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DIFFAMP-EVM: How to evaluate INA105?

Dan P
Prodigy 90 points
Part Number: DIFFAMP-EVM
Other Parts Discussed in Thread: OPA207

I'm trying to utilize the DIFFAMP-EVM to output the difference between two signals at approximately 2.3V. When I attach those two signals to VIN+ and VIN-, I see an output around 1.15V. I have an INA105KP in the socket on the board.

It appears that my V-out includes the buffered values from the onboard U1 OPA207 amplifier.

How do I bypass that amplifier to get a V_out that is centered (approximately) around 0V?

  • 0
    Prodigy 90 points

    I apologize, additional info: I configured the jumpers to match the configuration shown in the top left image of Figure 5 shown in the DIFFAMP-EVM user manual. www.ti.com/.../sbou211.pdf

  • 0
    TI__Genius 10850 points

    Hi Dan,

    Thanks for you post. If you don't want to level-shift the output, you can ground this pin. The reference voltage is set by J9, which either connects the reference to ground or ties it to the output of the reference buffer. Alternatively, this jumper can be left unconnected and the desired voltage can be applied directly to the VREF test point.

  • 0 in reply to Tamara M Alani
    Prodigy 90 points

    Hi Tamara,

    Thanks for the reply.

    If I monitor VIN+ or VIN- directly (no amplifier), they both report around 2.4V, as expected.

    I connect VREF, GND, and BUF together at J9. I set J4/Pin3 to VIN-, J8/Pin3 to VIN+, J2/Pin1 to VREF, J6/Pin5 to VOPA. VO hovers around 0.8V.

    If I remove BUF from GND and let it 'float' at J9, VO goes to 1.15V.

    I'd expect the V0 to be around 0.003V if the differential amplifier piece was working.

    The equation in the data sheet says VO = (Ra/Rb) * (VIN+ - VIN-) + VREF

    As far as I can tell, I have VREF at 0, but still see a static offset no matter what I do.

    Could you recommend jumper settings that will remove the offset?

  • 0 in reply to Dan P
    TI__Genius 10850 points

    Hi Dan,

    This seems like a input / output limitation.

    • What are your voltage supplies?
    • You have 2.4V on each input? (In+ and In-)
    • Is there a common mode voltage?
  • 0 in reply to Dan P
    Prodigy 90 points

    How do I apply VCC & VEE to the board?

  • 0 in reply to Dan P
    TI__Genius 10850 points

    Hi Dan,

    Please refer to section 4.1 of the users guide - Power:

    Power is applied to the device with test points VCC and VEE at TP1 and TP3, respectively. These test points power both the socketed diff-amp and the onboard reference amplifier. The board is initially populated with an OPA207 as the reference amplifier, but if a different device is used then take care that the new device can operate with the same supply as the diff-amp.

  • 0 in reply to Tamara M Alani
    Prodigy 90 points

    Got it, that's probably my issue (no power). I'll get those wired in and see if that works.