Diagram Guide

The input resistance of a.c.-coupled op amp circuits depends almost entirely on the resistance with which the d.c. setting is determined. If CMOS op amps are used, the input resistance is normally high, currently up to 10 MΩ. If a higher value is needed, a bootstrap circuit may be used. This enables the input resistance to be boosted artificially to a very high value, indeed In the circuit shown in the diagram, resistor R1 sets the d.c. point for IC1a. The terminal of the resistor linked to pin 7 of IC1 would normally be at earth potential, so that the input impedance would be 10 MΩ. Connecting the other terminal of the resistor to earth via IC1a and network C2-R3-R2 as far as d.c. is concerned results in the requisite d.c. setting of the op amp.


As far as alternating voltages are concerned, the input signal is fed back so that only a tiny alternating current flows through R1. Therefore, Rin=R1[(R2+R3)/R3]. With resistor values as specified, Rin is about 1 GΩ. One aspect must be borne in mind: the numerical value of (R2+R3)/R3 must not exceed 0.99. This means that the value of R3 cannot be less than 100 kΩ if the value of R2 is 10 MΩ. If these conditions are not met, the circuit will become unstable. 

The MOSFET input of the CA3140 makes it ideally suited for use in a high impedance DC voltmeter. Switch S1 is the range selector. The input impedance is limited to 10M by the resistors used in the voltage divider, but this is still a very acceptable value. Before switching on for the first time, the mechanical zero adjustment of the 100m A instrument should be set so that the pointer rests just below zero on the scale. The unit is then switched on and with the input shorted, TR1 is adjusted until an exact zero indication is obtained. The TR2 is used for calibrating the meter so that the full scale deflection in the three ranges is 1V, 10V and 100V. The supply voltage can be anything between 8 and 20V. The current consumption is less than 6mA at 12V. The accuracy of this voltmeter depends mainly on the quality of the 100m A instrument and the care with which the unit has been calibrated.

Part List

R1=8.2Mohms	R7=1Kohms	C2-3=10uF 25V
R2-3=820Kohms	R9-10=10Kohms	D1=1N4148
R4=82Kohms	TR1=10Kohms trimmer	IC1= CA3130
R5=100Kohms	TR2=25Kohms trimmer	VV1=100uA instrument
R6-8=47Kohms	C1=1.5uF 25V