The circuit shown in the figure can monitor resistance in a
noisy or otherwise hostile environment. A 1:1 transformer
provides isolation and a good common-mode rejection ratio
(CMRR). The resistance across the secondary winding is
reflected to the primary, where it forms a voltage divider with
resistor R1 (see the figure). This divider produces a reducedamplitude
clock signal that's coupled through C2, rectified by
D1 and D2, and filtered by C3 and R2. The result, delivered at
the inputs of the quad voltage monitor (IC1), is a dc voltage
that varies with the monitored resistance as shown in the table.
The monitored resistance can be in the form of a single resistor, like
an NTC resistor for measuring temperature, or a set of
resistors and switches, as shown in the example circuit. The slow
response of the rectifier and filter helps to mask the effect of any
switch bounce.
The component values shown support a 5-V, 0.1-MHz to 1-MHz
clock signal and resistance values from 0.5 kO to 10 kO. IC1's voltage-monitoring thresholds (1.8 V, 2.5 V, 3.3 V, and 5 V) are
suitable for this application, and T1 can be any 1:1 pulse
transformer with an inductance of approximately 0.5 mH. You can
decode the IC1 output with suitable discrete logic, or feed it to a
microcontroller as required by the specific application.