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INA381: Vout referenced to Input or Output Ground?

Keith Keller
Expert 7270 points
Part Number: INA381


Hello,

We are using the INA381 to measure current and had a couple of questions:

  1. Can we hook up the INA in the configuration shown in the image?  We want to keep the MCU and other circuits from seeing noise across the sense resistor (have them on the "quite ground").  The maximum voltage across the sense resistor will be 150mV.
  2. Will Vout be referenced to supply ground or input ground? (based on image attached)
  3. Is there essentially an instrumentation amp on the front end of the INA?

Thank you, Keith

  • 0
    TI__Mastermind 27715 points

    Hi Keith,

    I've asked Javier to give you a more detailed response tomorrow, but I have some initial answers now.

    1. I see no problem connecting the INA381 in the configuration shown. Just take care that the input common-mode voltage range and power supply voltage range are not exceeded. If there is a voltage different between "Quiet Ground" and "Noisy Ground," this will need to be considered for Vcm.

    2. Vout will be referenced to the ground connected at the INA381 GND pin, as drawn this is "Quiet Ground."

    3. You can think of the input to the INA381 (and most of our current sense amplifiers) functionally as an instrumentation amplifier, since they amplify a small-signal, differential input voltage into a full-scale, single-ended output voltage. However, the internal architecture is different than a traditional inst. amp as the input pins of our parts are not buffered, so the input bias current is not as low. Instead there is a special input bias stage which allows for Vcm much greater than Vs to be applied, which a traditional inst. amp cannot do.

    Best regards,

    Ian Williams
    Applications Manager
    Current & Magnetic Sensing

  • 0 in reply to Ian Williams
    TI__Mastermind 32480 points

    Keith,

    I will elaborate on some of what Ian said after reviewing the data sheet in more detail.

    Our specification for input common mode is verification is from Vin 0 to 26V.  Anything below Vcm = 0 and above -0.2 will function but some of the specifications may have a little degradation.  The expected degradation is small.  Please make sure you avoid the Vin- pin not going below -0.3V as this will exceed absolute max ratings.