Keithley 148 User manual

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~Nationat ~Bureau of Standards through equipment which is cal~@reted at~Nationat ~Bureau of Standards through equipment which is cal~@reted at
,~ fWrned ~Jnterv& by comparisqn to ,~ertlfii Standards maintained in ,~ @anned~Jntery&ls~by comparisqti !a ,~et$fii standafds nMin@nad in
the Laboratories of :Keithley lnstruments;~ Inc.the Laboratories of :Keithley Instruments;~ Inc.

INSTRUCTION MANUAL
Model 148
Nanovoltmeter
COPYRIGHT 1974, Keithley Instruments, Inc.
PRINTED JAN. 1979, CLEVELAND, OHIO U. S. A.
DOCUMENT #29029

CONTENTS

MOOEI, 148
ILLUSTRATIONS
ILlsJSTRATIONS
FIG. TITLE PACE
la Front Panel ................................ 1
lb Front Panel With Model 1481 Input Cable .................. 3
2 Front Panel Conrrole. ........................... 4
3 Rear Panel Conerols and Connections .................... 6
4 Model 1481 Low-Thermal Input Cable. .................... 10
5 Made! 1483 Low-Thermal Connection Kit ............ : ...... 10
6 Normal Wave Form at Demodulator with Input Sbor’ted. ............ 15
7 Wave Format Demodulator Sham with Some Pickup. .............. 15
8 Wave Format Demodulator when Amplifier is Saturated ............ IS
9 “sing Model 148 with 4-Terminal Connections ................ 17
10 Exploded “few far Rack Mounting ...................... 18
11 Block Diagram of Model 148 Amplifier Circuits ............... 19
12 Model 148 Input Circuit .......................... 20
13 Block Diagram of Model 148 Power Supplies ................. 23
14 Model 148 Input Compartment ........................ 26
15 Correct Wave Form in dc-to-dc Inverter. .................. 31
16 correct wave Form at Point F in Oscillator circuit. ............ 32
17 Improper wave Form af point F in Oscillator Circuit ............ 32
18 NOf Used. -- .................................
19 Nof Used. ................................. --
20 Top View of Model 148 massis ....................... 38
21 Bottom View of Model 148 Chassis. ..................... 39
22 Transistor Locations on Printed Circuit 76. ................ 40
23 Capacitor and Diode Locations on Printed Circuit 76 ............ 40
24 Resistor Locations on Printed Circuit 76. ................. 41
25 Resistor and Test Point Locations on Printed Circuit 76 .......... 41
26 Resistor and Test Point Locations on Printed Circuit 74. Bottom Face. ... 42
27 Component Locations on Printed Circ”it 74, Top Face ............ 42
28 Resistor and Test Point Locationa on Printed Circuit 75 .......... 4,
29 Capacitor and Transistor Locations on Printed Circuit 75. ......... 43
30 Resistor Locations on RANGE Switch (S102) ................. 44
31 Resistor LOcationS on RANOE SWitCh (5102) ................. 44
0375

SPECIFICATIONS MODEL148
SPECIFICATIONS
iv
0375

MODEL148 GENERALDESCRIPTION
SECTION 1. GENERAL DESCRIPTION
l-l. GENERAL.
a. The Keithley Model 148 Nanovoltmeter conveniently measures dc potentials
from 10 nanovolts (10 x 10-g volts) to 100 millivolts full scale. It makes
accurate and sensitive measurements without painstaking methods often previously
required. Meter accuracy is 2% of full scale on all ranges. Noise is less
than 1 nanovolt peak-to-peak on the lo-nanovolt range. Zero drift is less
than 10 nanovolts per 24 hours after warm-up. On the three most sensitive
ranges, line-frequency rejection is greater than 1OOO:l.
b. For reliable and versatile use, the Nanovoltmeter is of solid-state
design, except for the first two input stages. It has high line isolation
and battery or line operation.
FIGURE la. Front panel.
0972

