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INSTRUCTION MANUAL
MODEL 15OB
MICROVOLT AMMETER
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 and batteries) 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.
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 150B MICROVOLT AMMETER TABLE OF CONTENTS
TABLE OF CONTENTS
Section Page Section Page
MODEL 150B SPECIFICATIONS. .... .ii 4. MAINTENANCE. . . . . . . . . . . 19
1. GENERAL DESCRIPTION. ........ 1 4,-l. General. . . . . . . . . . . 19
4-2. Parts Replacement. . . . . . 19
l-1. General. ........... 1 4,-3. Mechanical Chopper
l-2. As a Microvoltmeter. ..... 1 Replacement. . . . . . . . . 19
l-3. As a Null Detector ...... 1 4,-4,. Troubleshooting. . . . . . . 20
l-4 .. As an Ammeter. ........ 1 4-5. Preliminary Troubleshooting
Procedures . . . . . . . . . 20
2. OPERATION. ............. 3 4,-6. Check Out and Calibration
Procedures . . . . . . . . . 20
2-1. Mode of Operation. . . . . . .3 4-7. Power Supply Check Out
2-2. Battery Operation. . . . . . .3 and Calibration. . . . . . . 22
2-3. Microvolt and Null Detector 4,-a. Amplifier Check Out
Operating Procedures . . . . . 4, and Calibration. . . . . . . 25
2-4,. Ammeter Operating
Procedures . . . . . . . . . .6 5. ACCESSORIES. . . . . . . . . . . . 35
2-5. Zero Suppress Operation. . . .6
2-6. Filter Switch. . . . . . . . 7 5-l. Model 4,006 Rack
2e7. Recorder Outputs . . . . . . : 7 Mounting.Kit . . . . . . . . 37
2-8. Synchronizing Terminals. . . .7 5-2. Model 4007 Dual Rack
2-9. Differential (Floating) Mounting Kit . . . . . . . . 37
Measurements . . , . F . . . .8 5-3. Model 370 Recorder . . . . . 39
2-10. Accuracy Consideration . . . .8
2-11. Thermal Noise. . . . . . . . 0 6. REPLACEABLE PARTS. . . . . . . . . 41
2-12. Input Resistance . . . . . . : 9
2-13. Therinal Emf's. . . . . . . . . 9 6-l. Replaceable Parts List . , . 41
Z-14,. Shielding. . . . . . . . . . .lO 6-2. How to Order Parts . . . . . 41
2-15. Operating From Source Other Model 150B Replaceable
Than 117 Volts, 60 cps . . . .ll Parts List . . . . . . . . . 42
2-16. Accessories For Input Models 14,83, 14.84
Connections. . . . . . . . . .ll Replaceable Parts List . . . 50
Schematic Diagram 20350E . . 53
3. CIRCUIT DESCRIPTION. ., . . . . . . .13 Schematic Diagram 20357D . . 55
3-1. General. . . . . , ...... 13 * Change Notice Last page
3-2. Input Circuit. . ....... 13
3-3. AC Amplifier . . ..... . .14.
3-4.. Demodulator. . . ... * .. .15
3-5. DC Amplifier . . ...... .15
3-6. Zero Suppression ..... . .15 * Yellow Change Notice sheet is included
3-7. Power Supplies . ..... . .15 only for instrument modFfications affect-
3-8. Battery Charging Circuit .. .17 ing the Instrument Manual.
0667R i
GENERAL DESCRIPTION MODEL 150B MICROVOLT AMMETER
TABLE 1. Model 1SOB Specifications.
AS A VOLTMETER AND NULL DETECTOR: AS AN AMMETER:
RANGE: 0.3 microvolt (3 x 10-' volt) full scale to 1 volt on a RANGE: 3 x lOLo ampere full scale to 1W3 empere on zero.
Zero-centw meter. 14 overlapping ranges in Ix and 3x center meter. 14 overlapping ranges In lx and 3x steps.
steps. ACCURACY:
ACCURACY: Meter: a3% Of full Scale on ali ranges.
Meter: *2g Of full scale on all ranges. I-Volt Output Terminals: *2%.
I-Volt Output Terminals: *I%. 100.Mllllvolt Output Terminals: Adjustable to +2q6.
