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INSTRUCTION MANUAL
MODEL 149
MILLI-MICROVOLTMETER
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 which,
within a year after shipment, proves defective
upon examination. We will pay domestic
surface freight costs.
To exercise this warranty, call your local
field representative or the Cleveland factory,
DDD 216-248-0400. You will be given assist-
ance and shipping instructions.
REPAIRS AND RECALIBRATION
Keithley Instruments maintains a complete re-
pair service and standards laboratory in Cleve-
land, and has an authorized field repair facility
in Los Angeles and in all countries outside the
United States having Keithley field repre-
sentatives.
To insure prompt repair or recalibration serv-
ice, please contact your local field representa-
tive or the plant directly before returning the
instrument.
Estimates for repairs, normal recalibrations,
and calibrations traceable to the National Bu-
reau of Standards are available upon request.
MODEL 149 MILLI-MICROVOLTMETER CONTENTS
TABLE OF CONTENTS
Section Page Section Page
1. INTRODUCTION . l-1 4. MAINTENANCE. . . . . . . . . 4- 1
Specifications . 1-2 Trouble-Shooting . . . . . 4-l
Excessive Output Noise . . . . 4- 1
2. OPERATION. . 2-1 Output not Zero with Input
Terminals Shorted . . . 4-2
Operating Controls . . . 2-l 220-Volt Operation . . . . . . . 4-3
Preliminary Set-Up . 2-l
General Precautions. . . 2-2 5. REPLACEABLE PARTS. . . . 5-l
Measuring Voltage. . 2-3
Other Applications 2-4 Replaceable Parts List . . 5-1
How to Order Parts . . . 5-l
3. CIRCUIT DESCRIPTION. 3-1 Model 149 Replaceable Parts List 5-2
Models 1483, 1484 Replaceable
Input Circuit. . 3-l Parts List. . . . . . . . 5-6
AC Amplifier . . 3-l Model 1491 Replaceable Parts List 5-7
DC Amplifier . . . . . . 3-l Model 1501 Replaceable Parts List 5-7
Zero Suppression . . . . 3-2 Model 1502 Replaceable Parts List 5-7
Other Controls . . . . . 3-2 Voltage and Resistance Chart . 5-9
Power Supply . . . . . . 3-2 Schematic Diagram. . . . . . 5-11
Green Repair and Calibration Form 5-13
* Change Notice . . . . . Last
Page
K Yellow Change Notice sheet is included only for instrument modifications
affecting the Instruction Manual.
1068R
MODEL 149 MILLI-MICROVOLTMETER INTRODUCTION
SECTION 1. INTRODUCTION
l-l. GENERAL.
a. The Model 149 Milli-Microvoltmeter is a stable, versatile instrument for measuring
low-level dc signals. It functions as a voltmeter from 100 nanovol s full scale to 100
millivolts. It also operates as a dc amplifier with gains up to 10 ' for driving recor-
ders.
b. The low noise level of the Model 149, together with its long-term stability, makes
it ideal for many measurements requiring extreme power sensitivity.
C. Typical applications include measuring the output from strain gages, thermopilcs,
thermocouples, bolometers, phototubes, ionization chambers, scintillation counters, and
barrier layer cells. Other applications are found in cell studies, measurement of elec-
trochemical potentials, electrolytic corrosion studies, molecular weight analysis and
Hall effect studies.
d. In addition to its use as a direct indicator of minute potentials and currents, the
Model 149 may also be used as a null detector in Wheatstone or Mueller bridges.
e. An important feature of the instrument is zero suppression up to 100 times full
scale in place of the usual more limited meter zero. This permits measurements of small
signals in the presence of large thermal emf's or other masking dc signals.
FIGURE 1. Keithley Instruments Model 149 Milli-Microvoltmeter.
l-l
INTRODUCTION MODEL 149 MILLI-MICROVOLTMETER
l-2. SPECIFICATIONS.
RANGE: 0.1 micro"olt (10 x 10-8 volt) full scale to 100 millivolts on zero-center meter.
13 overlapping ranges in lx and 3x steps.
ACCURACY: *2% of full scale on all ranges exclusive of noise and drift.
ZERO DRIFT: Less than 10 nanovolts per hour or less than 30 nanovolts in any *-hour per-
iod after approximately Z-hour warm-up with reasonably constant ambient temperature.
Long-term drift is non-cumulative.
INPUT NOISE (with input shorted): Less than 0.6 nanovolt rms (3 nanovolts peak-to-peak)
on most sensitive range.
INPDT CHARACTERISTICS:
Input Resistance Maximum Source1
Greater than, Resistance,
m ohms ohms
0.1 P" 10 k 100
0.3 ,I" 30 k 300
1.0 &I" 100 k lk
3.0 ,1" 300 k 3k
10.0 1" 1M 10 k
30.0 )A" 3M 30 k
100 pv and above 10 M 30 k
Note:l Source resistances higher than the recommended maximum will increase noise
and rise time.
LINE FREQUENCYREJECTION: Greater than 5O:l on the most sensitive r~ange. (Ratio of im-
pressed peak-to-peak line frequency voltage at input to indicated dc voltage.)
ISOLATION: Circuit ground to chassis ground: Approximately 10' ohms shunted by 0.05 mi-
crofarad. Circuit ground may be floated up to *400 volts with respect to chassis ground.
