17.6 EXAMPLE APPLICATIONS
17.6.1 3–Wire RTD
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FIGURE 38. Topology #1: 3-wire RTD Using 2 Current Sources
Figure 38 shows the first topology for a 3-wire resistive tem-
perature detector (RTD) application. Topology #1 uses two
excitation current sources, IB1 and IB2, to create a differential
voltage across VIN0 and VIN1. As a result of using both IB1
and IB2, only one channel (VIN0-VIN1) needs to be mea-
sured. As shown in Equation 2, the equation for this channel
is IB1 x (RTD – RCOMP) assuming that RLINE1 = RLINE2.
Equation 2 — VIN Equation for Topology #1
The PT-100 changes linearly from 100 Ohm at 0°C to
146.07 Ohm at 120°C. If desired, choose a suitable compen-
sating resistor (RCOMP) so that VIN can be virtually 0V at any
desirable temperature. For example, if RCOMP = 100 Ohm,
then at 0°C, VIN = 0V and thus a higher gain can be used.
The advantage of this circuit is its ratiometric configuration,
where VREF = (IB1 + IB2) x (RREF). Equation 3 shows that
a ratiometric configuration eliminates IB1 and IB2 from the
output equation, thus increasing the overall performance.
Equation 3 — ADC_DOUT Showing IB1 & IB2 Elimination
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