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OPA333: circuit review to detect the ASIC working mode

Neo.Wang
Intellectual 750 points
Part Number: OPA333
Other Parts Discussed in Thread: LMV761, OPA391, OPA369

Hi TI experts,

We are trying to use below circuit to detect the ASIC version, depending on the HBM_SNS pin status. Can you please review below circuit and check if below function can be achieved?

- For HBM version ASIC, HBM_VSNS is floating initially and will be a 1.2V after HBM rail ramps up. If initial status is floating, will amplifier output high?

- For MONO version ASIC, HBM_VSNS is shorted to GND inside of ASIC die. The amplifier output Low level.

Thanks

Neo

  • 0
    Guru 140200 points

    Hi Neo,

    I think what you need is a comparator and not an OPAmp? More, the OPA333 is a chopper amplifier which do not like very high ohmic sources. So, I would not take the OPA333 in this circuit but a TI's RRIO CMOS comparator.

    And no, the output of OPA333 will not guaranteedly emit high level when the ASIC output is floating. As being a CMOS OPAmp with ultra high input impedance, the input voltage will be undefined and the input bias current which is a very low leakage current of undefined polarity will make the input float to any undefined potential.

    If you want to have high level at the output of comparator in this situation, I would add a high ohmic pull-up to the output of ASIC. This would force the input voltage to high level and by this the output voltage to high level as well.

    Kai

  • 0 in reply to kai klaas69
    Intellectual 750 points

    Hello Kai,

    Thanks for the reply.

    Could you recommend any part number for RRIO COMS comparator please? 

    As for the high input impedance, how about to change the 2.8Mohm resistor to a buffer?

    Thanks

    Neo

  • 0 in reply to Neo.Wang
    Intellectual 750 points

    Also adding pull ups on the HBM_SNS is not feasible as it's the on-die voltage sensing pin.

  • 0 in reply to Neo.Wang
    TI__Guru 60570 points

    Neo,

    Why would you need to buffer a non-inverting input of CMOS amplifier or comparator? The input impedance of most CMOS op amps is above 1Tohm (>10^12).  Having said that, using OPA333 chopper is NOT a good choice especially since its input stage includes parasitic back-to-back diodes as shown below, which would get forward-biased when the inputs are taken more than 300 mV apart - this, in turn, would result in a large 'offset' error.

    I believe your best option would be to use a comparator like LMV761 - see below.

    If you absolutely need a lower quiescent current (IQ), lower voltage offset part, you could possibly use a non-chopper op amp like OPA391 or OPA369.

  • 0 in reply to Marek Lis
    Intellectual 750 points

    Thank you for the recommendations.

  • 0 in reply to Neo.Wang
    Guru 140200 points

    Hi Neo,

    Neo.Wang said:
    Also adding pull ups on the HBM_SNS is not feasible as it's the on-die voltage sensing pin.

    I haven't understood why you can't add a pull-up resistor. The pull-up could be very high ohmic.

    Kai

  • 0 in reply to kai klaas69
    Intellectual 750 points

    Hi Kai,

    Yes, got it. I am trying to have 1Meg ohmc pull-up on the input of the comparator. Thanks for your comments.

    Neo