Op Amps for Everyone - The Repeater Builder's Technical ...

the bias resistor, R 1 , is selected as a large value, V REF and R 1 act as a current source,
and the transducer resistance can be neglected in the calculations thus yielding Equation
12–4. When R 1 >> (R T + ∆R) Equation 12–3 reduces to Equation 12–4.
V OUT
V REF
R 1
R RT
(12–4)
Equation 12–5 is the equivalent of Equation 12–4, and it is obtained by exciting the transducer
with a bias current as shown in Figure 12–5. The bias current can be made very
accurate by employing op amps in a current source configuration as shown in Figure
12–6, thus the approximation R 1 >> (R T + ∆R) need not enter the calculations.
R 1
V OUT
∆ R
I
R T
Figure 12–5. Current Source Excitation for a Resistive Transducer
V IN
+
_
R
V CC
R 2
I V CC V IN
R
R T + ∆R
Figure 12–6. Precision Current Source
V OUT
I R RT
(12–5)
The Wheatstone bridge shown in Figure 12–7 is a precision device used to measure small
changes in resistance. One leg of the bridge is made up of a voltage divider consisting
of equal stable resistors (R 1 and R 2 ) and the reference voltage. When R X and ∆R equal
zero, R TX is selected equal to R T . As the transducer resistance changes ∆R assumes
12-7