Sometimes, we need to measure very tiny AC voltage and also high frequency. We cannot use a general meter.
This circuit can easily measure the very tiny AC voltage and high frequency 100 Hz to 500 Khz. Use an op-amp is the input is mosfet types, so has input impedance about 10 M ohms. The lowest full scale of this meter is 15 mV on a galvanometer 100uA.
A galvanometer is a very sensitive meter to measure tiny currents, usually 1mA or less.
It is analog multimeter, cheap and somewhat accurate.
Best for observing the trend of a slowly changing current.
This op-amp have 2 function are: an amplifier and an active rectifier. The gain can be selected by six the resistors (R1 to R6) to switch SW1, and these resistance can set full scale,be divided by 100uA. For example If we need 200 mV range we will use the resistor is 2 K ohms.
This circuit have 6 ranges are: 15mV,50mV,150mV,0.5V,1.5V,5V
The bridge diodes D1-D4 are the feedback of the amplifier circuit. Thus, must have the offset circuit of the diodes’ threshold voltage, to scale of the meter is a linear.
Adjustment zero of meter.
Short input,adjust P1 to read really zero. Then feed the reference to input, adjust P2 to correct reading. The accuracy of this circuit based on the value of R1 to R6.
The circuit use current very low. So should use two 9-volts batteries to long times life.
The parts you need.
D1-D4________1N4148__75V 150mA Diodes
C1,C2,C3,C5___100nF or 0.1uF 50V ceramic.
C6___________22p 50V ceramic
C4___________10p 50V Ceramic
R1___________40K ___0.25W resistors.
R2___________12K ___0.25W resistors.
R3___________4K ___0.25W resistors.
R4___________1.2K ___0.25W resistors.
R5___________400 ohms___0.25W resistors.
R6___________120 ohms ___0.25W resistors.
P1___________100K Pot (Preset)
P2___________1K Pot (Preset)