2-24
Voltage and Temperature Measurements
Compensating for thermal EMFs
— Although the 4-wire measurement method minimizes
the effects of lead resistances, other factors can affect low-resistance measurement accuracy.
Thermal EMFs, and other effects can add an extraneous DC offset voltage (V
OFFSET
in
) to the measured voltage.
The Relative feature of the Model 2182 can be used to null out the offset voltage. In general,
this is done by disconnecting the current source and zeroing the reading on the Model 2182 by
pressing the REL key (see “
Measuring voltage and temperature
,
DC offset voltage is effectively cancelled as long as it remains comparatively steady. If the offset
voltage varies, the DC current-reversal technique should instead be used.
The DC current-reversal technique to cancel the effects of thermal EMFs requires a source
that can output currents equal in magnitude, but opposite in polarity. In general, a voltage
measurement is performed on both the positive and negative alternations of the current source.
The averaged difference of those two readings cancels out the thermal EMF component of the
measurements. The Model 2182 can automatically perform the measurements, and then
calculate and display the result by using the Delta measurement mode. For Delta measurements,
a Keithley SourceMeter (Model 2400, 2410, or 2420) or the Keithley Model 220 Current Source
can be used to provide current-reversal. Details on performing Delta measurements are provided
in
.
Testing switch contacts
Low power switches
shows how the Model 2182 can be used to measure the
resistance of a switch contact. The constant current is provided by the Keithley Model 220
current source, which can source up to 100mA. To avoid oxide puncture, the voltage across the
switch contact should be
≤
20mV. Voltage is limited by choosing a current that will not result in
a larger voltage drop than 20mV. For example, with a contact resistance specified at 500m
Ω
, the
current should be no larger than 40mA.
Figure 2-11
Measuring switch contact resistance
With current known and voltage measured, resistance can be calculated using Ohms Law:
R = V/I.
Test Circuit
Switch
(DUT)
HI
HI
LO
LO
DCV1
CH 1
CH 2
2182
Model 220
Current
Source
Summary of Contents for 2182
Page 1: ...www tek com keithley Model 2182 2182A Nanovoltmeter User s Manual 2182A 900 01 Rev B May 2017...
Page 18: ......
Page 22: ......
Page 23: ...1 Getting Started Getting Started...
Page 41: ...2 VoltageandTemperature Measurements Voltageand Temperature Measurements...
Page 68: ...2 28 Voltage and Temperature Measurements...
Page 69: ...3 Range Digits Rate andFilter Range Digits Rate andFilter...
Page 82: ...3 14 Range Digits Rate and Filter...
Page 83: ...4 Relative mX b and Percent Relative mX b andPercent...
Page 91: ...5 RatioandDelta Ratioand Delta...
Page 117: ...6 Buffer Buffer...
Page 123: ...7 Triggering Triggering...
Page 140: ...7 18 Triggering...
Page 141: ...8 Limits Limits...
Page 149: ...9 SteppingandScanning Steppingand Scanning...
Page 168: ...9 20 Stepping and Scanning...
Page 169: ...10 AnalogOutput Analog Output...
Page 175: ...11 RemoteOperation Remote Operation...
Page 205: ...12 CommonCommands Common Commands...
Page 221: ...13 SCPISignalOriented Measurement Commands SCPISignalOri entedMeasure ment Commands...
Page 225: ...14 SCPIReferenceTables SCPIRefer enceTables...
Page 239: ...15 AdditionalSCPI Commands Additional SCPICom mands...
Page 260: ...15 22 Additional SCPI Commands...
Page 261: ...A Specifications Specifications...
Page 263: ...B Statusand ErrorMessages StatusandError Messages...
Page 268: ...B 6 Status and Error Messages...
Page 269: ...C Measurement Considerations Measurement Consider ations...
Page 278: ...C 10 Measurement Considerations...
Page 279: ...D Model182Emulation Commands Model182 EmulationCom mands...
Page 284: ...D 6 Model 182 Emulation Commands...
Page 285: ...E Example Programs ExamplePro grams...
Page 293: ...F IEEE 488 BusOverview IEEE 488Bus Overview...
Page 307: ...G IEEE 488andSCPI ConformanceInformation IEEE 488and SCPIConform anceInforma tion...
Page 310: ...G 4 IEEE 488 and SCPI Conformance Information...
Page 311: ...H Measurement Queries Measurement Queries...
Page 316: ...H 6 Measurement Queries...
Page 317: ...I Delta PulseDelta and DifferentialConductance Delta Pulse Deltaand Dif ferentialCon ductance...