LOW_DRAIN_METER_AMPLIFIER

Published:2009/6/24 22:16:00 Author:May | From:SeekIC

Meter amplifiers normally require one or two 9-V transistor batteries. Because of the heavy cur-rent drain on these supplies, the meters must be switched to the OFF position when not in use. The meter circuit described here operates on two 1.5-V flashlight batteries and has a quiescent power drain so low that no on/off switch is needed. A pair of Eveready No. 950 D cells will serve for a min-irnum of one year without replacement. As a dc ammeter, the circuit will provide current ranges as low as 100 nA full-scale. The basic meter amplifier circuit shown is a current-to-voltage converter. Negative feedback around the amplifier ensures that currents IIN and If are always equal, and the high gain of the op amp ensures that the input voltage between pins 2 and 3 is in the microvolt region. Output voltageVO is therefore equal to -IfRf. Considering the ±1.5-V sources (±1.2 V end of life) a practical value ofVO for full-scale meter deflection is 300 mV. With the master bias-current setting resistor (RS) set at 10 MΩ, the total quiescent current drain of the circuit is 0.6μA for a total power supply drain of 1.8 pW. The irtput bias current, required by the amplifier at this low level of quiescent current, is in the range of 600μA.



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