100.Millivolt Output Terminals: Adjustable to ~1%. Note: Accuracy specifications exclude noise and drift.
Note: Accuracy specifications excluie noise and drift. ZERO DRIFT A2 x lo'" ampere per 24 hours after l.hour
ZERO DRIFT: Less than 0.1 microvolt per 24 hours after I- werm.up.
hour warmup with reasonably constant ambient tempera. INPUT NOBE: Less than 3 x 10'12 ampere peak.to-peak on
ture. Long-term d!ift is ncwcumulatlve. the most sensitive ran,~e,
INPUT NOISE: `With input shorted, less than 5 nanovolts INPUT VOLTAGE DROP: 100 microvolts on the nsnoampere
rms (25 nanovolts peak-to-peak) on the most sensitive renges, 1 millivolt on the microampere ranges..
range. INPUT RESISTANCE: On the microampere ranges. the input
With a 10,000.ohm source resistance. lessthan 14 nanovolts resistance is equal to lo" divided by the range in amperes.
rms (70 nanovalts peak-to-peak) on the most sensitive On the nanoampere ranges, it is equal to lo" divided by
range. the range in amperes~
INPUT RESISTANCE: GENERAL:
Input Resistance Maximum Source*
Greater than, Resistance, ISOLATION: Circuit ground ,to chassis ground: Greater than
Range ohms Ohms 1O'O ohms shunted by 0.001 microfarad. Circuit ground
0.3 IN 1M may be floated up to ~400 volts with respect to chassis
10 k ground. On battery operation. may be completely isolated
1 P" 3M 30 k from power line end ground.
3, 10M 100 k RECORDER OUTPUT (1 volt):
10 pv 30 M 300 k Output: ztl Volt dc et up to 1 milliampere for full-scale
30 IN 100 M 1M meter deflection on eny range.
100 #v and above 100 M IM Resistance: Less than 5 ohms within the amolifiar oass
band.
Note: `Source resistances higher than the recommended Noise: Input noise times gain plus modulation products.
maximum will increase noise and rise time. Modulation Products: Less than 4% peak.to-peak of full
ZERO SHIFf WITH SOURCE RESISTANCE: Less than 1o-`o Scale with input shorted.
volt per ohm. RECORDER OUTPUT 1100mv):
LINE FREQUENCY RUECTION': A voltage of power line Output: ztlO0 mv adj;stabi;&er a 10%spa" for full-scale
frequency which is 75 db (p.p/dc) greater than full scale meter deflection on eny range.
affects reading less than 2% on the most sensitive range Output Rsslstancs: Less than 1000 ohms.
(decreasing to 60 db on the 10.microvolt range and to 20 Noise: Input noise times gain plus modulation products.
db on the l-volt range). Modulation Products: Less than `/z% peak.to.peak of full
COMMON MODE RLIECTIDN*: Greater than 180db at line scale with input shorted.
frequencv or dc. Using this output, response time is at least one second on
RISE TIME (10% to 90?&)? Using up to 1wO ohms source any IB"B.3.
reSistanCe, less than 0.5 second on the 30.mlcravolt and CONNECTORS: Input: Special Kelthley Model 1485. Output:
higher ranges. increasing to 3 seconds on the 0.3.microvolt Amphenol BOPCZFReceptacle.
range. POWER:
Using maximum source resistence up to 100 kllohms, rise Line Dperatlon: 105125 volts or 210.250 volts, 60 ops. 25
times increase to approximately 3 seconds on the 30. watts. 50.cps mode18 available.
microvolt and higher ranges. 6 seconds on the lo-microvolt Battery Operation: Rechargeable nickel-cadmium &volt
and lower ranges. battery pack. Ov?r 7 hours continuous operation from full
ZERO SUPPRESSION: Up to 10 millivolts available. Stability charge: recharges in less than 16hours from built.in charg.
is such.that 100 times full scale may be suppressed. ing circuit.
