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ANALOG SYSTEM DESIGN

by George Martin

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The simplest transducer is a resistive element that changes with the parameter you need to measure. You could just run a voltage out to the transducer, place another resistor in series, and measure the voltage at the junction of the resistors (see Figure 1). The problem with this approach is that any noise that is picked up on either the reference voltage or ground directly affects your signal in an adverse way.

Figure 1ýHereýs a simple voltage divider network that can be used to monitor the voltage across a transducer. Although itýs simple, any noise that is picked up will affect the signal you are measuring.

Consider a bridge for the transducer (see Figure 2). The reference voltage is connected to R1 and Rv and the return is connected to R2 and R3. If all resistors are nominally 1000 ohm, then the voltages at points A and B are the same and 50% of the reference voltage. If the reference voltage is reduced, the voltage at A and B are still the same, and A/B = 1.0. If resistor Rv changes from 1000 to 900 ohms, then the voltages at A is 50%, B is 47.4%, and the ratio of A/B is 1.06. If resistor Rv changes from 1000 to 1100 ohms, then the voltage at A is 50%, B is 52.4%, and the ratio of A/B is 0.95.

Figure 2ýA better method is to use a bridge. Any change in the reference voltage as a result of noise or some other factor will not affect the ratio of A to B.

Any change in reference voltage or ground does not affect the ratio of A to B; youýve got some noise immunity built-in. Also, by adjusting R3, you can change the starting ratio (zero engineering units) and the final ratio (max engineering units). So, you can get some offset and gain directly from the bridge. If you have a single-sided analog system (0 to 5 V), you might want the ratio to always be less than 1.0 (i.e., B is larger than A). Then you could put A/B (always a positive number) into an amplifier.

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