90mV per mA on the uCurrent

The day job had no useful resistors for measuring current, so tried to add a 10mV/mA range by doing this.



Trimmed it to nearly 0.1R, but the result was a very noisy waveform which spilled noise into the other ranges. The wire was an antenna. The day job had a 1R 5%, so whacked that in. It was still noisy. Left the 10k tacked on the end of it & it acted like another antenna. Removed the 10k completely, which gave a useful but still noisy signal. The 0805 10R wasn't a perfect fit for the 10k position. Fortunately, the day job has many 0402 10R's.

The result was 1mV/1uA, 90mV/1mA, or 0.99mV/1mA, very useful ranges for measuring the coveted 2ma-10mA range. The full range was 0mA-20mA. The oscilloscope seemed to have been doing better with 90mV than 9mV & it skewed the other ranges less. Could finally see how much an interrupt handler was using.

Also, the switches started interrupting the circuit with more usage, so had to go into SHORT mode to change ranges. There was now little point in dual contact switches with all the problems of requiring parallel resistors & not being able to have more ranges. It's something Dave probably couldn't foresee. Fortunately the OFF mode still passes current while not powering the LED, so you needn't drain the battery to keep the DUT going.

After much testing, the mane advantages are changing ranges without interrupting the circuit & plotting transients. The mane limitations are lack of ranges & peak voltage limited to +/- 2V by the high precision op-amps. High precision isn't a mane requirement. Less accurate op-amps with wider range, powered by several coin cells in series would have done a better job. 90mV/1mA was a quite useful range, as bad as it is.

The mane problem is noise. Any circuit could have shown the DC current usage, but it took precisely soldered surface mount parts with little room for changes to get the transients. Young Dave must have spent some time fine tuning the uCurrent for noise & making a better one from scratch would entail quite a bit of fussing.