DIMENSIONS. WEIGH(T: 7 inches high x 8% inches wide x
`Note: All specifications are measured with filter in. With 10inches deep; net weight, 16 pounds.
filter out. rise times forany source resistance up to maxi. ACCESSORIES SUPPLIED: Model 1506Low.Therma, Input
mum are less than 0.5 second on the 30.microvolt and Cable (4 feet, low-thermal triaxial cable, alligator clips).
higher ranges. increasing to 3 seconds on the 0.3.microvolt Mating output connector. Length of lowthermal solder.
range. With filter out, the rejection ratios are reduced Internally mounted nickel.cadmium battery pack. Model
about 30 db. 1499.
ii 0267R
MODEL 150B MICROVOLT AMMETER GENERAL DESCRIPTION
SECTION 1. GENERAL DESCRIPTION
l-l. GENERAL. The Keithley Model 150B Microvolt Ammeter is an extremely sensitive
instrument which measures voltages from 0.3 microvolt to 1 volt and currents from 0.3
nanoampere to 1 milliampere. It can be used as a voltmeter, null detector or an ammeter
with either battery or line power operation.
l-2. AS A MICROVOLTMETER.
a. The Model 150B is ideal as a microvoltmeter for measuring semiconductor resistivity,
thermopile and thermocouple potentials, Hall effect, contact resistances, biologically
generated emf's, electrochemical potentials and strain guages emf's.
b. The Model 15OB has input noise less than 25 na,novolts peak-to-peak; high input re-
sistance and loti zero shift; excellent zero stability; very high line-frequency rejection
and common mode rejection; battery operation; zero suppression; special low-thermal input
circuitry; two available outputs.
c. With these features, the Model 150B permits excellent resolution; maintains measure-
ment accuracy even from high resistance sources; is ideal for long-term measurements; de-
tects dc signals in the presence of large ac voltages and is virtually insensitive to nc
or dc voltages applied between circuit and chassis ground; disconnects from power lines
for improved isolation; measures small changes in dc signals: reduces temperature and shield-
ing problems; provides outputs where either filtering is needed or fast response or greater
output power is needed.
l-3. AS A NULL DETECTOR.
a. As a null detector the Model 15OB is excellent for use in ratio measurements and in
potentiometer and bridge circuits.
b. The Microvolt Ammeter's outstanding features as a null detector are: very good re-
solution; high line frequency and cormnon mode rejection; isolation from chassis ground to
input terminals; battery operation; high input resistance; floating capability; low zero
shift; zercl suppression.
C. Because of these features the Model 150B has resolution comparable to the maximum
usable resolution of most potentiometers and bridges; may be simply connected to the ter-
minals of a null circuit; is very insensitive to commnn mode voltages developed in the
null circuit; can be used in most potentiometer and bridge circuits without off-null load-
ing; accurately detects a null regardless of the setting on most potentiometers and bridges;
compensates for thermal emf's generated in the null circuit.
1-4. AS AN AMMETER.
a. The Model 150B has general use in research, design and production test facilities.
It is useful for making low voltage drop, in-circuit measurements as well as measuring the
.._ output of radiation detectors, phototubes and other current generating transducers.
b. The instrument has low voltage drop for measuring currents in very low-voltage circuits;
low zero drift for measuring long-term current; excellent floating capabilities for measur-
ing ungrounded sources; and low input noise,giving excellent resolution.
'. 0667R 1
OPERATION MODEL 150B MICROVOLT AMMETER
Control Functional Description Par.
JEER SUPPLY Switch Controls mode of operation for power supply 2-1,2-
4NGE Swi.tch Se.lects the full scale voltage or current which 2-3,2-
is to be measured
UNCTION Switch Determines whether the Model 150B measures voltage or 2-3,2-
current; selects input resistance on voltage ranges.
ERO SUPPRESS Controls Determines the amount of zero suppression 2-5
LLTER Switch Connects or disconnects a lfne frequency filter 2-6
at the input
gPLIT Receptacle Connection for input cable 2-16
TABLE 2. Model 150B Front Panel Controls. The table briefely describes each control, and
indicates the paragraph which contains instructions on the use of the control.
Control Functional Descriotion Par.
LINE VOLTAGE Switch Sets Model 150B for 117 or 234-volt ac power line 2-16
BATTERY FUSE Quick-Acting, .3/4 amp 3AG or MDL fuse __
Line Fuse 117-volt: Slow-Blow l/4 amp 3AG or MDL fuse --
234-volt: Slow-Blow l/8 amp 3AG or MDL fuse
Power Cord Provides ground connection for cabinet; 3-wire power --
cord with NEMA approved 3-pronged plug
1V OUTPUT Receptacle Power output; provides ?l volt at up to one.milliampere 2-7
for a full-scale meter deflection. Fast response.