RISE TIME (10% to 90%): -
O.l-microvolt Range: mess than 2 seconds when source resistance is less than 10% of
maximum; 4 seconds using maximum source resistance.
0.3-microvolt to lOO-millivolt Ranges: Less than 1 second when source resistance is
less than 10% of maximum; 2 seconds using maximum source resistance.
ZERO SUPPRESSION: Up to at least 1 millivolt on the microvolt ranges and up to at least
10 millivolts on the millivolt ranges. Stability is such that 100 times full scale may
be suppressed.
RECORDEROUTPUT:
output: *lO volts dc at up to 5 milliamperes for full-scale meter deflection.
Resistance: Less than 10 ohms within the amplifier pass band,
10 volts
Gain: Range setting in volts
1-2 1167R
MODEL 149 MILLI-MICROVOLTMETER INTRODUCTION
Noise: Input noise times gain plus modulation products.
Modulation Products: Less than 2% peak-to-peak of full scale with input shorted.
CONNECTORS: Input: Special connector. Front Output: Binding posts. Rear Output:
Amphenol 80-PC2F.
POWER: 105-125 or 210-250 volts, 60 cps, 50 watts. SO-cps models available.
DIMENSIONS, WEIGHT: 7 inches high x 19 inches wide x 13 inches deep; net weight, 24 pounds.
ACCESSORIES SUPPLIED: Model 1501 Low Thermal Input Cable with alligator clips; mating
output connector; length of low-thermal solder.*
-'t The solder is screwed to the right side of the copper input chassis, located inside the
Model 149. Remove the top cover to reach the solder.
1-3
MDDEL 149 KLLI-MICROVOLB OPERATION
SECTION 2 - OPERATION
A. OPERATINGCONTROLS
The controls of the Model 149 are simple and conveniently
placed. Their functions are as follows:
ON-OFF switch is located to the right of the panel
meter.
FUNCTION switch selects the function which Is to be
used: MilliVOltS, Or !dCrOVOltB.
2AN'm switch selects the full scale multiplier of
the function selected by the FUNCTION switch.
ZERC SUPPiWSS controls consists of the ZERO RANGE
switch which selects the coa.188 range of sup-
pressing voltage in discreet steps and the ZERO
SET potentiometer which gives continuously
variable fine control Including settings through
TieI-0.
B. PRELIMINARY SET-UP
Connect the instrument to the power line. Unless otherwise
marked the unit may be used on 117 volt, 60 cps line. To con-
vert to 220-volt operation, refer to the MAIN'lXK4NC!E section.
A three-wire line cord is furnished, which grounds the cabinet.
If a three-wire receptable is not available, use the two-pin
adapter furnished, and ground the third lead to an external
ground.
Set controls *a followa:
Function: Millivolts
Range : 100
zero suppress: OFF
Input: Short the input leads together.
2-1
OPE3ATION KNEL 149 MILLI-MICROVOL-R
C. GENERAL PRECAUTIONS
1. Source Resistance - 1n Section luncler the Input Re-
sistance SpeCifiCatYiOg the maximum source resistance for use with
each voltage range is specified. Reasonable operation is possible
with source resistance up to ten times greater than those specified,
however, the measurement will suffer from a considerable decrease in
speed of response, and measuring accuracy. If the instrument is
left completely open-circuited, the meter will generally drift off
scale on any range.
2. Shielding - Since the instrument operates with a modulator
frequency of 120 cps, it is not generally sensitive to 60 cps pick-
up unless it is large enough to overload the amplifier. The pickup
may be a source of difficulty when using the amplifier with high
impedances on ~the more sensitive voltage ranges. In these cases it
is desirable to shield the leads and the sources as completely as
possible. In some cases a simple low-pass filter at the input to
eliminate frequencies of about 1 cps and above will be helpful.
No use is made of an input filter in this instrument since any input
series impedance due to the filter will increase the input noise and
the thermal drift. When operating above ground, the case of the in-
strument must be grounded.
3. Determination of Excessive AC Pickup - A terminal attached to
the output of the AC amplifier at the point of the demodulator is
provided at the rear of the instrument. It is labeled DEMOD. OUTPUT.
If an inability to make consistent readings persists, it is possible
to check for the presence of excessive pick-up by observing the
wave-form at this point. With the input shorted the picture should
be approximately as shown in figure 2. If excessive pickup is ob-
served it will look as shown in figure 3. The circuit will operate
reasonably well as long as the wave-form does not clip as shown
in figure 4. At this point the operation will be erratic.
--I.-
lo v P-P
t
FIOUIlE2
c
40 v P-P
t
MODEL 149 MILLI-MICROVOLl'METI OPERATION
FIGURE 4
4. Thermal EMF - Extreme precautions have been taken in the input
circuit to minimize thermal EMF's so the residual RMF is usually less
than 0.5 uv. The material used in the input circuit is pure copper.
Any other metal will generate a thermocouple potential. Lead solder is
particularly troublesome. Where thermal W's are a problem, soldering
should be done with the cadmium-tin solder supplied with the instrument.
5. Input Noise - The noise at the input is a function of input
resistance and is approximately given by
E = 1.29 x 10-loum
where E is the RMS noise, and R is the source resistaixe. It is assumed
that the bandwidth of the instrument is about 1 cps and the temperature
is 80