Keithley 620 Manual de usuario

INSTRUCTION MANUAL

MODEL 620

MULTI-PURPOSE ELECTROMETER

AND ACCESSORIES

WARRANTY

We warrant each of our products to be free

from defects in material and workmanship. Our

obligation under this warranty is to repair or

replace any instrument or part thereof, except

tubes, transistors, fuses, and batteries, which,

within a year after shipment to the orig-

inal buyer, proves defective on examination.

DAMAGE IN SHIPMENT

Be sure to include the instrument model num-

ber and serial number in all communications.

If the instrument is damaged when received,

or fails to operate properly, a claim should be

filed with the carrier. Upon receipt of the claim

agent’s report, we will inform you regarding

repair or replacement.

REPAIRS

When returning an instrument for repair or

recalibration, it should be securely packed

against shipping damage and sent to the fac-

tory, freight prepaid. A brief letter describing

the difficulty should accompany the instrument.

CONTENTS I

-___-

SIWION

INTRODUCTION

...............................................

SPECIFICATIONS .............................................

OP&RATION

..................................................

Prel-iminary Steps

Voltmeter

Ammeter

(1) Normal

(2) Fast

Ohmmeter

(1) Normal

(2) External Voltage

Battery Check.

Using External Indicators

Miscellaneous Applications

(1) Current Source

(2) Static Charge Detector

CIRCUIT DISCUSSION..*..**.....,r.*....,....**.*.....*..,...

Voltmeter

EL Ammeter

(1) Normal

(2) Fast

C. Ohmmeter

(1) Normal

(2) EXternal Voltage

ACCESSORIES

. . . . . . . . . . . ..*........*............*...*........

MAINTENANCE

. . . . . . . . . . . . . ..*...~*.....*.*....*........*...*.

A. Trouble-Shooting

B. Schematic

?* Voltage-Resistance Diagram

D. Replacement Parts List

III

VI.

KEITHLEY INSTFiUMEXTS

620

CLEVELAND, OHIO

SECTIOI\T

I - INTROiXJCTION

The Keithley Model 620 Electrometer

is an

ultra-high impedance volt-

meter with full-scale ranges of 0.1,

0.3, 1.0, 3.0

and

10 volts.

The maximuminput resistance is greater than 1014 ohms; in additi

the input resistance may be varied in decade steps from 106 to n

YOO,

ohmsby means of the shunt resistors built into the instrument.

Thus, the 620 is not only appropriate for measurement in high-impedance

circuits, but also can be used with decreased input impedance where

a high input impedance would merely introduce unwanted pickup.

The 620 may be used as a direct-reading ammeter from 10-5 to-lo-11

amperes full scale. This (j-decade range is covered in overlapping

3x and 10x scales. Two current measuring methods are available to

the user of the 620. Normally, current is determined by measuring

the voltage drop across7a resist&r shunted from input to ground.

Alternately, on the ,lO to 10 ampere ranges, negative feedback

can be applied.to the input of the voltmeter through the current

measuring resistor. This largely eliminates the input drop and in-

creases measuring speed, particularly on the more sensitive ranges.

The 620 measures

105 to 10"

ohms full scale with a two-terminal

input. Unlike conventional ohmmeters, the resistance is read on

the samelinear scales used for current and voltage,

As a dc preamplifier, the 620 has gains of 0.1,

0.3,

and 10.

Continuing gain stability is assured by a large feedback factor on

all ranges.

The output is 1 volt, open circuit, with an internal impedance of

16.67K for fW.l. scale input on all ranges. Thus, the output may

be used for driving oscilloscopes or pen recorder amplifiers, or

the output may be shunted down to provide lo- or

50-1~

outputs

for

servo-rebalance recorders. Note that a phase reversal occurs from

input to output.

620 I-l

SECTIONII - SPECIFICATIONS

Ranges:

b” Voltage: 0.1

0.3,

10-5 to 10-11 and 10 volts full scale.

Current: amperes full scale in lx and 3x

overlapping ranges.

105

ohmsto lo" ohms full scale on linear lx and 3x

(d) overlapping ranges.

Meter Scales: Left Zero 0 to

and 0 to 10.

Accuracy:

Voltage:

Current:

of full scale on all. r3nges.

of full scale fz$m 10 amperes to 10-Y amperes.

x 10 0

t LO-l1 amperes.

of full scale from 10 ohms to 109 ohms.

full. scale from

x 109 ohms to LOU ohms.

Resistance Standards:

109 and lOlo ohms, 2%accuracy.

oh~resistors may be expected to decrease in

value at about & per year.

