Posted on Nov 5, 2012

This circuit relies on extremely high input impedance for effective operation. The HA-5180 with its JFET input stage, performs well as a preamplifier. The standard three amplifier configuration is used with very close matching of the resistor ratios R5/R4 and (R7 + R8)/R6, to insure high common-mode rejection (CMR). The gain is controlled through R3 and is equal to 2RI/R3. Additional gain can be had by increasing the ratios R5!R4 and (R7 + R8)!R6.

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The capacitors C1 and C2 improve the ac response by limiting the effects of transients and noise. Two suggested values are given for maximum transient suppression at frequencies of interest. Some of the faster DVM"s are operating at peak sampling frequency of 3-kHz, hence the 4-kHz, low-pass time constant. The 40kHz, low-pass time constant for ac voltage ranges is an arbitrary choice, but should be chosen to match the bandwidth of the other components in the system. C1 and C2 might however, reduce CMR for ac signals if not closely matched. Input impedances have also been added to provide adequate de bias currents for the HA-5180 when open-circuited.

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