• State Resolved
  • Locked Locked
  • Replies 8 replies
  • Subscribers 82 subscribers
  • Views 767 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LAUNCHXL-F28379D: Questions about ADC Reference Circuit

stevenh
Guru 20035 points
Part Number: LAUNCHXL-F28379D

Hello,

I have a couple of questions about LAUNCHXL-F28379D's ADC reference circuit:
1. Why does the ADC reference circuit use a unity gain op-amp circuit?

2. Why are there two ADC reference circuits: One for VREFHIA and VREFHIB?

3.  What is the purpose of the 1k Ohm resistor at VREF5030's VOUT pin?

  • 0
    TI__Mastermind 30185 points

    Hello Steven,

    I will look into this and try to get back to you by tomorrow.

  • +1 in reply to Omer Amir
    TI__Mastermind 30185 points

    Sorry, to clarify is this a question on the LaunchPAD's schematics or the ADC chapter of the F2837xD reference manual?

  • 0 in reply to Omer Amir
    Guru 20035 points

    My questions are about the LAUNCHXL-F28379D schematic.

  • +1 in reply to stevenh
    TI__Mastermind 42966 points

    Hi Steven,

    See F2837x device TRM section for reasoning:

    Best,

    Kevin

  • 0 in reply to Kevin Allen18
    Guru 20035 points

    Thanks.

    More questions:

    1. Is the 1k Ohm resistor at VREF5030's VOUT pin necessary?

    2. Why are two separate RC loads required at the Reference output if only one unity gain op-amp is shared between VREFHIA and VREFHIB?

    3. The REF5040 datasheet doesn't give a requirement for a unity gain op-amp at REF5040's output. Why does the TMS320F28379 require it?

  • +1 in reply to stevenh
    TI__Mastermind 32826 points

    Steven,

    Please see the Precision Labs Series: Driving the reference input on a SAR ADC

    1. I don't think that exactly 1kOhm is necessary. Although it seems high, this is ok since the OP-AMP has very high input impedance. As the REF50xx datasheet mentions, it is recommended to add some series resistance to capacitors on the reference output for better noise performance.

    2. I'm not sure what you mean, there are two unity gain op-amps on the launchpad. 

    3. I think this can be answered by the precision labs series linked above. Pretty much, there will be very high frequency current spikes into the VREFHI pins. The REF50xx will not be able to respond to these as quickly as the op-amp, which will result in some performance loss. The REF50xx parts may not need an op-amp to drive VREFHI on some standalone ADCs, but they are needed to drive VREFHI on C2000 ADCs for 16-bit conversions. 

    Best Regards,

    Ben Collier

  • 0 in reply to Benjamin Collier
    Guru 20035 points

    For question 2 see Figure 11-25 in the technical reference manual.  I thought the series RC on the output of the op-amp was for phase margin stability.  Therefore, why would there need to be two series RC on the output of each op-amp, which would mean two of the series RC would be in parallel.  Also, is it possible to use one op-amp for all four VREF inputs?

  • +1 in reply to stevenh
    TI__Mastermind 32826 points

    Steven, 

    You are correct, the series RC on the output of the op-amp is meant to reduce the phase shift from the capacitive load from the VREFHI pins. I think it is fine to have a single RC at output of the op-amp. If you look at the launchpad schematic, that is what's done.

    stevenh said:
    Also, is it possible to use one op-amp for all four VREF inputs?

    Yes, but you may have slightly worse performance with 16-bit differential, based on characterization data that we took. 

    Best Regards,

    Ben Collier