GENERALDESCRIPTION MODEL148
1-2. FEATURES.
a. Battery operation permits complete isolation from power line, eliminat-
ing many grounding problems. Battery operation also allows flexibility and
convenience in use. The Model 148 automatically recharges the battery if
needed when the ac power cord is connected.
b. The Nanovoltmeter has a 21 volt at 1 milliampere output at full-scale
meter deflections for driving a recorder or oscilloscope. Accuracy is 1% of
full scale for output.
c. A zero suppression circuit permits measuring small changes in a larger
dc signal.
l-3. APPLICATIONS.
a. The Model 148 Nanovoltmeter measures very small dc potentials or
small changes in dc potentials from low impedance sources. These are found
in fundamental or applied research, laboratory standards work, cryogenic
experiments and instrument development for space research. It can also
serve as an amplifier in these uses.
b. Typics.1 uses include measuring small temperature differences and
small temperature changes indicated by thermocouple outputs, small changes
in conductance, super conductivity in the lo-6 ohm range, and thermopile
outputs used in narrowband spectrum analysis. Other uses are determining
the thermoelectric power of metals, conducting Hall effect studies, and
making Bolometer measurements.
Also, the Model 148 is suited for use with
potentiometers, ratio sets and resistance bridges, including Wenner, Wheatst
and Kelvin Double bridges. It can be used to make 4-terminal resistance
measurements.
2 0972

MODEL148 GENERALDESCRIPTION
FIGURE lb. Front Panel With Model 1481 Input Cable.
0972 3

GENERALDESCRIPTION MODEL148
ZERO s”PPmssIoN
POWER
SWITCH *
5201 COARSE 5103 FINE R168
4
FIGURE 2. Front Panel Controls.
0972

MODEL148 NANOVOLTMETER
SECTION 2. OPERATION
OPERATION
.2-l. FRONTPANEL CONTROLS. (See Figure 2.)
a. AC CONNECTEDLamp. The Lamp is lit whenever the unit is connected to
the ac line and the POWERSUPPLY Switch is in the AC or OFF position.
NOTE
The AC CONNECTEDLamp indicates only that the instrument
is connected to the ac power line; it does not indicate
that the Nanovoltmeter is operating. Also, when the
POWERSUPPLY Switch is turned from AC to OFF, a difference
in Lamp brightness is normal.
b. BATTERYCHARGINGLamp. When lit, this Lamp indicates the battery is
charging. The charge current determines its brightness. If the lamp is
not lit, then the battery is charged.
c. POWERSUPPLY Switch. The Switch controls the mode of operation for
the power supply.
1. AC position: The Nanovoltmeter will operate from the ac power lint.
The battery will be charged if needed; then, the BATTERYCHARGINGLamp
will light.
2. OFF position: The Model 148 is not operating. However ) tk! battery
will be charged, if needed and if the power cord is connected.
3. BATTERYposition: The Nanovoltmeter is operated from its battery.
The ac power line is internally disconnected; the AC CONNECTEDLamp is off;
the battery cannot be charged.
4. BATT.TEST position: When the POWERSUPPLY Switch is held in this
position, the Model 148 shows the state of the battery charge directly
on its meter. All circuits within the instrument are the same as for
battery operation except at the meter terminals.
Switch Settin
TABLE 1. Indicating Lamps and POWERSUPPLY Switch Settings.
The table shows the relationship between the front panel
lamps, the power cord and the POWERSUPPLY Switch setting.

