Keithley 7015-C, Instruction Manual

Page: 59 / 97

Operation
4-13
Minimizing thermal EMFs caused by solid-state relays
Since thermal EMFs generated by the solid-state relays are
largely a function of the operating temperature of those re-
lays, thermal EMF generation can be minimized by keeping
the power dissipation in relays to a minimum. (To do so, sim-
ply minimize the current flowing through the card at a given
voltage.) Not only will the relays switching the power be af-
fected, but adjacent relays on the relay card circuit board can
also be affected by heating, although usually to a lesser de-
gree. Also, minimizing on time will reduce thermal EMFs.
Nulling residual thermal offsets
Even if all reasonable precautions are taken, some residual
thermal offsets may still be present. These offsets can be
minimized by using the measuring instrument’s offset-com-
pensated ohms feature if available. Also, the zero or relative
feature can be used to null them out. To do so, place the in-
strument on the range to be used for the measurement, and
short the end of the connecting cable nearest the measured
source (first disconnect the cable from the source to avoid
shorting out the source). After allowing the reading to settle,
press the zero or rel button to null the offset, then make your
measurement as usual. Note that it may be necessary to re-
zero often to counteract thermal drifts, and the rel process
should be repeated whenever the range is changed for best
accuracy.
4.5.2 Channel resistance
Channel resistance is the equivalent resistance between the
input and output terminals of the multiplexer card. With con-
ventional mechanical relays, the channel resistance is gener-
ally low enough to be negligible. However, the solid-state
relays used in the Model 7015 have an on resistance of
<210
Ω
(<130
Ω
typical), which can be a consideration in
many applications.
The channel resistance of the multiplexer card is in series
with the equivalent resistance of the DUT, as shown in Fig-
ure 4-10, and it adds to R
DUT
. When measuring resistance
through the card, the channel resistance can significantly af-
fect measurement accuracy, particularly for DUT resistance
values less than 100k
Ω
. For other types of measurements,
any current that flows through the card will cause a voltage
drop across R
CHANNEL
, resulting in similar measurement er-
rors.
Figure 4-9
Thermoelectric generation
Voltmeter
T
2
HI
LO
T
1
A B A
E
T
= Q
AB
( T
1
– T
2
)
The thermal EMF developed by dissimilar metals A
and B in a series circuit is:
Temperature of the A to B
junction in
°
C
Temperature of the B to A
junction in
°
C
Thermoelectric voltage
coefficient of material A with
respect to B,
µ
V/
°
C
E
T
Figure 4-10
Channel resistance
R
DUT
R
CHANNEL
DUT MUX
Card
= DUT Resistance
= Channel Resistance
= Measured Resistance
= R
DUT
+ R
CHANNEL
R
DUT
R
CHANNEL
R
M
R
M
R
M
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