Keithley 640 Service manual
Instruction Manual
Model 640
Electrometer
Keithley Instruments, Inc.
Cleveland, Ohio, U.S.A.
MODEL 640 ELECIROMETER
1. GENERAL OESCRIPTION------------------------------------------------- 1
2. OPERATION----------------------------------------------------------- 5
CONTENTS
3. CIRC”IT DESCRIPTION-------------------------------------------------
4.
5. SER”ICING----------------------------------------------------------- 20
6.
7.
*CCESSORIES--------------------------------------------------------- 1,
CALIBRATION--------------------------------------------------------- 30
REPLACE&&E PARTS---------------------------------------------------
SCHEMATICS---------------------------------------------------------- 51
14
35
0472B
SPECIFICATIONS
MODEL 640 ELECTRO”ETER
AS A MICROVOLTMETER:
RANGE :
ACCURACY: il% of full scale on 30-volt to 300-microZERO DRIFT:
METER NOISE: Less than 0.4 microvolt rm‘m8 (2 microvolts
INPUl’ IMPEDANCE : Greeter than 1016 ohms shunted by
RISE TINS (IO%-90%, with up to 100 megohms source re-
AS AN AMmiTER:
RANGE :
ACCLmcY: 23% of full scale on 3 x 10-5 to 10-11 am-
METER
DAMPING :
30 microvolts full scale to 30 volts in thir-
teen lx and 3x ranges.
volt ranges, decreasing to 25% on 30-microvalt range.
each succeeding 24-hour period after l-hour warm-up.
Less than 35 wVv/‘C.
p-p) with 1 megohm or less input resistance on most
sensitive range.
less than 2 picofarads.
be selected in four steps from 106 to 1012 ohms.
sistance and no external capecitance): Less than 10
milliseconds on 1-mv and higher ranges, increasing
to 6 seconds on the 30-p’! range.
twenty-two lx and 3x ranges using built-in high-
megoh,,, resistors and range switch.
pere ranges using the smallest recommended multiplier 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.
NOISE:
:;;“li-:&,::,‘:. (5 x lo-16
critically damped.
hour as observed on the 30-millivolt range.
ing with 20 picofarads shunting the high-megohm ra-
seas than 35 6” in the first hour and in
Input reaistsnce may also
10-15 ampere fuL1 scale to 3 x 10-5 ampere in
mess ,rhan 2 x 10-l’ ampere rms (lo-16
-15 ampere range when overdamped
ampere p-p) when
Less than 24 alpha pulses per
Variable from critical damping to overdamp-
sister.
CURRENT STABILITY:
after stabilization.
lative.
MAX. EXTERNAL CAPACITANCE (Feedback c”rrent ranges):
500 pf.
RISE TIME: Seconds, from 10% to 90%.
Recommended Resistor Damped;
Full-Scale Value, no external
Ranges
10-15 to 3x10-11
10-12 to 3x10-9 1010 0.2
10-11 to 3x10-7 108 0.05
10-9 to 3x10-5
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.
Setter than 5 x 10-L’ ampere/day
Long-term drift is non-cumu-
ohms
1012
106
Critically Overdamped;
cepeeitenC* capecitance
1.5
0.01
up to
maximum
44
0.5
0.05
0.01
ACCURACY :
+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:
RECORDER 0"TPLPl':
scale input.
DOlaritY.
Gain: 6.033 to 3.3 x 104.
Frequency Response (Within 3db): dc to 0.07 cps at
e 9 ain of 3.3 x 104, rising to 35 cps at a gain of
10 or below.
NOiS*:
ified function.
output p-p on the 30-v to lo-mv ranges, increasing
to 10% on the 1-w and lower ranges.
UNITY GAIN ODTPW:
within .Ol% or 10 p’f, excLuaiva of zero drift, for
output C”rre*ts of LOO * or less.
ZERO cHEcx:
low through L kilohm in volt8 position, to feedback
in current or integrate position.
ISOLATION:
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
does not reverse polarity of output.
CONNECTORS : Input: Special type, metes with many
commercially available ion chambers and other accessories (adapter to “SF included). Low: Binding
pst. Recorder output: Amphenol SO-PCZP. “nityGain 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 6volt battery pack, S hours full charge to complete
discharge.
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=.
tery pack and charging circuit.