Input Impedance:

On the VOLTSposition, the input impedance is greater than 1014

ohms resistive, shunted by approxiasteu

micromicrofarads.

Drift:

Less than 3 millivolts per hour after 2 hours warm-up.

Recorder Output:

one volt for fur. scale meter deflection,l6.6m source impedance.

Amplifier:

Frequency response is Cc to 100 cycles on all ranges. Maximum

gain is 10. Noise is less than 2%peak to peak of fWl scale.

Front Panel Controls and Terminals:

INWJ! comector is a teflon insulated. UHFtype receptacle. An

accessory binding post which plugs into the cente?Zof the con-

nector is furnished. A ground binding post is mounted on the

panel above the input connector.

RANGEswitch is located in the center of the front panel. This

control selects VOIIp9, OHMS,or AMPERFS. On the AMPERESposition,

620

II - 1

a shunt resistor whose value is the rec%procal of the designated

range may be used to decrease the input resistance as well as to

measure current.

MULTIPLIERswitch, located in the center of the front panel above

the RANGEswitch, determines the voltage sensitivity of -the dc

amplifier, and sets the voltage range when the RANGEis set on

VOLTS. On OHMSor AMPERES,the setting of this knob multiplied. by

the OHMSor AMPIXESsetting gives the full scale meter reading.

ZERCcontrol, located at the left under the meter, is used to set

the meter to zero.

METER-BATTERYswitch, at the right under the

meter, turns

the

instrument on, determines meter polarity, and tests the batteries.

ZEROCHECK-OPERATEswitch, located at the bottom right, shorts the

input terminal through

megohmswhile the amplifier input is shorted.

Rear Panel Controls:

OIJTPCTreceptacle for external recorders, Amphenol 8opC2F; Mat-

ing plug Amphenol 8oPC2M.

Normal-Fast control is locked in NORMALposition. In FASTpositi-

tion, current measurements are made with feedback around the shunt

resistor.

COARSEZEROis used to bring the front panel ZEROcontrol in range

if the aaplifier is quite badly unbalanced.

Tubes:

one

5886, two 6418

Cabinet:

@" wide by &' high by

6 314"

deep. Weight

5 314

lbs.

Batteries:

Battery !422.? Expected Life

Bl Mallory FMl2R or

350

hours

equivalent

req'd)

E, B3, B4 tieready- #4l2 or

500 hours

equivalent

620 II - 2

SECTION

III - OPERATION

A. PREPARINGTEEINSTRUMENTM)ROPERATION

(1) Set controls as follows:

MULTIPLIER: 10

RANGESWITCH: VOLTS

ZEROC~K SWITCH: Zero check position (horizontal).

(2) Turn the METERswitch to meter +. The instrument should

cometo zero in approximately 5 seconds.

(3) Rotate the MULTIPLIERswitch toward the high sensitivity

end, adjusting ZEROas required. If it is impossible to zero

the meter with the front panel ZEROcontrol use COARSEZERO

control on rear panel to,bring the instrument within range of

the ZEROcontrol.

(4) Connect leads as required for measurement. If high im-

pedance is involved, the input should be shielded using s. co-

axial connection or shielded enclosure. The various 600 series

accessories may be used. See Section V.

If the impedance is low (below about lOlo ohms) and leads can

be kept short, the binding post adapter furnished with the in-

strument may be used.

B. MEASURINGVOLTAGE

Place RANGEswitch.at VOLTS. Turn MULTIPLIERswitch to expected

sensitivity and check meter zero. If the sensitivity of the in-

strument is increased., recheck the zero reading.

C. MEASURINGCURRENT

(1) NOFMATJmethod (lo-5 anperes to lo-l1 amperes full scale),

Turn PANGEswitch to desired AMPERESrange. Make sure the switch

&t the rear of the instrument is on the NORMALposition. Connect

current source to the input. Ful& scale sensitivity is the

product of the settings of the RANGEswitch and.MULTIPLIER.

Check zero with the ZEROCHECKswitch and then read the unknown

current. The full. scale voltage drop is the setting of the

MULTIPLIERswitch.

(2) FASTmethod (10-T to LO"= amperes full scale).

Proceed as above except with the FAST-NORMALstitch at FAST.

The input drop is now negligible and the input circuit time

constant is reduced at least 100 times. Observe the following

cautions:

620 III - 1

(4 U

se only the ZEROCHECKswitch to check zero. Do

not short the input.

(b) The low side of the output is no longer at ground.