1OOMV OUTPUT Receptacle Recorder output; provides >OOmv, adjustable within 2-7
1Omv span, for full-scale meter deflection. Filtered.
1OOMV ADJUST Adjusts 1OOMV OUTPUT within 1Omv span 2-7
GND Terminal Connection to chassis ground 2-3,2-9,2-14
LO Terminal Connection to circuit low; circuit low will not 2-3,2-9,2-14
be at chassis ground unless LO is linked to GND
SYNCHRONIZING Jacks Eliminates any interaction between two ad~jacent 2-8
TABLE 3. Model 150B Rear Panel Controls. The table briefly describes each control, and
indicates the paragraph which contains instructions on the use of the control.
2 0567K
MODEL 150B MICROVOLT AMMETER OPERATION
SECTION 2. OPERATION
2-l. MODE OF OPERATION,
a. When the POWERSUPPLY Switch is in the OFF position, the Model 150B will not operate,
and the red dot cannot be seen through the switch knob.
b. The Model 150B operates either from an ac power line when the POWERSUPPLY Switch is
in the AC position, or from its battery with the Switch in BATTERY position. In either po-
sition the red dot can be seen. For most uses the instrument functions well from ac. use
battery operation, however, if the ac power line will create ground loop or isolation prob-
lems. Isolation from low to ground is complete for battery operation when the power cord
is'disconnected; it is greater than 101" ohms shunted by .OOl microfarad with the power
cord connected. Also, battery operation is useful to reduce modulation products (usually
8 cps) which may appear at the output for certain low-level measurements. (See paragraph
2-14,.)
NOTE
Before using the battery operation, thoroughly read paragraph 2-2. Improper
battery operation can damage the battery pack and lead to inaccurate measurements.
2-2. BATTERY OPERATION.
The Model 150B is supplied with a rechargeable 6-volt, 4, ampere-hour nickel-cadmium
battery pack. Do not use the battery more than seven consncutive hours without recharging.
,FIGURE 1. Model 150B Front Panel Controls. FIGURE 2. Model 150B Rear Panel Controls.
Circuit designations refer to Replaceable Circuit designations refer to Replaceable
Parts List and schematic diagram. Parts List and schematic diagram.
0667R 3
OPERATION MODEL 150B MICROVOLT AMMETER
NOTE
Permanent damage to the battery pack may occur if it is used for more than 8 con-
secutive hours witbout recharging. At this discharge rate, the number of recharge
gycles is greatly reduced. Before using the Model 150B, check the state of the
battery charge.
b. Check the battery charge before making a measurement. Hold the POWER SUPPLY Switch
in the BATT. TEST position; the red dot will show. In this position the Model 150B shows
the state of the battery charge directly on the meter. The minimum acceptable charge is
a meter indication of approximately +6 on the upper meter scale.
1. Tine terminal voltage of a nickel-cadmium battery changes very little from full
charge to almost crmplete discharge. The +6 meter indication for minimum charged ter-
minal voltage will vary a few minor divisions for different batteries. After a few
charge-discharge cycles, the exact value of the charged terminal voltage for any in-
dividual battery wil.1 be apparent.
2. Recharge the battery if needed. Otherwise, battery operation is the same as for
the ac power line operating mode; refer to paragraph 2-3.
NOTE
When the battery is used beyond its capacity, two effects are seen. There is
a large shift in zero offset from ac to battery operation. Also, the power
supplies do not regulate and high ripple voltages appear at the supply outputs.
C. To recharge the battery, connect the power cord to an ac power line. Turn the POWER
SUPPLY Switch t,o AC or OFF. The battery will be automatically charged in either of these
positions. The charging circuit is such that the battery cannot be overcharged.
d. It is suggested that the battery be used during the day and be recharge& at night.
Leave the instrument always connected to the ac power line; then turn the POWER SUPPLY
_ .