Integrating capacitance ia 20 picofarads
+l vole at up to 1 ma for full-
output polarity is opposite input
Below 1 cps: same 88 meter noise for spec-
Circuit ground to chassis ground: Greater
W3.StiVa)
Above 1 cps: less than 2% of full
At dc, output ia equal to input
Remote “zero” solenoid shorts input to
or center-zero sce3les. Mete; switch
For maximum battery life, battery oper-
InternaLly mounted nickel-cadmium bat-
LL’LR
GENERAL DESCRIPTION
SECTION 1.
l-l.
GENERAL.
Electrometer is an ultra-stable, solid-state microvolt
electrometer.
a. As a Microvoltmeter.
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.
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.
guarded capacitor in rhe feedback loop the instrument
is useful as a coulombmeter or current integrating
amplifier.
ment has ten ranges Pram 2 x lo-14 coulomb full scale
to 6 x l’~?-~~ coulomb.
d. As an Amplifier.
of the instrument as e very stable, variable gain
amplifier.
The Model 640 Vibrating Capacitor
When used as a microvolt-
When used with the built-in,
By switching en accurate
In the CURRENT INTEGRATE mode the instru-
The analog OUTPUT permits we
GENERAL DESCRIPTION
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.
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
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.
1016
A compact Remote Input pre-
ohms shunted by less than 2 pico-
1170
GENERAL DESCRIPTION
MODEL 640 ELECTROMETER
TABLE l-l.
Front Panel Controls.
CO”trOl
POWER Switch (5301)
FINCTION Switch (S402)
RANGE Switch (S403)
METER Switch (S404)
Controls the power to the instrument.
Selects the mode of operation.
Selects the meter sensitivity.
Selects meter polarity, center scale, and meter off.
Functional Description
Paragraph
2-3, a
2-3, b
2-3, c
2-3, d
ZERO Controls
KEDIUM (S407)
FINE (R431)
ZERO CHECK Switch (S401)
Adjusts meter zero.
Adjusts meter zero (fine control).
Permits a meter zero check.
2-3, e
2-3, e
2-3, f
!
TABLE 1-2.
Input Head Controls and Terminals.
Control
SHUNT RESISTOR Switch (5102)
Selects P shunt or feedback resistc.r frw 106 to
Functional Description Paragraph
1Ol2 ohms.
ZERO CHECK Switch (SlOl) Permits a meter zero check.
FEEDBACK Terminal (5103) Useful for unity gain or guarded wsauremsnts.
Input Receptacle (5105) Provides connection to Input High
DAMPING Control (RlOS) Adjusts damping for CURRENT INTEGRATE function.
I
2-2, a
2-2, b
2-2, d
2-2, c
2-2, e
2
1170
MODEL 640 ELECTROELETER
FUNCTION
Switch
(5402)
GENERAL DESCRIPTION
POWER
Switch
(S301)
ZERO
Switch
(S407)
METER
Switch
(S404)
FIGURE 2. Front Panel Controls.
FEEDBACK
(5103)
RANGE
Switch
(S403)
ZERO
CHECK
(S401)
SHUNT RESISTOR
1170
FIGURE 3.
Input
(JlO5)
ZERO
CHECK
(SlOl)
Input Head Controls.
3
GENERAL DESCRIPTION
CO”tr01 Functional Description Paragraph
MODEL 640 ELECTROMETER
TABLE l-3.
Rear Panel Controls and Terminals.
REMOTE HEAD Receptacle (5405)
OUTPUT Receptacle (5404)
GND Terminal (5406)
LO Terminal (3402)
FEEDBACK Terminal (5403)
COARSE ZERO 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 OIJTPLW current 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,
2-4, h
2-4, i
2-4, j
2-4, k
g
FIGURE 4. Rear Panel Controls and Terminals.
4
1170
MODEL 640 ELECTROMETER
GENERAL DESCRIPTION
SECTION 2.
2-1.
INPUT CONSIDERATIONS.
Input Head Connections.
a.
Remote Cable. A shielded coaxial cable (5
1.
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.
tom mounted as described in paragraph 2-12.
Input Assembly.
3.
of B” insulated input High terminal (center post)
and a machined housing which is input Low. High in-
put resistance (over 10~6
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. Dimensions 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 accessory 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
riels such as sapphire, teflon, polyethylene or poly-
styrene for insulation of the input circuit.
