Keithley 151 User manual

INSTRUCTION MANUAL
MODEL 151
MICROVOLTMETER

CONTENTS
INTRODUCTION
SPJXIFICATIONS
Ranges
Accuracy
Input Xesl3t3ncs
Overvoltage
Zero Adjust
Noise
Zero Drift
Response Speed
Input Rejection
In-Phase Rejection
Impedance
output
Tube Complement
power
Connector3
Accessories Supplied
Dimension3
OPERATION
Operating Control3
Preliminary Set Up
Operation
CIRCUIT DESCRIPTION
Input Circuit
AC Amplifier
Demodulator Circuit
DC Amplifier
Loop Description
Power Supply
MAINTENANCE
General
Trouble Shooting Procedure
Replaceable Part3 List
Voltage Resistance Diagram
Circuit Schematic
SECTION
I
II
III
Iv
151
0763

SECTION 1 - INTRODUCTION
The Keithley Model 151 Null Detector is a stable, low-level DC
amolimwith high-~<~utesistance and low noise. Careful shielding.
fiitering,
and gu&di.nk permit floating operation, as in a bridge nuli-
detector, with excellent rejection to extraneous voltages.
With a power sensitivity of lo-l7 watt, a re3pon3e speed of ""3
second, lack of overshoot and a zero-centered meter, the 151 may be used
,;n applications Inhere a suspension galvanometer might be employed, or
in other applicaticns where a galvanometer is not sufficiently sensitive,
fast or rugged.
Five non-linear ranges are provided which have the same center-
scale sensitiv'ty as the linear ranges, but are compressed in a quasi-
logarithmi: way to three decade3 on each range. The3.e range3 permit
speedier null-searching th3.n the l~inear scales, without sncrlfizing
sensitivity at null.
Two zero controls for opera 2.r.d short-circuit inputs ai~low prop3r
zeroin,- for any Source resistance even in the presence of Bridge thermal
emfts eliminating the need for disconnecting bri~di=e voltage cc set zerr).
Although de?-:gned for nu?l detector applications, the Model 151 is
also useful as a dc voltmeter with 3 maximum full-scale sensi~isi~y ?f
100 microvolt3 and as an ,ammeter with a maxlm::m full-s,x1e ren:ii~tivL::.
of 10 micro-microamperes.
The 151 has i full-scale ouL.out of 10 volts at up to one mlliiamperc.
Thiv is sufficient Lo drive one milliar5~ere reco!.ders 'ii .well as ~iervo-
rebalance recorder3 and oscilloscopes.
I
I-l
0763

MODEL 151 ~fICROVOLTMETER
RANGE:
Linear: 0.1 millivolt full scale to 10 volts on zer*-center meter. ILL overLappin,:
ranges in lx and 3x steps.
Non Linear: Five ranges of 0.001, 0.01, 0.1, 1.0 and 10 volts full scal.c, eaci1 covering
3 decades.
ACCURACY:
Linear ranges: +3% of full scale exclusive OE noise and drift.
Non-linear ranges: ?lO% of input exclusive of noise and drift
ZERO DRIFT: Less than 10 microvolts per day after 30-minute warm-up.
INPUT NOISE (with input shorted): Less than 2% of fui.1 scale on ~11 r;,,r::cs
INPUT RESISTANCE: 10 megohms on all ranges.
LINK FREQUENCYREJECTION: 2,OOO:l
COMMONMODEREJECTION: With l-megohm source resistance. dc:5,000,000:1. l.i~,le rriqut,ncv:
500,000:1.
ISOLATION: Circuit ground to chassis ground: Approximately 109 ohins ijllillted 5'; ll.i!L
microfarad. Circuit ground may be floated up to ?500 volts dc or ~p~!ak:<i!:il rcspcc: KC,
chassis ground.
RISE TIME (10% to 90%): Approximately L second on all ranges.
OVERVOLTAGE: 200 volts steady, 400 volts transient
ZERO ADJUST: For both open and short-circuit inputs.
RECORDEROUTPUT:
Output: ?lO volts dc at up to 1 milliampere for full-scale meter deflection.
Resistance: Less than 50 ohms within the amplifier pass band.
Gain on linear ranges: 10 volts
Range setting in volts
LINE STABILITY: A 10% change in line voltage will cause less than a 2,X of full-scale
shift on all ranges.
CONNECTORS: Input: Binding posts. output: Amphenol 80-PC2F.
POWER: LOS-125 or 210-250 volts, 60 cps, 50 watts. 50-cps models available.
DIMENSIONS, WEIGHT: 10 inches high x 6-l/2 inches wide x 8-l/2 inches deep; net wcighc,
11-l/2 pounds. I
ACCESSORIES SUPPLIED: Mating output connector.
0166R II-1