Therefore, a recorder attached to the output must not be

pounded to the Model 620.

leakage.

D. MEASURINGOHMS

(1) NOW method (lo5 to 1Ol-l ohms full scale).

Turn RANGEswitch to desired OIQ4Srange. Make sure that NOBMAL-

FAST switch is in the NORMALposition.

Connect resistance sample to be measured only after Z&W CHECK

switch has been returned to zero check position (horizontal).

The testvoltage at full scale is the setting of the MULTIPLIER

switch. Before reading OHMS,turn RANGEswitch to the approxi-

mate range of the unknown resistance. By manipulating the WLT-

IPLIER

and the RANGEswitch, the sample can be tested at a num-

ber of test potentials, if desired.

(2) EXTERNALVOLTWEmethod.

Any dc voltage may be used. The unknown is connected between

the external source and the electrometer input. The RANGE

switch is set to AWEE and the resistance may be calculated

from the applied voltage and the indicated current.

Proceed as follows:

(a) Set the Z&XI CHECKswitch in the zero check (horizontal)

position.

(b) Connect unknown between IEHJT terminal and source

of potential. A switch should be connected In the high

voltwe line so that when the sample is disconnected from

the potential, the low impedance end of the sample is grounded,

(d) Set RANGEswitch at 10e6 AMPERE. Apply potential

to sample before setting the ZlDiUCKEXKswitch to OPEB-

ATE. Advance the sensitivity until a reading is obtained.

If the potential applied is at least 100 times the ammeter

drop (the setting of the MULTIPLIERswitch), the resistance

is equal to:

620 III - 2

FCTEJ!JTIALAPPLIED

cuRREm READING

If the potential applied is not large compared to the sm-

meter drop, the resistance is equal to:

WTENTIAL APPLIED-

INPUT

DROP(VOLTS),

CURREIKCPt.&WING

(e) If the noise in the source is low enough it is possible

to have the FAST-RCFNALon FASTand the input drop need

not be connidored in the calculation.

E. BATI'ERY

CmK

Turn METER-BATTERYswitch to Bl, B2, etc. All batteries should read

half scale or higher on the meter.

All batteries are tested directly. Bl consists of three.l.34 volt bat-

teries in series and all three, should be replaced if the reading

in Bl position is less than half scale.

F. USIX EXTERNALINDICATORS

The output of the Model 620 may be used to drive servo rebalance

recorders as well RS high impedentie devices such as oscilloscopes

and dc amplifiers.

(1) For use with servo rebalance recorders:

The output circuit consisto~of a 16.G’71<resistor through which

microamperes flow for full scale deflection of the panel

meter. Shorting the output does not affect the meter reading

of the Model 620; low impedances placed across the output ter-

minals will lower the terminal voltage for full scale deflection,

Thus, if it is deslred that the output voltage for full scale

millivolts is required, 878 ohms should be used.

(21, For use with oscilloscopes and high impedance dc smpllfiers,

the output is one volt for full scale input on

any range, with a phase inversion.

The frequency response is dc to 100 cycles on all ranges. The

maximumamplitude which can be delivered by the amplifier is

approximately 2 volts peak to peak.

CURRENTSOURCE

When

measuring ohms, the instrument is designed to supply R constant

current to sny device placed across its input terminals. The magnitude

of the cbrrent is equal to the reciprocal of the designation on the

OHMSsegment Of the MEGE switch. Therefore the instroment may be

usedas a current source for calibration of other instruments if

desired.

620

Turn RANGEswitch to OHMSand ZEROCHfXKswitchto OPERATE. The

current that is supplied on each range is the reciprocal of the OHMS

III - 3

setting, and is not affected by the setting of the MULTIPLIERswitch.

H. STATIC CHARGEMkXSUF@NEWIS

The Instrument Is zeroed and the RANGEswitch placed on VOLTS. The

MUI/l'IPLIEFl Is placed at 3 or 10 volts f7iJ.l scale. The charged object

is then brought near the uncovered, unshielded input connector of the

620. Depending on the distance between the charge and the Instrument

a voltage will be induced on the input terminal and can be read on

the panel meter. The instrument zero should be checked frequently

since accumulation of charge due to the electrometer tube grid cur-

rent will cause a slow,drift of input voltage.

Connecting a capacitor across the input reduces the drift due to

grid current and also the sensitivity to charge. An electrode con-

nected to the INPUT terminal which increases the capacitance between

the INPUTterminal and the charged object will increase the sensitivity

to charges.

620 III - 4

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