OPERATION MODEL148 NANOVOLTKETER
d. RANGESwitch. The RANGESwitch selects the full-scale meter sensitivity
(either microvolts or millivolts) for one of nine ranges, from 0.01 to 100.
e. FUNCTION Switch. The FUNCTION Switch selects the function - MICROVOLTS
or MILLIVOLTS - which is to be measured.
f. ZERO SUPPRESSControls. Two controls determine the amount of zero suppression.
1. The COARSEControl disconnects the suppression circuit (in OFF position)
or selects one of four suppression voltages in decade steps. Refer to Table 3.
2. The FINE Control is a continuously variable adjustment for the suppression
voltage set by the COARSEControl. It adjusts the range between the positive
and negative values of the maximum voltage set by the COARSEControl.
g. INPUT Receptacle. The INPUT Receptacle is of a special low-thermal design.
Use only the Models 1481, 1482 and 1486 for mating connectors.
WER
5103: RD
FIGURE 3. Model 148 Rear Panel Controls and Connections. Circuit designations
refer to Replaceable Parts List and schematic diagrams.
2-2. REAR PANEL CONTROLSAND CONNECTIONS.
a. Line Voltage Switch. The screwdriver-operated slide switch sets the
Model 148 for 117 or 234~volt ac power lines.
b. Fuse.
1. For 117~volt operation, use a 3 AG or MDL Slow-Blow l/8-ampere fuse.
2. For 234volt operation, use only a MDL Slow-Blow l/16-ampere fuse.
c. Power Cord. The 3-wire power cord with the NEMAapproved 3-prong plug
provides a ground connection for the cabinet. An adapter for operation from
2-terminal outlets is provided.
6 0464

MODEL148 NANOVOLTMETER OPERATION
A note above the power cord shows the ac power line frequency
for which the rejection filter is adjusted. The instrument
will work at any line frequency from 50 to 1000 cps, but ac
rejection is best at the indicated frequency.
d. DEMODULATORTEST. A phone jack provides access to the demodulator for
test purposes.
e. OUTPUT. The OUTPUTReceptacle provides fl volt at one milliampere for
a full-scale meter deflection on any range.
f. GNDand LO Terminals. The ground terminal (GND) is connected to the chassis
and the third wire of the power cord. The low terminal is connected to
circuit ground and the low side of the input connection.
2-3. MODEOF OPERATION. The Model 148 operates either from an ac power line
or from its battery. For most uses, it functions well from ac. Use battery
operation, however, if the ac power line will create ground loop or isolation
problems. Isolation from low to ground is complete for battery operation
when the power cord is disconnected; it is greater than 10' ohms with the power
cord connected. Also use battery operation to reduce the 8-cps ripple which
may appear at the output with the input shorted in ac operation. See
paragraph Z-13.
NOTE
Before using the battery operation, thoroughly read paragraph 2-4.
Inproper battery operation can damage the battery pack and lead
to inaccurate measurements.
2-4. BATTERYOPERATION.
a. The Model 148 is supplied w;th a rechargeable 6-volt, 4 ampere-hour
nickel-cadmium battery pack. Re<,c~mmended: Do not use the battery more
than eight consecutive hours without recharging. At this discharge
rate / the battery should last about 1000 recharge cycles.
NOTE
Permanent damage to the battery pack occurs if it is used
for more than 14 consecutive hours without recharging.
At this discharge rate, the recharge cycles are greatly
reduced. Before using the Model 148, check the state
of the battery charge.
b. Check the battery charge before making a measurement. Hold the
POWERSUPPLY Switch in the BATT. TEST position. The minimum acceptable
charge is a meter indication of +8; full charge is shown by the BATTERY
CHARGINGLamp not being lit. Recharge if needed. Otherwise,
battery operation is the same as for the ac power line operating mode;
refer to paragraph 2-5.
1067'
7