SECTION III -- OPBlATION
A. OPERATINGCONTROLS
The operating controls of the Model 151 are listed below:
ON - OFF SWITCH;
RANGESWITCH: Selects both the linear and the logarithmic ranges.
SHORTCIRCUIT ZERO: Sets the meter zero ,with the input shorted.
This control compensates for thermal EMF's in
the connected circuit and the amplifier input.
OPENCIRCUIT ZERO: Sets the meter zero with open or high resistance
input. This control compensates for an:, leakage
currents present in the external circuit.
B. PRELIMINARY SET UP
--
Connectto the power line. Unless otherwise marked, the unit is wired Co.-
117 v., 60 cps power. To convert to 220 volt and/or 50 cps operation,
consult MAINTENANCEsection. A three-wire line
cord is furnished which
grounds the cabinet. If a three-wire receptacle is not available, use
the two pin adapter furnished, and ground the third lead to an external
ground for the best operation. If open circuit unbalance is encountered,
reverse line cord.
Set the range switch to the 10 volt linear range. Xrn on the power.
In about 30 seconds the meter will zero. Short the
input and turn to the
.1 millivolt range. Zero the meter with the SHORTCIXCUIT ZERO. Open
the input and zero the meter with the OPEN CIRCUIT ZERO. In the first
30 minutes of operation some zero drift may occur and it may be desirable
to reset the zeros.
C. USE OF ZEROCONTROLS
----
The stability of the Model 151 is such that, after 30 minutes of operation,
only infrequent attention need be given to either zero co::trol except
occasionally on the most sensitive ranges. The twc zero controls, however,
allow the user to maintain a constant zero with varying input resistances.
The short circuit zero bucks ou,t low impedance voltages such as generated
by thermal EMF's. The open circuit zero is a current buckout control for
balancing any current generated by chemical effects or leakage. In normal
practice, if zeroing is required, the SHOR
T CIRCUIT ZERO is sufficient.
However, in the case of a critical null balance application where it is
necessaw to eliminate zero shifi; over a range of inuut re4istances, this
can be assured by balancing the null detectcr using i,ne SHORTCIRCIJIT
ZiTiO at the lowest impedance involved and the OPEN CIHCIIIT ZEROat. the
hiphest impedance involved.
0763
III - 1

D. INPUT RFSISTAlsCEAND SOURCEIMPEDANCERIWTRICTIONS
-
The input resistance of the Model 151 is 10 megohms, within 5% on the
most sensitive range and within 1% on all ether ranges. Other than the
consideration of circL.'.t loading and speed of respcnse (See F.), there
are no source impedance restrictions.
E. CURRENTMEASUREMENT
Since the input resistance is a constant 10 megohms, ibe Model 151 may
bt- used '.? measure current with a maximum sensitivity of lo-11 amperes
full scale. Since the tolerance cn input resistance is 5% on the mcst
sensitive range, an accuracy of about 7% can be expected there. On
all other ranpes an accuracy of k% is possible.
F. SPEEDOF RESPONSE
--
The specification in Section II is for low impedance input. With an
open-circuit input, speed is about 5 seconds to 90% of final value.
G. LINE FREQUENCYREJECTION
--
The specification for 60 cps rejection are given in Section II. The
rejection is high enough so that usually no precautions are necessary
with regard to 60 cps pick-up. While the input filter is specially
peaked at 60 cps for maximum rejection, there is sufficient rejection
at other harmonics to make pickup troubles unlikely.
H. FLOATING OPERATION
The common mode rejection to DC as well as 60 cps is extremely high in
the Model151 as spwified in Section II. The instrument will operate
with no difficulty up to 500 volts from pround. For floating cperation,
remove the shorting link between the LO and G terminals at the front of
the instrument.
I. RECORDING
The output at the recorder terminals for ful; scale is 10 ~clts at up to
1 milliampere. Since the output has
a common ground with the input, when
recording, either the Model 151 must be grounded or the recorder input
capable of being floated. It shoud be remembered that, with a recorder
COMected to the output, the system rejection will be no better than that
of the recorder.
III - 2