Switch to OFF at night. After a fully charged battery is used icor seven ConseCutiVe Hours, ..~
it will recharge within 16 hours. Leaving the power cord connected has little effect on
the isolation: loLo ohms with the low-ground link disconnected. If the battery is used
longer than eight hours, it may take considerably longer than 16 hours to recharge.
-.,
2-3. MICROVOLT AND NULL DETECTOR OPERATING PROCEDURES.
a. Set the front panel 'controls as follows: -.
POWER SUPPLY Switch OFF
RANGE Switch 1oooMV .-
FUNCTION Switch INPUT R lOOK
ZERO SUPPRESS COARSE Control OFF
FILTER Switch -IN
-
NOTE
Make sure rha ZERO SUPPRESS COARSE Control is OFF. If it is not, a suppression
voltage is introduced, which may cause an error in measurements. See paragraph
2-5 for zero suppression.
4 0667R
MODEL 150B MICROVOLT AElMETER OPERATION
b. Check the voltage shown on the rear panel LINE VOLTAGE Switch; connect the Model
150B to the ac power line. The battery will now be charging.
c. If the circuit low is to be at ground, put the low-ground link between the LO and GND
terminals on the rear panel. The ground terminal (GND) is connected to the chassis and the
third wire of the power cord. The low terminal (LO) is connected to circuit ground and the
low side of the INPUT Connector.
d. Turn the POWERSUPPLY~Switch to the desired mode of operation, AC or BATTERY. For
moat stable operation, allow the Model 150B to warm up for 1 hour.
e. Connect the unknown voltage to the INPUT Receptacle using a Model 1506 or 1507 Low-
Thermal Input Cable. (See paragraph 2-16)
f. Set the FUNCTION Switch to INPUT R OPEN if high input resistance is desired. In this
position the Model 150B input resistance varies bv range (See Table 4,). If the input is
left completely open circuit, the meter mav drift off scale on any range within a few sec-
onds. Set the FUNCTION Switch to INPUT R 1OOK if it is desired to maintain on-scale read-
ings as the input circuit is opened. In this position the Microvolt Ammeter measures volt-
age with 100 kilohms shunting the INPUT Terminals.
g. Increase the sensitivity of the Model 150B with the RANGE Switch until the meter
shows the greatest on-scale deflection.
1. Check the source resistance to make sure'that it is within the maximum value
specified for the range being used. (See Table 4). If the maximum is exceeded, the
Model 150B may not perform within its specifications.
2. Zero offsets with the ZERO SUPPRESS Controls off will vary with the quality of the
circuit's thermal construction. When a Model 1488 Low-Thermal Shorting Plug is connected
-~
to the Model 15OB INPUT Receptacle, offset
Input Resistance
should be less than 0.5 microvolt.
Maximum Source Line
Minimum
Frequency
Range Greater Than Resistance Rejection
0.3 microvolt 1 Ma 10 kQ 55OO:l
1 microvolt 3 Ma 30 kR ---
3 microvolts 10 MQ 100 kR ---
10 microvolts 30 Ma 300 kR 1OOO:l
30 microvolts 1 00 MO 1 Ma ---
100 microvolts 100 MR 1 MR ---
300 microvolt8 100 MO 1 MR ---
1 millivolt
I through 100 MR 1 MR 3O:l
TABLE 4. Model 150B Input Resistance, Maximum Source Resistance, and Minimum Line Fre-
quency Rejection by Range. The rejection is the ratio of impressed peak-to-peak line
frequency (50 or 60 cps) voltage at input to the indicated dc voltage. The above line
frequency rejections are reduced about 30 db on all ranges with the filter out.
0667R 5
-
I
OPERATION MODEL 150B MICROVOLT AMMETER __;
3. Shifts in source resistance may also affect the zero offset, if the source resis-
tance approaches the maximum value given in Table 4. This effect is negligible for
source resistances less than 10% of the maximum value.
h. At low levels, spurious emf's may be generated simply by conta.ct between the input
leads and the circuit under test. These may be compensated for by the zero suppression
circuit. If possible, always leave the instrument connected, Andy adjust the zero after
estabLishing a zero reference in the apparatus under test. For example, in bridge measure-
ments, disconnect the bridge, exciting voltage, or with a phototube, shield the tube from,
light.
