AC bridge using LF356

Posted on Jul 7, 2012

The circuit provides a simple and cost-effective solution to matching resistors and capacitors. Impedances ZR and Zx form a half-bridge, while OSC and OSC excite the bridge differentially. The external op amp is a FET input amplifier (LF356) with very low input bias current on the order of 30 pA (typical). CI allows ac coupling by blocking the dc common mode voltage from the bridge, while Rl biases the output of LF356 to 0 V at dc. Use of FET input op amp insures that dc offset due to bias current through Rl is negligible.

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Ac output of the demodulator is filtered via the uncommitted amp to provide dc voltage for the meter. The 10 k potentiometer, R5, limits the current into the meter to a safe level. Calibration begins by placing equal impedances at ZR and Zx, and the system offset is nulled by the offset adjust circuit so that Pin 1 is at 0 V. Next, known values are placed at Zx and the meter deviations are calibrated. The bridge is now ready to measure an unknown impedance at Zx with Â±0.05% accuracy or better.

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