This uses a MOSFET op-amp to provide near infinite input impedance, followed by a virtual earth summing amplifier to adjust the d/c offset. The meter output is driven by a x2 voltage gain op-amp providing outputs for 1mA or 100µA meter. The input has switchable input resistance of 1G, 10G, 100G and 'infinite' ohms as well as capacitance of 100nF, 10nF, 1nF and 0. All the capacitors are discharged in the SHORT position, and the amplifier grounded for setting the zero.