The Input Head Chassis can be cus-
This assembly (5105) consists
ohms shunted by less than
resistance,
low-loss mate-
OPERATION
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 insulation materials.
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.
guish between stray and signal voltages since it measures the “et voltage.
consider the presence of low-level electrical phenomena 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.
(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.
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 Connection Kit contains all necessary materials for
making very low thermal copper crimp connections for
minimizing thermal effects.
Even the best insula-
The instrument does “ot dtsttn-
When using the microvolt ranges
Thermoelectric potentials
Drift ca” rlso be mL”immLzed by
1170
FIGURE 5.
Dime”sio”s of Input Housing.
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” erroneous 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 magnetic fields can be a potential source of BC noise.
Magnetic flux lines which cut a conductor can produce large ac noise especially at power line frequencies. 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 several 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.
Electric Fields. Shielding is usually nec-
1.
essary “he” the instrument is in the presence of
very large ac fields or “hen very sensitive measure-
ments are being
ment
circuit and
made.
The shields of the measure-
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 circuit, 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.
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
INTEGRATE operation.
When the control is set fully
CURRENT
clockwise to “MAX” damping,the rise time is approximately 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 CURRENT FAST 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.
POWER Switch (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 except for the battery charging circuit which operates
in this position.
3. BATTERY Position.
normal operation of the
battery
pack
is satisfactorily charged.
This position permits
insteument
when the internal
2-2.
INPLT HEAD CONTROLS AND 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 selecte 5 positions corresponding to the shunt resistor
(acro88
ment.
and “OPEN”.
input of feedback) require% by the me~8ure.
The switch positions are 10 , lOa, lOlo, 10lz
The “OPEN” position has no resistor
connected.
by 1000 ohms.
c, Input ReceDeecle (JlOSl. This receptacle provides input connection to the Model 640 Input High
and Input Low.
4. BAIT TESTY Position.
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,
C”RRENT
INTEGRATE.
CURRENT
FAST, and
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 neutralizes the effect of input capacitance and incream 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 sensitivity from 30 microv01ea to 30 volts.
d. METER Swftch (S4041. This switch has 4 poaitions 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.
“i” and ‘I-” Positions. These p0sitiDne select
2.
the polarity af the meter only. The
OUTPUT
voltage
is not affected by these pasitions.
CENTER ZERO. This position sets the meter
3.
circuit so that zero is indicated at center scale
(mid-scale). The deflection of the meter corresponds to one-half RANGE setting, The OUTPUT “oltage 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.
FINE Confrol (R431). This control is the
2.
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.
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 linepower. 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 ”
234 V
1.
Battery Power Fuse (F302). This fuse protects
l/4 smp, 3AG
l/B nmp, 3AG
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.
REAR PANEL CONTROLS AND TERMINALS. The rear
2-4.
panel controls and terminals sre shown in Figure 4.
The operation of each control or terminal is described
as follows.
a. REMOTE HEAD 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 input palarity.
The output polarity is ~posite
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. COARSE ZERO Switch (5405). This switch hes ten
positions far adjusting the meter-zero circuit. This
switch should only be used when the FINE and MEDIUM
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 POWER Switch (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 POWER Switch to
. (The bettery will automatically recharge
when the POWER Switch is in either “AC” or “OFF”
positians).
Battei-y charging-time is approximately 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 POWER Switch can
be see et either ‘AC” or “BATTERY”.
1170 7
OPERATION
MODEL 640 ELECTROMETER
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 COARSE ZERO
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.
The
b. Input Impedance.16
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 input 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 RANGE Switch (5403) represented
by RA.
wltage eA is defined by the following expression,
The input voltage is represented by ei.
The
eA = ei (j&)
where K is the amplifier loop gain.
Therefore iA = *A 2 ei where KA is selected by
the RANGE Switch (S403).
XK
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
resistor selected for 10 , 10 , 1010, or 1012 ohms.
088 a known shunt
k; li
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 CURRENT FAST 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 -
(INPUT
HEAD)
FIGURE 6. Voltage Function With Shunt Resistor KS
< FEEDBACK
(MAIN CHASSIS)
MODEL 640 ELECTROMETER
-
INPUT
OPERATION
LO
FIGURE 7.
CURRENT FAST FUNCTION.
2-7.
a. General.
