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TMS320F28377DWT ADC Reference

Pavan Panchamukhi
Intellectual 260 points
Other Parts Discussed in Thread: REF3230, LMP7709, LMP7704, OPA320, OPA350

Hi

We are working on the ADC reference section of TMS320F28377DWT and we are sampling the input signal  at 1MSPS rate (16-bit ADC)

We are using Ref3230 IC as the reference IC. and is connected to LMP7709MT op-amp which is configured as non-inverting buffer . It is connected to the Vref pin of ADC with the 22uF as suggested in DS of TMS320F28377DWT.

The DS of LMP7709 doesn't mention about the max capacitive load. Is it feasible to use LMP7709 as buffer IC ?

  • 0
    Guru 192875 points
    Hi Pavan,

    The buffer seems to be alright but you don't need a buffer; you can very well do without it as in F28377D controlCard.

    Regards,
    Gautam
  • 0 in reply to Gautam Iyer
    Intellectual 260 points

    We are using 3 pairs (3 Channels) of differential ADC(16-bit)  and  5 Single ended inputs at 4th channel (12 bit) of F28377D ADC.

    The ADC will be sampled at 1MSPS. 

    If we directly connect Ref IC to the ADC Reference pin, the time required to fully charge 22uF to 3.0V is  6.6msec (10mA output current of Ref IC) .

    And time required to charge 22uF to 3.0V using buffer IC is  1.3msec (50mA output current of Buffer IC) which is less when Ref IC is directly connected. 

    Do we have to check any other parameters while selecting the buffer op-amp at the vref pin of ADC?  

     

  • 0 in reply to Pavan Panchamukhi
    TI__Guru 60865 points

    Hi Pavan,

    As far as the LMP7709, this part is not recommended, as it is not unity gain stable (this is a decompensated op-amp). There was an error on an earlier F2837xD ControlCard revision where this part was used, but the intended part was LMP7704.

    This recent post was also looking for a reference for a F2837xD family device. Based on TI characterization, a good choice for the reference driver would be OPA320 or OPA350 with a 2.2uF capacitor on each VREFHI pin (with a 560m ohm series resistor) - you can share two VREFHI pins per buffer if desired. e2e.ti.com/.../477506

    If you are interested in determining whether some other TI op amp can drive the recommended 22uF VEFHI load, the best place to post that question would probably be in the op-amp sections of the e2e forum. (I would still recommend that you use 22uF for 16-bit operation for any circuits not explicitly described in the linked forum post above, and also use one op-amp channel per VREFHI pin).

    Gautam,

    I believe that every version of the F2837xD ControlCard has used some kind of buffering on the VREFHI pin? In any case, as you can see in the linked post, we have experimentally determined that driving the VREFHI pin without a buffer will result in significant performance degradation, even in 12-bit mode.