Overvoltage protection for meters - 10 MS2 input impedance

7-32 — elektor july/august 1982 Normally, the high impedance input of the "front end" amplifier in a digital voltmeter is protected against excessive voltages by means of two diodes. One diode is connected between the input and the positive supply rail, while the other is connected between the input and the negative supply rail. In principle, this form of overvoltage protection is perfectly satisfactory. However, the diodes used would have By replacing the diodes with FETs, the following result is obtained. With a reverse bias voltage of 15 V the diode has a leakage current of 5.2 nA, whereas the leakage current of the FET "diode" is a mere 12 pA! This means that the input impedance of the meter can be increased to 10MS2 with no difficulty. The circuit of the input section of a high impedance voltmeter based on the principle outlined above is shown in figure 1. Resistor R1 constitutes the 10 MS2 input impedance. Transistors T1 and T2 are the protective FET "diodes". They can wit...