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