OPERATION MODEL148 NANOVOLTMETER
NOTE
When the battery is used beyond its capacity, two effects
are seen. There is a shift in zero offset from ac to
battery operation. Also, the power supplies do not reg-
ulate and high ripple voltages appear at the supply outputs.
(See paragraph 4-7.)
c. To recharge the battery, come-t the power cord to an ac power line. Turn
the POWERSUPPLY Switch to AC or OFF. The BATTERYCHARGINGlamp will light. The
battery will be charged only if needed, and the circuit automatically prevents
it from being overcharged.
d. It is suggested that the battery be used during the day and be recharged
at night. Leave the instrument always connected to the ac power line; then
turn the POWERSUPPLY Switch to OFF at night, After a fully charged battery
is used for eight consecutive hours, it will recharge within 14 hours. Leaving
the power cord connected has little effect on the isolation: 109 ohms with
the POWERSUPPLY Switch in BATTERYposition and the low-ground link disconnected.
2-5. OPERATINGPROCEDURES,
a. Set the front panel controls as follows:
POWERSUPPLY Switch OFF
FUNCTION Switch MILLIVOLTS
RANGESwitch 100
ZERO SUPPRESSCOARSEControl OFF
NOTE
Make sure the ZERO SUPPRESSCOARSEControl is OFF. If it is not,
a suppression voltage is introduced, causing an error in measurements.
b. Connect the unknown voltage source to the INPUT Receptacle. Refer to
paragraph 2-6 for suggestions.
c. Check the voltage shown on the rear panel Line Voltage Switch; connect the
Model 148 to the ac power line. Make sure the frequency shown above the power
cord is the frequency of the ac power line. At this point, the AC CONNECTEDLamp
will light, as will the BATTERYCHARGINGLamp if the battery is being charged. If
the circuit low is to be at ground, put the low-ground link between the LO and
GNDterminals on the rear panel.
d. Turn the POWERSUPPLY Switch to the desired mode of operation, AC or BATTERY.
e. Increase the sensitivity of the Model 148 until the
meter shows the greatest
on-scale deflection.
1. Check the source resistance to make sure it is within the maximum value
specified for the range being used. (See Table 2.) If the maximum resistance
is exceeded, the Model 148 may not be within its specifications.
1067

MODEL148 NANOVOLTMETER OPERATION
2. Zero offsets with the Zero Suppress Controls off will vary with the
quality of the circuit's thermal construction. See paragraph 2-14. when
a Model 1486 with a copper-wire short is on the Model 148 INPUT Receptacle,
offset should be less than 0.2 microvolt.
3. Shifts in source resistance also affect the zero offset, if the
source resistance approaches the maximum value given in Table 2. This
effect is negligible for source resistances less than 10% of the
maxiwm value.
4. If the input is left completely open-circuit, the meter will drift
off scale on any range.
5. Refer to Table 4 if problems exist during the measurement.
Minimum
Input Resistance Maximum Source Line Frequent
Range Greater Than Resistance Rejection
0.01 microvolt 1 kcl 10 n 3OOO:l
0.03 microvolt 3 kcl 30 n 1OOO:l
0.1 microvolt 10 kn 100 n 1OOO:l
0.3 microvolt 30 kn 300 11 5OO:l
1 microvolt 100 lcrl 1 lu? 5OO:l
3 microvolts 300 kn 3 kc1 1OO:l
.O microvolts 300 kll 3 kn decreasing
i0 microvolts 300 kn 3&l to
10microvolts 300 krl 3kn 5O:l
0.01 millivolt 1 I%,2 10 kn 100: 1
0.03 millivolt 1lQ 10 kcl 5O:l
0.1 millivolt l?Sl 10 kn 2O:l
0.3 millivolt 1 Ma. 10 kn 2O:l
1 millivolt 1Kl 10 kn 2O:l
3 millivolts 3Kl 30 m 1O:l
.O millivolts 5 Ml 50 Icn 1O:l
IO millivolts 5 I%1 50 k!J 5:l
10 millivolts 5 Nl 50 kn 5:l
TABLE 2. Model 148 Input Resistance, Maximum Source Resistance,
and Minimum Line Frequency 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.
f. Three millivolt and microvolt ranges overlap: 0.01, 0.03 and 0.1
millivolts and 10, 30 and 100 microvolts. use the millivolt ranges when the
source resistance is high or if large 60-cps fields are present. The micro-
volt ranges are more convenient to use if subsequent measurements require more
sensitive ranges. Refer to Table 2 for maximum source resistance and line
frequency rejection by range.
g. At low levels, spurious emf's may be generated simply by contact
between the input leads and the circuit under test. If possible, always
leave the instrument connected, and adjust the zero after establishing
a zero reference in the apparatus under test. For example, in bridge measurements,
1067 9