SECTION IV - CIRCUIT DESCRIPTION
The Model 151 is a narrow-band chopper amplifier employing negative feed-
back to stabilize the gain and increase the input resistance.
A. INPUT CIRCUIT
---
The input circuit contains the dc to ac modulator, the range Switch, and
input filter to reject spurious ac Signals, and the zero controls.
The modulator used in the Model 151 employs two photoconductive cells PD 101
and PD 102 (refer to circuit schematic diagram at the rear of this manual)
which are alternately switched by two neon lamps NE 101 and NE 102 operated
from the ac line. The action is similar to a Single-pole double-throw
mechanical chopper with the result that the dc input is converted into an
ac signal.
Spurious ac signals are prevented from entering the input by meanS of a
low-pass filter consisting o,f RlO8 and ClOL and a "twin-tee" filter con-
sisting of RlOl +,;lrough R105 and Cl01 through Cl03 which is tuned to the
line frequency. RlOh and R102 are set at the factory for maximum rejection
to line frequenc,v.
Below 10 millivolts, the sensitivity is changed by altering the feedback
factor. Above 10 millivolts an input divider is used. The l~nput divider
is formed by R153 through Rl59. The total resistance of this divider is
10 megohms which is always across the input.
The zeroing circuits consist of batteries MO1 and MO2 and resistors
Rl38 through Rlir8. R139 together with Rl38 and the bias batteries form
a current source which bucks out any spurious currents appearing at the
invut. Rlk8 and either R1117or Rlb8, depending on range, place a voltage
in series with the low side of the modulator and buck-out any spurious
EMF's appearing at the ingut.
As indicated on the circuit schematic, critical parts of the input circuit
are surrounded by a Separate shield connected to the LO terminal. This
shield largely accounts for the high in-phase rejection.
a.
AC AMPLIFIER
-
The AC amplifier consists of Vl and V2. The total gsin is approximately
500,000. The first stage tube is not Specially selected, although a tube
may have to be rejected for excessive hum pick-up which results in exces-
sive zero offset. Due to the excellent internal shielding of the first
stage tube, it is not necessary to use dc on the first stage filament.
The time constants of the amplifier are selected to give as narrow a
paSs-band as possible around the carrier frequency. ,
IV- 1

