NanoAmperMeter High Voltage part 2

Puplished 6th May 2019

Most digital multimeters can measure a current between a few microamps and 20 amps. They usually cannot measure a current below the microamp range. There are two basic techniques for measuring low currents: the shunt technique, and the feedback ammeter technique. Here an electric scheme consisting of a very simple feedback ammeter is presented (right panel).

The input current (100nA full scale) is connected to the inverting input of an op-amp (IC1G1, ½TLC272 LinCMOS Precision Dual Operational-Amplifier, Input impedance: 1TΩ, Texas Instruments). The output voltage of the IC1G1 can be shown as follows: V[output] = -I[input]*R1 where R1 is the resistance of the feedback resistor, equal to 10MΩ. Here 100nA on the input produces -1V of output. The second op-amp (IC1G2, ½TLC272) inverts the negative voltage to positive for the ADC. A negative 5V power supply for the TLC272 is created using an ICL7660 (CMOS switched-capacitor voltage converter/inverter). An Arduino Nano microcontroller ( with a breadboard-friendly board based on the ATmega328P) is used to digitize the signal (10bit ADC, 1.1V internal voltage reference) and to display it on the OLED display. The prototype was built on a breadboard (shown on the left panel).

The prototype was tested using a programmable reference voltage (Electronic Explorer, Digilent) and 100MΩ/3kV/1W/5% resistors (CHV-series, Bourns). The data is presented as a table (shown below) and as a graph with a log-log scale (shown on the right panel). Additionally, a video of measurements of current flowing through a lighter flame when an external 5V voltage is applied is presented on the top of this page.

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