MODEL148 NANOVOLTMETER
disconnect the bridge exciting voltage; or with a phototube, shield the tube
from light.
2-6. ACCESSORIESFOR INPUT CONNECTIONS.
a. The easiest way to connect the
voltage source to the Model 148 input
is with the Model 1481 Low-Thermal
Input Cable supplied with the instru-
ment. Use the Cable for temporary
setups, for measurements at several
points, and when fast connections are
needed. The Model 1481 connects
directly to the INPUT Receptacle.
I
FIGURF:4. Model 1481 Low-Thermal b. Where more permanent setups are
Input Cable. possible or where very low thermal
connections are needed, use the Model
1482 Low-Thermal Input Cable. It is similar to the Model 1481, except it
has bare copper leads instead of alligator clips. Clean the bare wire with
a non-metallic abrasive, such as Scotch Brite or its equivalent, before making
the connection. Crimp connections to the voltage source, as possible with
the Model 1483 Kit, provide the best low-thermal connections.
C.
If cadmium solder is used for a connection, make sure the soldering
iron used is clean and that it has not been used with regular solder before.
Use only rosin solder flux. If possible, heat sink all cadmium-soldered
joints together to reduce generated thermal emf's.
d. Use crimp connections with copper
wire and lugs for the best low-thermal
joints. The Model 1483 Low-Thermal
Connection Kit contains a crimp tool,
shielded cable, an assortment of copper
lugs, copper wire, cadmium solder and
nylon bolts and nuts. It is a complete
kit for making very low-thermal measur-
ing circuits. The Kit enables the user
of the Model 148 to maintain the high
thermal stability of the Nanovoltmeter
in his own circuit.
'IGURE 5. Model 1483 Low-Thermal
Connection Kit.
e. The Model 1486 male low-thermal
input connector is for connecting custom-
made circuits to the Model 148. It also
makes a good low-thermal shorting plug
for testing the Nanovoltmeter: crimp a
short length of pure copper No. 18 or
No. 20 wire between the two pins of
the connector.
f. Other available accessories are: The Model 1484 Refill Kit, which
contains replacement parts for the Model 1483. The Model 1485 female low-
thermal input connector to use with the Model 1481, 1482 or 1486 for building
shielded low-thermal circuits.
10 0464.

MODEL148 NANOVOLTMETER OPERATION
2-7. ZERO SUPPRESSOPERATION.
a. Purpose: The zero suppression circuit cancels any constant voltage in
order to use a more sensitive range to observe a superimposed signal. up to
100 times full scale may be suppressed on the ranges from 0.1 millivolt to
0.01 microvolt. For example, the Model 148 can measure changes of less than
one microvolt in a lOO-microvolt steady signal on its l-microvolt range.
b. Suupression Voltaxes Available: The COARSEControl sets the suppression
voltage to one of eight values, depending upon its setting and the FUNCTION
Switch setting. (Refer to Table 3.) The FINE Control continuously adjusts
the voltage between tile positive and negative value of COARSEControl setting.
For example, if the COARSEControl is at 1 for a suppression voltage of
0.24 millivolt, the FINE Control adjustment span is from -0.24 mv to 0 to
+0.24 mv.
FUNCTION Switch
Setting
MICROVOLTS
MICROVOLTS
MICROVOLTS
MICROVOLTS
ZERO SUPPRESSCOARSE
Control Setting
1
2
3
4
Maximum
Suppression
Voltage
0.24 microvolt
1.2 microvolt6
12 microvolt6
120 microvolt6
MILLIVOLTS 1 0.24 millivolt
MILLIVOLTS 2 1.2 millivolts
MILLIVOLTS 3 12 millivolts
MILLIVOLTS 4 120 millivolts
TABLE 3. Suppression Voltage by Control Settings. The zero suppression
voltage shown is the maximum value, ?15%, for each FUNCTION Switch and
COARSEControl setting.
c. Operation,
1. Keep the COARSEControl in OFF position. Adjust the RANGEand FUNCTION
Switches for the most sensitive meter reading.
2. Completely turn the FINE Control in the direction opposite to the meter
deflection (counterclockwise for positive deflections and clockwise for
negative deflections).
3. Increase the COARSEControl setting until the meter needle passes
through zero. Adjust the FINE Control for zero deflection.
4. Set the RANGESwitch to a more sensitive range, up to 100 times more
sensitive than the original range (four RANGESwitch positions). Readjust
the FINE Control to zero, if necessary.
0365 11