CURRENT FAST
“se Of FEEDBACK Connection.
When the FUNCTION Switch is set to the
position,the Model 640 operates es e feedback 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.
TABLE 2-1.
Inwt Resiacance in CURRENT FAST Function.
RANGE
Current Input
Resistance
1ov 1x10-11
1V
;;:;I$
1OOmV
1OmV
lmv
:g: ::
108
108
108
108
108 O.lvV
Input
Voltage
lmv
1OO~V
1ovv
4Jv
c
0472B
INPUT HI >-+
INPUT LO >
FEEDBACK >
I
I
5
RF
RM
( OUTPUT
t
a0
I
< LO
FIGURE 8.
Current Feet Function.
OPERATION
MODEL 640 ELECTROMETER
2. Current
e.) Rise Time.
particular meeeuremene depends on the shunt resistor end residual capacitance ecroes the feedbeck loop.
is given in the specifications for each resistor
value.
stcondition where no external capacitance is
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 indicate the voltege develaped ecrose Rx (un!aowo
resistance).
Rx
CURRRNT INTEGRATE FUNCTION.
2-8.
a. General.
CURRENT INTEGRATE position the Model 640 operates es
e feedback ammeter with damping.
b. Feedback Ammeter Measurements.
1. Theory.
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,
Measurement.
The actual rise time for a
The specified rise time (10 to 90%)
These rise times era for a criticallv
, Additional external capacitance ten be
= Eo =
T
When the FUNCTION Switch is eat ta the
The Model 640 operation ten be illus-
%K RB
2-9.
use as en smueter accurate to 20.25%.
10 ) shunt resistors can be accomplished using a
current integrating technique.
voleege eource can be connected in series with the
shunt resistor forming e current source where I =
V/R.
the meter reading EM is e function af cepacitance C
end the integral of the current.
arAEM =
Solving for R,
Where R -
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.
acy, maesure the enalog OUTPUT using a 0.01% digitel
voltmeter.) Refer to Figure 11 for circuit connec-
tions.
SHUNT RESISTOR CALIBRATION.
8. General. The Model 640 ten be calibrated for
& Theory. Calibration of
With FUNCTION Switch set t0 CURRENT INTEGRATE
c J
$ AT =(!-) AT
shunt resistance, ohms.
V
= eource voltage.
C = integrsting capacitor (20 pf).
E-E0 - chenge in wltage indication.
T-T0 = time ineervel for voltage change.
(To obtain the best possible eccur-
c. Calibretion Procedure.
1. Set the FUNCTION Switch to CURRENT?NTZGRATE.
rhe
high value (lo”,
An
accurately known
where I =
AE
At
2. Variable Damping.
(R108) is adjusted.counter-clockwise,the Model 640
can be used fot current meesuremente with veriable
damping.
10
cm-rent in amperes.
C = feedback capacitance (2 x 10-11).
=
change in the meter resdtng during time
interval
=
time interval of me*aurement.
At.
When the DAMPING Control
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.
7. Calculste the value of R using equation.
Record time interval T-To.
04728
MODEL 640 ELECTROMETER OPERATION
INPUT HI
< OUTPUT OUTPUT
INPUT LO
<
1
e. =
l/C
I
LLO LO
--c
FEEDBACK,
>
!
RI.!
1
I
p+k
E -
-
FEEDSACK~
P106
INPUT HI
FIGURE 10.
Equivalent Current Integrator.
-< OUTPUT
RA,
% *o
t
= l/C J E/R dt
‘< Lo
04728
FIGURE 11. Current Integrate - Shunt Resistor Calibration.
11
OPERATION
MODEL 640 ELECTROMETER
Z-10.
ANALOG OUTPUTS.
a. OlPPPUT
en analog output for recording or monitoring purposes.
eo161”~“,~i?&,,
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
Noise: Below lcps: seme es meter noise for spec-
ified function. Above 1 cp8:
output p-p an the 30-v to lo-mv ranges, increasing
co 10% an the 1-w and lower ranges.
dtit~ the o”tp”t is approximately 1X4 for a full
scale input.
b. onio Gain output.
set to VOLTAGE the FEEDBACK terminal can be used for
measuring a potential from a very high impedance
SO”X.2.
in .01X or 10 M”, exclusive of zero drift, for output
current of 100 pA or less.