2-4. AMMETER OPERATING PROCEDURES. Set the FUNCTION Switch to the AMPS position. Select
the range using the RANGE Switch. Make sure low resistance leads are used to connect the
source to the Model 150B input to minimize input voltage drop. The maximum allowable cur-
rent overload on any range is 100 mii'liamperes. If this is exceeded, or if the maximum in-
put voltage drop is exceeded (see specifications), the current-sensing resistor may be dam-
aged. The Model 150B rise time (10% to 90%) as an ammeter is less than 1 second on the 3- ..,
nanoampere and higher ranges, increasing to 3 seconds on the 0.3-nanoampere range.
2-5. ZERO SUPPRESS OPERATION.
a. Purpose: The zero suppression circuit cancels any constant voltage in order to use
a more sensitive range to observe a superimposed signal. Stability is such that up to
100 times full scale may be suppressed. For example, the Model 150B can measure changes
of less than one microvolt in a lOO-microvolt steady signal on its l-microvolt range.
b. Suppression Voltages Available: The COARSE Control sets the suppression voltage to
one of four maximum values. (Refer to Table 5). The FINE Control continuously adjusts the
voltage between the positive and negative value of COARSE Control setting. For example, if
the COARSE Control is at 3 for a maximum suppression voltage of l 1.2 mv, the FINE Control
adjustment span is from -1.2 mv to +1.2 mv. -,
I
~C. Operation: Maximum
ZERO SUPPRESS COARSE Suppression
1. Keep the COARSE Control in OFF Control Setting Voltage
position. Adjust the RANGE Switch to
the range that gives the closest to a 1 .*3.6 microvolts
full scale meter deflection. 2 *120 microvolts
3 t1.2 millivolts
2. Completely turn the FINE Control 4. *12 millivolts
in the direction opposite to the meter
deflection (counterclockwise for posi- TABLE 5. Suppression Voltage by Control
tive deflections and clockwise for neg- Settings. The zero suppression voltage
ative deflections). shown is the maximum value, *15%, for each -
COARSE Control setting. The Level of sup-
3. Increase the COARSE Control set- pression voltage for each setting is the
ting until the meter needle passes same on every voltage range.
through sero. Adjust the FINE Control
for zero deflection.
4. Set the RANGE Switch to a more sensitive range, up to 100 times more sensitive -J
than the original range (four RANGE Switch positions). Readjust the FINE Control to
zero, if necessary.
6 0667R _;
MODEL 150B MICROVOLT AMMETER OPERATION
2-6. FILTER SWITCH.
a. The input filter is adjusted at the factory for 60 cps line frequency, unless 50 cps
is indicated on the rear panel of the Model 150B.
b. When the FILTER Switch is at the IN position, a line frequency (Twin-Tee) filter is
used at the INPUT. With the Switch at the IN position a higher level (about 3~0 db) of 60
cps can be tolerated at the INPUT without affecting the accuracy or the sensitivity of the
Model 150B. Nomally , it is best to leave the FILTER Switch at the IN position for all
cases except where the source resistance exceeds 300 kilohms.
c. The filter has a 5-microfarad capacitor. When the FILTER Switch is set to IN, this
capacitor is in use and produces an RC time constant. For a source resistance of 300 kil-
ohms or greater, the rise time of the Model 150B is affected by the RC time constant and
increases above that given in the specifications.
d. With the Switch set to OUT, the capacitor is not in use. If rise time is important,
set the FILTER Switch to~the OUT position for source resistances greater than 300 kilohms.
HOWeVer, with the filter out, the line frequency rejection is reduced 30 db on all ranges.
2-7. RECORDEROUTPUTS. The Model 150B has two'recorder outputs; fl volt at up to 1 mil-
liampere and a filtered *lOO millivolts.
a. The *l volt, 1 milliampere output is accurate to i-l% of full scale. Output resis-
tance is less than 5 ohms within the amplifier pass band. This output may be used during
eithef battery or ac operation. I'f the Model 150B is used for differential measurements,
do not ground the recorder connected to the output.
1. When recording with the 1 volt 1 milliampere output, the Keithley Model 370 Recorder
offers complete compatability with the Model 150B. This output is sufficient to drive
the Model 370 without the use of any recorder preamplifiers. The Model 370 allows maxi-
mum capability of the Model 150B. It has 1% linearity, 10 chart speeds and can float up
to 1500 volts off ground. Using the Model 370 with the Model 150B avoids interface
problems which may be encountered between a measuring instrument and a recorder.