OPERATION MODEL148 NANOVOLTMETER
NOTE
On the highest zero suppression range - 120 millivolts maximum - a zero offset
will be apparent when changing the RANGESwitch settings. On this zero suppres-
sion range, first set the RANGESwitch to the range intended to be used. Then
zero the meter with the ZERO SUPPRESSFINE Control. This offset is introduced
only when the ZERO SUPPRESSCOARSEControl is set to 4 and the FUNCTION Switch
is set to MILLIVOLTS. There is no significant offset on any other zero supprss-
sion range.
2-8. DIFFERENTIAL MEASUREMENTS.
a. The Model 148 will measure the difference between two voltages, neither or which is
at power line ground. It can be floated up to 1-400 volts off ground in ac operatior When
the Model 148 is battery operated it is completely isolated from line.
Unplug the Model 148 power co? and use battery operation before measuring a
source which is more than f400 volts instantaneous off ground. Damage to the
instrument can result if the power line is connected under these conditions.
CAUTION
The front panel controls are electrically connected to the case. If the power
"cord is unplugged, the case may be at a voltage equal to the off-ground voltage.
Use necessary safety precautions.
-
b. For best results in making differential measurements, follow the steps below:
1. Remove the link from the LO or GND terminal on the rear panel.
2. Connect the voltage source to the Nanovoltmeter i,L,wt. Make measurements as de-
scribed in paragraph 2-5. The zero suppress controls may be used for differential meas-
urements. Do not ground any recorders used with this operation, since the low of the
Model 148 output is no longer grounded.
3. If power line frequency pickup is a problem, battery operation usually provides
better results.
2-9. RECORDEROUTPUT. The output of the Nanovoltmeter for a full-scale meter deflection
on any range is *l volt at one milliampere. Accuracy is 1% of full scale. Output resis-
tance is less than 5 ohms within the amplifier pass band. Output may be used during both
ac and battery operation. If the Model 148 is used for differential meas?xements, do not
ground the recorder connected to the output.
a. When recording the Keithley Model 370 Recorder offers complete compatibility with
the Model 148. The output is sufficient to drive the Model 370 without the use of any re-
corder preamplifiers. The Model 370 allows maximum capability of the Model 148. It has
1% linearity, 10 chart speeds and can float up to *500 volts off ground. Using the Model
370 with the Model 148 avoids interface problems which may be encountered between a meas-
uring instrument and a recorder.
b. The Model 370 is very easy to use with the Model 148. All that is necessary is con-
necting the two units and adjusting an easily accessible control for full-scale recorder
12 1067

