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Original question:

INA190: Vout at zero input current

INA190: Vout at zero input current

Ashvin Srinivas15
Prodigy 135 points

Part Number: INA190
Other Parts Discussed in Thread: LM7705

Hello,

The voltage output at zero current is very important for me to be as low as possible. 3mV leaves me very less margin.

Without using an offset that can be subtracted out later, what other highe side opamps can I onsider to use? My supply is 3.1Volts, I need to sense upto 25mA. At a minimum current of zero, I would like to have an output voltage of less than 1mV.

  • 0
    Guru 140200 points

    Hi Ashvin,

    although the INA190 can go with the output voltage very close to the negative supply rail, it does not perform very well in this condition. You can see this from these specifications:

    Reading between the lines, this means that the output voltage should not go too close to the supply rails, if you desire the best performance of INA190. Other specifications like timing and speed will also suffer when going too close to the supply rails. This is not olnly valid for the INA190 but for almost any other amplifier.

    Kai

  • 0
    TI__Mastermind 32395 points

    Hey Ashvin,

    Trying to get any amplifier (current sense, operational, or instrumentation) to linearly swing to Vout < 1mV at 0-V Vshunt is just about impossible when using a single-ended power supply. All amplifiers will be limited by their linear operating output region and accuracy probably won't be specified here. 

    The easiest solution is to provide a reference voltage of > ~100mV and then read the differential output as Vout_dfif = VOUT-to-VREF where Vout_diff = 0V when load = 0A. You could probably place a buffer on the output to convert this differential signal into a single-ended signal where the output-to-ground will actually be 0; however, then you run back into the problem of needing a dual supply for the buffer/op amp.

    If you truely want a 0V VOUT when load is 0A, then you will need to provide a negative voltage at the GND pin of INA190 or negative supply pin of an operational or instrumentation amplifier. For the INA190, it is important that while GND is a negative voltage, the REF pin should be tied to system ground of 0V. We don't usually recommend giving dual supply voltages for our current sense amplifiers (e.g., INA190) because the VCM (high-side voltage) rating is reduced and the output voltage could go negative during specific power up sequences and  damage downstream circuitry if proper isolation or level-shifting are not employed.

    I will note that the INA190 is nice in this application because of its ENABLE pin. So during system power up, INA190 can be disabled (VOUT is high-Z) and thus even if VOUT is a negative voltage during a power up condition, then it won't damage any downstream ADC or circuitry because VOUT is high-Z and thus any turn-on currents are limited by MΩs of impedance. 

    Hope this helps.

    Best,

    Peter

  • 0
    Guru 140200 points

    Hi Ashvin,

    you could use the LM7705 to provide a small negative supply voltage.

    Kai