High impedance DC Voltmeter by IC CA3130

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

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