltmeter
gain or sensitivity from 30 microvolts to 30 volts
full scale.
a.
Power Supply, (As shown on Schematic 213823).
The power for the Model 640 is provided by either a
rechargeable 6-volt battery pack or a recitifier cir-
cuit operated by 50-60 Hz line power. Power Switch
S301 selects four positions: “AC” (line power), “OFF”,
“BATT” (battery power) and “SATT TEST” (battery volt-
age check). The power supply utilizes a -6 volt un-
regulated voltage from the battery pack or a rectifier
circuit composed of transformer T303 secondary (yellow
end green taps) and diodes X317-D318. A 5-volt regu-
later is composed ofcapacitor C321, transistors Q317-
Q319 (Darlington series regulator), and reference
diode D319. Transistors 9321 and Q322 compoee an out-
put sensing amplifier to regulate the series transistor
stage. Potentiometer R338 is a.” internal adjustment
of the regulated output (approximately -5 volts). The
regulated voltage is applied to an Inverter circuit
consisting of transistors Q301-Q302 end saturable core
transformer T301.
1. + 4OV Supply. Power is tapped from B second-
ary winding on Craneformer T302 (brown/yellow,
brown/white, brown). Diodes D301-0304 and capact-
tars C302-C303 provide + 40 volts for the dc ampli-
fier output stage.
2. - 2ov supply. This voltage is not used in
the Model 640.
3. + 12” supply. Power is tapped from B eecand-
ary winding on transformer T302 (red/yellow, yellow/
white, yellow). Diodes 0307-0310, resistor R304,
and capacitor C307 form a rectifier circuit. A
voltage doubling circuit cdnsisting of capacitors
C305-C306, and diodes D30S-D309 forms a bootstrap
voltage). Transistors Q303-Q304 form a Darlington
series regulator circuit. Feedback is obtained by
sampling the + and - 12 volt outputs at the junction
of resistors R316 and R317. Transistors Q30S and
Q309 form a differential amplifier which senses a
change in either the + or - outputs. Another dif-
ferential pair (transistors Q306-Q307) drives the
base of transistor 9304 to complete the feedback
100p. Transistor Q305 provides overload-current
protection by sensing the current through resistor
R305.
4. -12v Supply. Power is tapped from a aecond-
ary winding on transformer T302 (red, blue/ white,
blue). Diodes D311-0312, resistor R318, and cap-
acitor form a rectifier circuit. Transistors Q310-
Q311 form e Darlington series regulator circuit.
Feedback is obtained by sampling the -12 volt out-
put et the wiper erm of potentiometer R327. This
potentiometer adjusts the output voltage. Transis-
tors Q114-Q115 form a differential amplifier with
the base of Q314 referenced to diode D314. Trans-
istor 9313 drives the base of Q310 to series regu-
late the output. Transistor Q312 provides overload
current protection by sensing the current through
resistar R345.
5.
Line Power to Battery Switching. The Model
640 can be modified so that a failure of line power
(with POWERSwitch set to “AC”) will ca”se an auto-
matic switching to battery operation to occur. A
diode (0.75A. 5OV, Keithley Part No. RF-17) can be
connected at the POWERSwitch 88 shown in Figure
When line power is present the diode is turned off
and the battery is not used.
f. Battery Charuinp. Circuit. The charging circuit
functions whenever the POWERswitch is eet to “AC”
or “OFF”. Charging c”rrent is provided
by a rectif-
ier circuit conslating of diodes 0315-0316 and re-
si*tor 107.9. Fuse F302 is rated for 314 ampere and
used to protect the battery end circuitry during
charging or discharging.
FROM BATTERY
I
FIGURE 17. Line to Battery Switching.
3-5.
VOLTAGE FUNCTION. With the FUNCTION Switch
set to “VOLTAGE”, the Model 640 operates as a sensi-
tive voltmeter with input “OPEN” or shunted by any
one of four resistors,RlOl through R104.
3-6. CURRENTFAST FUNCTION. With the FUNCTION Switch
eet to “CDRRENT FAST”, the Model 640 operates as e
feedback ammeter with a Shunt Resistor connected
acrose the amplifier (from High to Feedback). An
external resistance can be connected ,in place of the
four Shunt Resistors “he,, switch 5102 ie *et to “OPEN”.
This method minimizes the slowing effect of capacit-
ance across the input.
3-7. CURRENT INTEGRATE FUNCTION. With the FUNCTION
Switch set to “CURRENT INTF.GRATE”,the Model 640
operates ee a feedbaok anmeter or coulomb-meter.
With (1 Shunt Resistor connected,(RlOl through R104)
a 20 pF capacitor (ClO5) shunte the amplifier to
therefore slow the response and filteenoisy signals.
With switch S102 set to “OPEN”,capacitor Cl05 acts
as an integrating capacitor for charge or current
integration measurements. A simplified diagrams of
the current integration amplifier is shown in Figure
11 . When the FUNCTION Switch is set to “CURRENT
INTFJXATE”, switch S4O2 connects -5 volts to solenoid
K102 which in turn closes e contact. The contact
connecte the Damping Control (RlOS) such that Capac-
itor Cl05 is connected in the feedback loop. The
Damping Control adjusts the effective capacitance
connected in the feedback loop and thus controls the
amount of damping. When the Damping Control is ad-
justed fully clockwise the maximum damping (20 pF
+ .25%) is provided.
16
1170

MOOEL640 ELECTROMETER ACCESSORIES
SECTION 4. ACCESSORIES
4-1. GENERAL. The follo"ing Keithlsy accessories
can be used with the Model 640 to provide additional
convenience and versatility.
4-2. OPERATING INSTRUCTIONS. A separate Ins~ructio"
Manual is supplied with each accessory giving com-
p1ete operating information.
Model 6401 Remote Cable
Description:
The Model 6401 is a shielded coaxial cable with a
Keithley B-195 (male) connector on each end. The
cable is 25 feet long.
Application:
The Model 6401 permits remote location of the Input
Head up to 25 feet from the Main Chassis with no de-
gradation to the specifications.
Model 6402 Adqter
Description:
The Model 6402 is special adapter which replaces the
UHF adapter supplied with the instrument.
Application:
The Model 6402 adapts the Input Receptacle for GR074
series Of coaxial accessories. (General Radio Co.).
This adapter is limited to measurements above 10-13
amperes or source resistances belo” 1014 ohms. The
adapter can be connected to the Input Head as shown
in the illustration.
Model 399 Isolating Amplifier
Description: Application:
The Model 399 is a unity-gain amplifier which permits The M,,del 399 can be used for ‘FIFO” operation (float-
operation with the instrument output floated at up to ing input, floating output) or when it is necessary
1500 volts off ground while the Model 399 output is ea break ground loops within a system. The 1 volt at
grounded or floated up to 100 volts off ground. up to 1 mA output enables use of the Model 399 as a
preamp far driving most analog recorders.
1170