Terminal
P
ain of 3.3 x 10 , rising to 35 cps at e gain
3
or below.
1MA Output. With the IV-1”A Switch set to
At dc, the output is equal to the input with-
(54041. This terminal provides
With the l”-WA SWitch (S406) set
is + 1 volt corresponding to a
less than 2% of full
When the FUNCTION Switch is
The peak-to-Peak noise is approximately five times the
rms value (from experimental measurements), therefore
the equation can be expressed as follows.
EPP =
If the ambient temPerat”re is 300°K (room ambient)
then the peak-peak noise can be expressed as follows.
EPP =
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 *
EPP (typically)
KPP =
*AF =
2-12.
8. casting Dimensions.
the Input Head Casting are shown in Figures 12 and
13.
5 x %ms
6.45 x 1O-Lo s
R - 1012 ohms.
RANGE see to 1 MY.
180 vV Peak-Peak
= 6.45 x lo-lo
1
kiK=
MOUNTING DIMENSIONS.
2% lo’& 2 x lo-‘f 2 .OB
The overall dimensions of
2-11. THERMAL NOISE.
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
noise equation as follows.
%m =
JZXZ
the
Johnson
where
Q-m =
T =
R =
F = amplifier bandwidth, Hz.
K =
rm8 voltage noise generated in the
resistence.
temperature, OK.
ideal resistance, ohms.
soltzmenn CO*Ste”t (1.38 x 10-1ojaulea/4()
b.
Input
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.
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.
this hardware).
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 four 8crews replaced.
ed should provide sufficient clearance for the 6-32
Phillips screw heeds.)
contact. The input COntaCt is spring
The Base Plate is fastened Co the
(The deesicant beg is also attached “sing
In order to mount the Input Head
The rubber feet should be removed and
(Note that the holes drill-
12
04728
MODEL 640 ELECTR‘WSTER
1
.I ._.-._.-.-.
:-.i,-‘-‘-‘-‘-‘-.-’
1 i
i
I
i
I
I
FIGURE 12.
6.28
>ut Head Castins.
I”1
4
L5.13 I
FIGURE 13.
1170
Base Plate Dimensions, FIGURE 14.
Mounting Hole Locations
CIRCUIT DESCRIPTION
MODEL 640 ELECTROMETER
SECTION 3.
3-l. GENERAL.
Head (Remote Preamplifier) end e Main Chassis (Amplifier end Power Supply).
8. High Impedance Microvoltmeter. When the FLNCTION 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
Model 640 operates es a stable current enh charge
measuring instrument.
3-2.
amplifier utilizes e vibrating-capacitor input preamplifier end variable-sensitivity emplifier. The
overall amplifier operates es a very sensitive dc
amplifier using e vibrating cepecitor es en input
signal modulator.
amplified end demodulated in the preamplifier circuit.
The dc signal is filtered end amplified further
by the main dc amplifier.
ly to provide gain accuracy end stability, A block
diagram of the overall amplifier is shown in Figure 15.
3-3. INPm HEAD.
Heed contains the input modulator, high-gain ec emplifier, oscillator end demodulator. The Shunt Resistors are connected across the overall amplifierfeedbeck using Switch S102.
Switch is set to either CURRENT position,the
ELECTROMETER AMPLIFIER. The basic electrometer
The Model 640 is composed of an Input
The input signal is modulated,
Feedback is used extensive-
(Remote Preamplifier). The Input
CIRCUIT DESCRIPTION
a. Vibrating Capacitor.
used consisting of two stationary pletes and e vibrating 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”.
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
ics does not appreciably effect the amplifier circuit.
b.
to receuescle 5105
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 capacitor. (Cl05 which is 20 pF 5 0.25%).
c.
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 resistence (RS) is less then 100 K in accordance with the
following equation.
% Error = 100
frequency, the terrier frequency end harmon-
Input circuit. The input High signal is spplied
Zero Check Circuit. The ZERO CHECK control is
RS’1’
This unique capacitor provides very
on
where KS is expressed in kilohms.
A
special capacitor is
the Inwe Heed. The inout LO
r-
-INPUT HEAD -MAIN CHASSIS -INPUT HEAD -MAIN CHASSIS
FEEDBACK FEEDBACK
INPUT INPUT
FIGURE 15. Block Diagram of Model 640
14
1170
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