Posted on Apr 11, 2012

This circuit shows amplifier Al as a sensor amplifier in a bridge configuration. Amplifiers A2 and A3 are configured as a voltage to frequency converter and A4 is used as the transmitter. This entire sensor/ transmitter can be powered directly from a 4 to 20 mA current loop. The bridge configuration produces a linear output with respect to the changes in resistance of the sensor. The voltage at the output of Al causes the integrator output A2 to ramp down until it crosses the comparator threshold voltage of A3.

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A3 turns on Ql and Q2. Al causes the output of A2 to ramp up at a rate nearly equal to its negative slope, while Q2 provides hysteresis for the comparator. 1n addition, Ql and Q2 help eliminate changes in power supply loop voltage. Amplifier A4 and Q3 are configured as a constant current sink which turns on when the comparator current increases. The resulting increase in loop current transmits the frequency of the VI F converter back to the control circuitry.

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