µCurrent Adaptor for multimeters

The µCurrent is in many cases also able to improve upon your meters current range accuracy by using your meters more accurate mVDC or mVAC voltage ranges instead to display the DC or AC current. With AC the frequency response extends flat up to 10KHz, but THD increases substantially above 2KHz. Still a very respectable AC response range, surpassing that of many meters on current and voltage ranges. Typical accuracy of the µCurrent itself is better than 0.2% on the µA and nA ranges, and 0.5% or better on the mA range. Unfortunately it is not easy to obtain a 0.1% precision shunt resistor for the mA range, as the 10 milliohm value is too low.
µCurrent Adaptor for multimeters - schematic

The heart of the design is U1, a Maxim MAX4239, a special ultra-low offset/drift, low noise precision amplifier”. Whew, what a mouthful! As the name suggests, it’s a pretty high spec device. The key figure in this application is the nearzero offset voltage. And not just low offset” like many precision op-amps, this one has almost no practical offset voltage at all. 0.1uV typical, with a maximum figure of 2.5uV over the entire temperature range, if you want to know the actual numbers. Specifications 3 Current ranges: +/- 0-300mA (70µV / mA burden voltage typical) +/- 0-1000µA (10µV / uA burden voltage) +/- 0-1000nA (10µV / nA burden voltage) Output Voltage Units: 1mV/mA 1mV/µA 1mV/nA Resolution (nA range): 100pA (3.5digit meter), 10pA (4.5 digit meter) Accuracy (typical): <0.2% on µA and nA ranges, <0.5% on mA range. Output Offset Voltage: Negligible on 4.5 digit meter Bandwidth: 2KHz nominal (+/-0.1dB) Temperature Drift: Insignificant over normal ambient range Noise: < -90dBV THD: < -60dB Battery: CR2032 Lithium coin cell Battery Life: >200 hours (LED OFF), >50 hours (LED ON)

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