Q.series
Gantner Instruments GmbH
143
6 Functional Procedures → Connecting sensors with sensing leads
6.3 Connecting sensors with sensing leads
Resistive sensors require an excitation voltage to be able to pro-
vide their output signal. For sensor exci
tation a current is passed
through the connecting lead, which however causes a loss of volt-
age due to the resistance of the conne
cting lead. Consequently,
the sensor is then not supplied with the voltage set on the ampli-
fier module, but rather with a slightly lower voltage. This leads in
turn
to a lower output signal and, depending on the cable resis-
tance, losses in the single-figure percentage range can occur
even
with just a few meters. Therefore, high quality amplifier
modules for the excitation of resistive sensors use so-called sens-
ing leads which can measure the loss of volt
age, since only a very
small current flows in them. This is because the inputs for the
sensing leads have very high input resistances (usually over 10
6
compared to a sensor resistance of a few 100 . The amplifier
module can therefore acquire the voltage arriving at the sensor
error-free and increase its excitation voltage to compensate for
the losses in the connecting cable. This is particularly the case
when the temperature of the connecting cable changes. In this
case the cable resistance changes and the sensor output signal
would therefore also change if no sensing leads were used.
We therefore recommend the use of sensing l
eads. This is mainly
necessary when several meters of cable are used, low measure-
ment deviations are to be obta
ined or when the temperature of
the cable may vary.
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