Measurement Considerations; Thermocouple Measurement Error Sources - Keithley 7014 Manual

Common-emitter characteristic tests
Common-emitter characteristics are determined bv
setting the base current, I,, to specific values. At each 1;
value, the collector-emitter voltage, VCE, is swept
across the desired ranee at soecific intervals. and the
4.5.1
Thermocouple measurement error
sources
The reference accuracy specification of the Model 7014
is the sum of the following error sources:
collector current, I,, is then measured. When the data
are plotted, the result is the familiar family of common-
emitter curves (Figure 4-10).
* Reference junction sensor error
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Temperature gradient across the card
The same test configuration that is used for current
gain tests can be used for measuring common-emitter
characteristics. The Model 224 is used to set the base
current, IB, to the desired values. The Model 230 Volt-
age Source provides the collector-emitter voltage, VCE,
and the Model 196 DMM measures the collector cur-
rent, Ic.
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Relay self-heating
You can achieve better card performance by under-
standing how these error sources contribute to the
specification. The reference junction temperature and
its associated circuitry are shown in Figure 3-6.
Reference junction sensor
The primary factor determining reference junction ac-
curacy is the operating temperature. By using the Mod-
el 7014 in the 18°C to 28°C range, maximum sensor
perf"rmance is achie"ed,
A temperature gradient of 0.32"C develops across the
connector board (*O.l6"C on each side of the sensor).
This gradient contributes to the rcfcrence error.
Channels that are close to the sensor have the least tem-
0
1 2 3 4 5
per&we gradient error (channels 5,6, 15, 16,25,26,35,
VCE , volts
and 36); channels farthest away have the most temper-
ature gradient error (channels 2 and 40).
Figure 4-10
Typical common-emitter characteristics
When making relative tcmperaturc measurements and
not absolute measurements, it is advantageous to use
adjacent channels, as follows:
4.5 Measurement considerations
Many measurements made with the Model 7014 are
subject to various effects that can seriously affect low-
level measurement accuracy. The following para-
graphs discuss these effects and ways to minimize
them.
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Adjacent channels on the same terminal block (e.g.,
channels 2 and 3) will have no more than 0.035"C
error behveen them.
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Adjacent channels on different terminals block
(e.g., channels 15 and 25) will have no more than
0.05T
a-i-Oi-
4.13