C. DEMODUIAMR CIRCUIT
The demodulator circuit employs a four-diode bridge circuit with silicon
diodes. A balanced configuration is used so that careful balance of the
transformer secondary is not necessary.
The demodulator is driven synchronously with the neon lamps which witch
the input modulator. The modulator output is a pulsating dc signal
which is fed through R119 to the input grid of the dc amplifier.
D. DC AMPLIFIER
-
The dc amplifier is required for two reasons. At the carrier frequency,
the dc amplifier is a feed back integrator. The integrating capacitor,
C115, is in a local feedback loop from output cathods to input grid.
The gain of the dc amplifier is about 500 so that the value of Cl15 is
effectively multiplied by 500, eliminating the need for a large value
of capacity for filtering. At dc, the demodulated signal is amplified,
increasing the loop gain by about 500. This additional feedback results
in a ver.v high input resistance for the null-detector. Exclusive of the
divider, the input resistance is in excess of 300 megohms on most ranges.
The dc amplifier circuit is conventional and consists of V3 connected as
a differential amplifier, Vb used as a dc an.7lifier, and the output cathode-
follow@r. Rl28, DC AMP BAL, adjusts the balance of the dc amplifier.
Once set, this control requires very infrequent adjustment. A misadjust-
ment is evident if, on the 10~ log range the meter is not exactly on zero.
E. THE WHOLELOOP DESCRIPTION
---
Sections of the amplifier are combined. The input fil.ter re-
moves any high frequency components from the input signal.
The modulator converts the filtered dc signal to ac, which is amplified
by the ac amplifier. The output of the ac amplifier is converted into a
pulsating dc signal filtered by the dc amplifier acting as an integrator,
and further amplified by the dc amplifier.
The output signal is fed back to the input by means of ~160 through R16h
and Rlh9 through Rl52. The feedback is applied to the low end of PD102,
the modulator diode. Applying feedback at this point not only stabilizes
the gain, but raises the input resistance Vera substantially.
The sensitivity is changed from .l millivolt to 10 millivolts by changing
the feedback factor. An input divider is used above 10 millivolts.
The ~*logarithmicfl ranges are obtained through the use of a non-linear
"thyrite" resistor Pi166 in the feedback loop. ?
IV - 2 0763

On some ranges, when the feedback factor becomes too large, it is necessary
to decrease the gain of the ac amplifier. This is done by shunting pin
7 of Q2 to ground with a small value of resistance, ~167.
F. POWERSUPPLY
Despite the high sensitivity, no regulation is employed in the power supply,
The plus and minus supplies are derived from standard rectifier-filter
transformer ccmbinst!.cn. Points 'X nnd V on the eame transformer winding
are used to drive the modulator and demodulator.
IV - 3

SECTION V -- MAINTENANCE
A. GENERAL
The only periodic maintenance the Model 151 requires is replacement OC
EKDl and BAXZ, the zero set batteries, at inter-&Is of about two or three
years. If the zeroing controls become inoperative, the batteries need
replacement.
The modulator and other components have an indefinite life and should
not be tampered with unless there is a failure.
B. TROUBLE-SHOOTINGPROCEDLIRE
If the instrument is inoperative but replacement of tubes doe:: not cure
the trouble, the following step-by-step procedure is indicated.
1. Check the S-plus and B-minus voltages at the junctions of R170,
C120-B and R171, C119B. These should agree within about
of the voltages indirated on the schematic. If these arelOg
markedly different, check the fuse and 'hen the ac voltage frcm
the transformer (about '23i.~v to ground). Then check the selenium
rectifiers and the filter capacitors.
2. If the proper voltages are present in the power supply
a.
Remove V-h, the 12AT7, from its socket, sh.crt the clltcut
terminals and the input termi.nals.
5. Connec' an oscillosope to the plate of W-B (pin 6).
Rotate the SHORTCIRCLTIT ZERO to one end of its travels.
If the ac amplifier is functioning and the bias batteries
BAl and BA2 are providing voltage, a 60 cps carrier signal,
proportional Ian amplitudeto the rotati.on of the potentic-
meter (until the amplifier saturates), should be seen.
By returning the short circcit zero con'~.rcl tr midymsition,
it should be possible to reduce the signal ir i.he point
where it disappears 51; the fi~rst stage tube n, i~se. If
th4.s occurs, the ac amplifier is functioning. Proceed
:n the instructions in paragraph D.
C.
If there is an ac signal present at the output which is
not affected by the short-circuit zero control, short
pin 9 of V-l to ground. You will have to remove the in-
put stage shield to do this. Replace it when 0bserv:r.g
the output with the oscilloscope cn pin 6 of V-2. If the
signal is no longer present and just background tube noise
is seen, either the modulator is defective yr the input cir-
cuit is open somewhere between Cl05 and the input terminals.
V-l