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AMC7834: AMC7834

Thomas Heidemann
Prodigy 40 points
Part Number: AMC7834
Other Parts Discussed in Thread: AMC7832, AFE20408, AMC7836, AFE10004

We have used the AMC7834 extensively for power amplifier control. I am evaluating a new GaN power amplifier device which requires -5.3V negative voltage. I have adjusted the AVSS negative voltage supply to -5.5V (from original -5V) and was expecting the DAC negative output voltage range to be able to extend to the lower negative voltage (ie -5.5V) but it seems to limit at -5V. Is it possible to extend the negative rail? If so, how? Note the clamp voltage is operating correctly. I have gone through the datsheet and schematics but can't find what sets the negative limit of the bipalor DACs.

Thanks in advance,

Thomas

  • 0
    TI__Expert 6320 points

    Hi Thomas,

    The title page of the datasheet has the DAC ranges. The ranges are 2 * VREF. You could theoretically extend the DAC range by applying a higher reference voltage to the VREF pin, though I'm not sure how stable the output would be myself.

    We have some newer devices that can output with a range of 0 to -10V for GaN PAs, namely the AMC7832, AMC7836, AFE10004, and the AFE20408. 

    Let me know if you have any other questions!

    Thanks,
    Erin

  • 0 in reply to Erin Guthrie
    Prodigy 40 points

    Hi Erin. Thanks for the feedback. Still a bit of uncertainty. Will the range extend further negative by applying a slightly higher Vref? In other words, programming range to -5 to 0V and applying 2.75V to Vref will extend negative rail to -5.5V?

  • 0 in reply to Thomas Heidemann
    TI__Expert 6320 points

    Hi Thomas,

    Yes, that is correct. I've tested this on an EVM today and when applying a voltage of 2.75V to the reference, the DAC output went down to -5.5V. This is a feasible solution, though I'm not sure it was how the part was intended to work.

    There are going to be two things that limit the DAC output. 1) The reference voltage. A higher reference voltage provides a higher DAC output range. 2) The AVSS rail. The DAC is limited by what you apply to AVSS. If you set AVSS to -5.5V, the DAC will not be able to reach the full output and will cap around 5.47V unloaded. You will want to lower AVSS below -5.5V, while taking care that the absolute min for this supply is -6V.

    Again, this likely wasn't how the part was intended to use, but it is possible to operate it in this configuration. Let me know if you have any other questions.

    Thanks,
    Erin

  • 0 in reply to Erin Guthrie
    Prodigy 40 points

    Hi Erin. Your great support is much appreciated.