Resistance-based transducers, such as strain gauges and piezoresistive devices, find common use in the measurement of several physical parameters. For applications in which digital processors or microcontrollers serve for data acquisition and signal processing, the transducer's response must assume a form suitable for conversion to digital format. It is desirable to convert the resistance change of such sensors into a proportional frequency or a time interval so that you can easily obtain an output in digital form, using a counter/timer. The circuit of Figure 1 linearly converts the sensor resistance, R_S, into a proportional time period. The circuit is essentially a relaxation oscillator, comprising a current source, a bridge amplifier, a comparator, and a flip-flop. The current, I_S, divides in the paths of R₁ and R₂ as if the two resistors were connected in parallel. Assuming ideal op amps, the circuit functions as an oscillator when R_X (R₄+R_S) is greater than R₁R₃/R₂.

The circuit produces waveforms at the input and output of the comparator, IC₂ (Figure 2). T₁ and T₂ are the time intervals for which the comparator's output assumes levels V_S1 and –V_S2, respectively. The output voltage from IC₂, with its levels changed via a zener-diode circuit, serves as clock input to a D flip-flop. From the 7474 flip-flop, you obtain a square-wave output that is high and low alternately for a time period T=4C(R₂R_X–R₁R₃)/R₁. This equation indicates that the circuit converts a change in sensor resistance into a proportional time period ΔT with sensitivity ΔT/ΔR_S=4C(R₂/R₁). The following salient features of Figure 1 merit mention:

• The sensor is grounded; you can easily vary the conversion sensitivity by varying either R₁ or R₂.
• You can adjust the offset value, T₀ (about which changes in T occur because of a change in the sensor's resistance), by changing either R₃ or R₄ without affecting the conversion sensitivity.
• The offset voltages of the op amps alter T₁ and T₂ in opposite ways, such that their overall effect on T(T₁+T₂) is not appreciable.
• Thanks to the current source, the output is largely insensitive to noise voltages in the line of the current source and to changes that occur in V_S1 and V_S2.

Consider the example of converting a Pt-100 (platinum RTD) sensor in the range of 119.4 to 138.51Ω, which corresponds to a temperature range of 50 to 100°C, into time periods of 10 to 12.5 msec. The design is simple. Because the current through the sensor is a fraction of I_S, I_S should be low enough to keep the self-heating error to an acceptably low level. This design uses an IN5287 current regulator; it provides an I_S of approximately 0.33 mA and has a dynamic impedance better than 1.35 MΩ. For a better current source, you could use a circuit based on a voltage-regulator IC. In the next step, with suitable and practical fixed values for R₁ and C, you adjust R₂ until you obtain the needed sensitivity: 130.82 µsec/Ω. Following this step, with a fixed R₄, you adjust R₃ to obtain the offset required in the output (T). Figure 1 shows the values of components for this example. The resistors all have 1% tolerance and 0.25W rating, and C is a polycarbonate capacitor.