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Looking for current sense / power amp capable of measuring 1nA to 1mA with +- 100pA resolution

Nick Burczyk
Prodigy 30 points
Other Parts Discussed in Thread: TIDA-00639, INA226, OPA333, ISO224, OPA2180, INA2180, INA821, INA111, ISO1541

Hi,

I have a quirky academic device to design,

I will be be passing 0-150V at 1nA to 1mA through a piece of organic tissue.

I need to measure both the voltage ( to ensure its precisely what is required - resolution +- 100mV)

as well as current between 1nA to 1mA.

I was looking at a design similar to TIDA-00639 with OPA333 and INA226

however I was looking at modifying the design to use the isolated ISO224 and OPA2180.

Please let me know what you think. Any help is welcomed.

Best

Nicholas

  • 0
    TI__Mastermind 31195 points

    Hey Nick,

    Welcome to the forum and thanks for your question.

    Measuring 1nA to 1mA with 100pA resolution will require a low-noise and low input bias (high input impedance) amplifier. I think you may want to consider using an amplifier with lower input bias current as the INA2180 is already at 1nA max.

    There will not be a viable current sense amplifier to act as the front-end of your signal chain. It will either need to be an op-amp or instrumentation amplifier. If you look up low-noise in-amps and sort by noise (http://www.ti.com/amplifier-circuit/instrumentation/products.html#p770typ=1;10&sort=p770typ;asc), you can see that the INA821 may be a good option with low noise and similar IB to OPA2180. Using an instrumentation amplifier could be helpful in that you won't add any noise/error with the current sensing resistors or level shifting FETs used in TIDA-00639.

    You can also search all in-amps and sort by input bias current (http://www.ti.com/amplifier-circuit/instrumentation/products.html#sort=p769max;asc), the INA111 seems like a good option at 20pA input bias current and 10nV/rtHz noise at 1kHz.

    Overall, you will need to consider some fundamental aspects to the design so you understand what exactly you want to optimize. So maybe consider performing an offset calibration (zero-current) to negate offset errors. Meaning you can choose an amplifier with higher offset since you know you will calibrate it out anyway allowing you to choose an amplifier with low noise. Keep in mind that total noise is also related to amplifier bandwidth. I recommend watching these training videos to fully understand these concepts:

    Also, moving the ADC to the high-voltage side and then using the ISO1541 to level shift digital signals down to ground level is also another option. This is done in: http://www.ti.com/lit/ug/tidu361a/tidu361a.pdf

    Hope this helps.

    Best,

    Peter