Products > Test Equipment
AC Picoammeter
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slugrustle:
I'm working on design of a transformer with very low coupling capacitance between primary and secondary, intended for 30kHz sinusoidal voltage excitation, and I would like to measure the common mode leakage current through the transformer insulation. The current is anticipated to be quite low, maybe sub-nanoamp. It doesn't register on an Amprobe AM-140-A, so the leakage on my first prototype is <5nA if the meter is considered perfect.
Is there an instrument made to measure AC current (at 30kHz or higher) with picoamp resolution? It strikes me that picoammeters are likely meant for DC or close to it.
I suppose I could use an opamp like the LTC6268-10 (20fA input bias max @ 25°C, 0.45pF typical input capacitance) as either a transimpedance current sense amplifier or with a high shunt impedance and send the output to an AC millivoltmeter. I have a Tonghui TH1912A AC millivoltmeter, but the input impedance is 1MΩ in parallel with 30pF, so that would shunt a lot of the signal I'm trying to measure.
I haven't thought of of other options for this.
TimFox:
One (un-cheap) off-the-shelf solution is a current-input preamplifier, of the type used with lock-in amplifiers, which can drive any normal AC voltmeter.
Stanford Research Systems is a good (un-cheap) source: https://www.thinksrs.com/products/preamp.html
The SR570 is a free-standing (with AC power supply, also usable from battery) unit.
The SR555 and SR556 are designed to be powered from a lock-in amplifier through a DSUB cable.
These have different bandwidths (and the SR570 has selectable filters), and the input impedance depends on range setting (see specifications).
I believe the inputs (virtual ground) are all DC-coupled, so depending on your application you might need a series capacitor at the input.
In my university days, decades ago, I used similar units from Princeton Applied Research (PAR), which may be available on eBay.
slugrustle:
Thanks. SR555 looks like it would work for this, and they explain how to use it separately from a lock-in amplifier, which is nice. Also very cool that they show block diagrams in their manuals.
Although, given the price, I'm pretty tempted to lay out a board in Rogers RO4350B with an LTC6268-10 (and maybe a post buffer amp), power it from 9V batteries and quiet regulators, and put it in a metal box. Doesn't need to be pretty.
Come to think of it, I should probably run an impedance sweep on the AC millivoltmeter input before trying to hook a preamp up to it. Haven't had the occasion to do that to test equipment before, haha.
mawyatt:
You realize that 1pa at 30KHz is equivalent of 1V across 5.3aF, or 0.0053fF, or 0.0000053pF!!!
Best,
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