MODEL148 NANOVOLTMETER OPERATION
L
Trouble (seen on meter)
Change in offset between ac
and battery operation
Possible Cause
Low Battery
Refer to
paragraph 2-4
Very slow response time
Excessive drift
High source resistance paragraph 2-12
Improper shielding paragraph 2-13
I I
Thermal emfs I paragraph 2-14
Improper connection to input paragraph 2-15
Excessive noise or needle High source resistance paragraphs 2-11,2-l
instability improper shielding paragraph 2-13
Improper connection to input paragraph 2-15
Thermal emfs paragraph 2-14
Excessive temperature sensitivity Thermal emfs paragraph 2-14
presence of large, constant zero suppress Controls on paragraph 2-5
signal Thermal emfs paragraph 2-14
Improper connection to input paragraph 2-15
Excessive 8-cps beat at output I Improper location or poor I paragraph 2-13
or meter magnetic shielding
I
TABLE 4. Troubleshooting Operating Procedures. *he Table gives some possible sources of
errors while using the Model 148 and refers to instructions to correct the situation.
deflection. The furnished Model 3701 Input Cable mates with the output connector on the
Model 148. On the most sensitive ranges of the Model 148, under some conditions, a" 8-
cps beat may appear. This condition can be eliminated by mounting a 100~microfarad capac-
itor across pins 14 and 17 in the back of the Model 370 Recorder.
2-10. ACCURACYCONSIDERATIONS. For sensitive measurements - 10 millivolts and below -
other considerations beside the voltmeter affect accuracy. The Model 148 reads only the
signal received at its input; therefore, it is important that this signal be properly
transmitted from trle source. The following paragraphs indicate factors which affect ac-
CllrWy: thermal noise, loading, shielding, thermal emfs and circuit connections. Table
4 also offers a quick reference to correct troubles which may occur.
2-11 THERMALNOISE.
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 show" in
the following equations.
1. The thermal noise in any ideal resistance can be determined from the Johnson noise,
equation:
-As =4kTRF Eq. 1
where Er,, is the rms noise voltage developed across the voltage source;
T is the temperature in degrees Kelvin;
1067 13

OPERATION MODEL148 NANOVOI~'~ETEl?
R is the source resistance in ohms;
F is the amplifier bandwidth in cps;
k is the Boltzmann constant (1.38 x lo-23 joules/OK).
For an ideal resistance at room temperature (300°K), equation 1 simplifies to
E
rms = 1.29 x lo-lo (R F)112 Eq. 2
2. Peak-to-peak meter indications are of more interest than the rms value. Experi-
mentally, the peak-to-peak Johnson noise is about five times the rms value. At room
temperature, equation 2 becomes
EPP = 6.45 x lo-10 (R F)li2 Eq. 3
where Epp is the peak-to-peak noise voltage developed across the voltage source.
3. The Model 148 bandwidth, F, can be estimated from the response speed, tr, by:
F = 0.35/tr Eq. 4
The response speed varies with the range used and the source resistance. On the O.l-
microvolt range when the source resistance is less than 10 ohms, for example, the band-
width is greater that 0.7-cps. The maximum specified response speed for this situation
is 0.5 second, so the 0.7-cps bandwidth is a minimum value.
4- As an example, determine the Johnson noise of a lo-ohm ideal resistor. Measured
with the Model 148 on the 0.1~microvolt range and using equation 3, this becomes
17.1 x lo-10 volts peak-to-peak or 1.71 nanovolts peak-to-peak minimum.
b. In general, good wirewound or low-noise metal-film resistors approximate ideal resis-
tors, and equations 2 and 3 are nearly correct. If the source resistance is.composed of
other materials, it may be necessary to include other terms in the equations to account
for flicker, l/f, and current noise over and above the thermal noise.
c. As seen in equations 2 and 3, the noise of even low resistance values becomes signi-
ficant in the microvolt region. The noise in non-ideal resistors is even greater. There-
fore, keep the source resistance as low as possible. Other effects of high source resis-
tances are decreased response speed and added pickup of extraneous voltages.
2-12. INPUT RESISTANCE. The Model 148 is a feedback amplifier, and its input resistance
is obtained by using high feedback factors. When the source resistance exceeds its physi-
cal input resistance - the amplifier without feedback - the feedback is partially de-
stroyed. Normally, do not exceed the maximum source resistance specified in Table 2;
higher resistances may be used, but noise, offset, slow response time and instability may
result. On the most sensitive ranges, the maximum source resistance is consistent with
Johnson noise considerations.
Z-13. SHIELDING.
a. Due to its narrow bandwidth, the Model 148 is somewhat insensitive to ac voltages
superimposed upon a dc signal at the input terminals. However, ac voltages which are
large compared with the dc signal may drive the Model 148 ac amplifier into saturation,
erroneously producing a dc output at the demodulator. Therefore, shield the circuit to
the Nanovoltmeter input, particularly for low-level sources.
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