2. The Model 370 is very easy to use with the Model 150B. All that is necessary is
connecting the two units and adjusting an easily accessible control for full-scale re-
corder deflection. The furnished Model 3701 Input Cable mates with the output connector
on the Model 150B. On the most sensitive ranges of the Model 150B, under some conditions,
an 8 cps beat may appear.. This condition can be eliminated by mounting a lOO-microfarad
capacitor across pins 14. and 17 in the back of the Model 370 Recorder.
b. The other recorder output is ilO0 millivolts. This output can be used in conjunction
with servo balance recorders. The 1OOMV ADJUST, which is a screwdriver adjust potentiom-
eter on the rear panel of the Model 150B, is used to adjust the 1OOMV output over a lo-
millivolt span. The resistance of this recorder output is less than 1000 ohms. The 1OOMV
output Receptacle is filtered to provide less than l/2% peak-to-peak ac voltages in the
output signal. The rise time will be no less than 1 second on any range. This output
may be used during either ac or battery operation. If the Model 150B is used for differ-
ential measurements, do not ground the recorder connected to the output.
2-8. SYNCHRONIZING TERMINALS. All Model 150Bs have nearly the same chopper frequency.
When two or more Model 150Bs are close together, a beat may develop between the instruments
due to the slight difference in their chopper frequencies. To eliminate this interaction,
0667R 7
7
/
OPERATION MODEL 150B MICROVOLT AMMETER .,
use the two synchronizing terminals on the rear panel of the instrument. These terminals ~'
synchronize the chopper, frequencies. No polarity is necessary; just connect leads from the
terminals of one Model 150B to the same terminals on an adjacent Model 150B. - I
2-9. DIFFERENTIAI. (FLOATING) MEASUREMENTS.
a. The Model 150B will measure the difference between two voltages, neither~ of which is
at power line ground. It can be floated up to *400 volts off ground.
CAUTION
The instantaneous voltage between circuit low and case ground must not exceed *400
volts at any time. The front panel controls are electrically conne,cted to the case.
If the power cord is unplugged, and the off-ground voltage exce'eds *400 volts, the
case may be at any voltage. Use necessary
- safety precautions.
b. For best results in making differential measurements, follow the steps below:
1. Remove the link from the LO terminal on the rear panel.
2. Connect the source to the input. Make ,measurements as described in- paragraph 2-3.
The zero suppress controls may be used for differential measurements.
3. If power line frequency pickup is a problem, use battery operation.
4. When recording from the Model 150B with the LO to GND link removed, be sure to
-use a recorder which also has LO isolated from GND by a high imp&dance, and is also
capable of withstanding the necessary voltage with respect to ground. The Keithley
Model 370 Recorder meets these requirements.
Z-10. ACCURACY CONSIDERATIONS. For sensitive measurements - 10 millivolts and below -
other considerations besides the instrument affect accuracy. Effects not noticeable when
working with higher voltages are very important with microvolt signals. The Model 150B
reads only the signal received at its input; therefore, it is important that this signal
be properly transmitted from the source. The following paragraphs indicate factors which
affect accuracy: thermal noise, input resistance, thermal emf's, shielding and circuit
connections.
2-11. THEIU@L NOISE.
a. The lower limit in measuring small potentials occurs when the Johnson noise, or
thermal agitation, becomes evident. The amount of noise present in the source is shown
in the following equations.
1. Thermal noise in any ideal resistance can be determined from the Johnson
noise equation:
E2 rms - 4kTRF Equation 1
where E,, is the rms noise voltage developed across the voltage source;
T is the temperature in degrees Kelvin;
R is the source resistance in ohms;
F is the amplifier bandwidth in cps;
k is the Boltzmann constant (1.38 x 10mz3 joules/OK).
8 0667R
MODEL 150B MICROVOLT AMMETER OPERATION
For an ideal resistance at room temperature (300◦ Jabse Service Manual Search 2024 ◦ Jabse Pravopis ◦ onTap.bg ◦ Other service manual resources online : Fixya ◦ eServiceinfo