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High Side, High Voltage(50 to 200VDC), Low current(100nA to 1 mA) sensing

Other Parts Discussed in Thread: LMC660, TIPD104, TIPD129, TIPD135, DDC112

I am looking to measure the current through a high impedance load (around 1 Giga-Ohm) from a voltage supply of 50 to 200 Volts. I was hoping for a single chip solution but with the common mode voltage being so high, and the current to be measured being so low, I am not seeing a single chip that will work.

So my question is, is this even possible? I once read an article that said the DDC112 could be used to measure small currents, but I'm not sure how I would actually use it for my application.


Another option might be using op-amps as a difference amplifier. The LMC660 has very low bias currents, but I'm not sure how to interface it with such a high voltage other than maybe float the supply to the LMC660 with a zener.


Any ideas?

Thanks,

Derek

  • Hello Derek,

    Can you please give us more information about your application?  Also, is it possible to measure on the low side instead of the high side?  

    Please be aware that measuring 4 decades of load current is difficult enough itself and will likely require changing the gain of the sensing circuit.  Here are a few current sensing reference designs that you may be interested in reading:

    TIPD104 (Reference Level-Simulation only)

    TIPD129 (Verified Level-PCB tested)

    TIPD135 (Verified Level-PCB tested)

    I caution you on building your own difference amplifier.  Please note that the resistor tolerances will greatly degrade your CMRR, which is very important in your design if you have to sense on the high side.

    I don't support the DDC112, so if it looks like that is the solution you want to go forward with I can move your post to the appropriate forum.

  • The application involves a substance with probes on both sides of the substance. The goal is to measure the current through the substance. I have been told to expect currents on the order of 100nA to 1mA.

    High side sensing is highly preferred over low side, so I would like to extinguish all options before switching to low side. I was expecting to have to switch the gain, and there is a digital interface to this so I should be able to do that with software if I go down the PGA route.

    I'm not sure the DDC112 is right for my application so I do not think I will be pursuing that.

    Another part of the project that may help is that the power supply is programmable, and I may be able to get away with a couple volt drop over the sense resistor if that will make it easier to obtain this range.

    I will look into TIPD135 for the time being as that seems to have most of what I am looking for.

    Please let me know if you find anything else that may work .

    Thanks,

    Derek