Input circuit for DC and AC voltmeter - has amplifier with high input resistance whose output goes partly to inverting input of second, operational amplifier

摘要

The first amplifier (1) of the input circuit has a high input resistance. Its output signal is supplied on one hand to a summation point (2), via a resistor (R6), and on the other hand to the inverting input of a second operational amplifier (5). The latter output is coupled to a terminal of a diode (6) whose other terminal is connected to the summation point via a second resistor (R9). This diode terminal is connected to the inverting input of the operational amplifier via a negative feedback resistor (R5). The circuit is so rates that with the diode in conductive state the current (I2), flowing across the second resistor, is approximately twice as high as the current flowing over the first resistor. Preferably the first amplifier is also of operational type and has several negative feedback resistors which can be selectively switched. Thus the output signal polarity is independent of that of the input signal.