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LMH6882 driving ADS4142 design questions

Rick Bogdan
Intellectual 930 points
Other Parts Discussed in Thread: LMH6882, ADS4142, LMH6682

I have a situation I’d like to ask you about.  In my application I have a differential amplifier (LMH6882) feeding an ADC (ADS4142).  The ADC is in differential mode.  The VCM from the ADC is fed back to the LMH6882 to set the common mode voltage.  My signal is perhaps unusual in that it swings only positive, meaning ideally, the differential lines never cross.  However, the ADC, which is 14-bits then reports only positive voltages and the 14th bit acts as a sign bit so we never take advantage of it.  What I would like to do is set the ADC in offset binary mode (so 0000 = the negative rail) and then bias my signal so it swings from the negative rail to its positive peak.  This solves a couple problems, not just recovering the 14-th bit.

 I am considering setting the LMH6882 stage to full gain (Av = 20) and biasing the negative input with a pot.  It should require only a 50mV variation.  Alternatively, I could apply the negative offset to the output between some coupling resistors but I would have to come up with about 1V.  What do you think about this?  Perhaps you have a better idea?

 

  • 0
    TI__Mastermind 44570 points

    Hi Rick

    I'm moving your post to the High Speed Amplifiers Forum where an expert on the LMH6882 can help you with this question.

    Best regards,

    Jim B

  • 0 in reply to Jim Brinkhurst1
    TI__Genius 15975 points

    Hello Rick,

    To re-iterate your question, you seem to have a positive full-scale only signal which does not do negative full-scale at the ADC which you want to flip it starting from the negative full-scale of the ADC, so that in offset binary format the negative full-scale becomes code 0000?

    In such a case, as you suggested, you can use the LMH6882 to offset at the inputs by 50mV so that you can introduce a negative offset of 1V at the ADC. But in this scenario, just make sure that the differential signal does not cross at all or else there will be clipping at the ADC. However, I doubt that you will be able to re-cover the 14th bit at the ADC because to get that, you must have the differential signal crossed over. I think your code in offset binary will be limited only from 000..000 to 100..000. Something might be good to check with the data-converter guys.

    One thing I noted is that the ADC input common mode is 0.95V, which makes it tough for the LMH6882 to be used in single-supply operation. In such a scenario, I would definitely suggest split supplies for the LMH6682 with supplies centered to output common mode of 0.95V. So, the VCC pins connected to 3.45V and the GND pins connected to -1.55V. The digital pins will now be referenced to the GND pin or -1.55V in this case. The OCM pin has a gain of x2, so you would want to divide down the ADC CM by 2 as shown below.

    Best Regards,

    Rohit