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OPA350: to Drive DSP ADC Vref Pins

Keith Keller
Expert 6260 points
Part Number: OPA350
Other Parts Discussed in Thread: OPA4350

Hello,

We have a question regarding using the OPA350 to drive voltage reference pins on the TMS320F28375DZWT.

  1. Is it better to drive the different ADC VREF pins with 4 separate OPA350 (single opamp) or 1 quad device like OPA4350?
  2. Can noise internally couple from internal devices in an OPA4350?
  3. What kind of offset difference could we expect between 4 OPA350 versus 1 OPA4350?

We’re using the four ADCs to take four simultaneous samples with the same sampling window.  Every half PWM period we trigger each ADC to take a sequence of four conversions for a total of 16 samples.  Our concern is that each op-amp buffer is going to have its own offset.  Wouldn’t it just be better for each ADC to have exactly the same reference?

The reference path we have going out to analog circuits adds another layer of complication.  Those circuits now see a buffered reference that is different from the references presented to the ADCs.  Theoretically it’s the same but again, each buffer is going to have its own offset and other non-idealities.  So, are we hurting ourselves by having this separate buffered path?  Can we get away with a single buffered reference that goes everywhere?

According to the DSP Technical Reference Manual (http://www.ti.com/lit/ug/spruhm8g/spruhm8g.pdf )..

Thank you, Keith

  • 0
    TI__Guru 60610 points

    Keith,

    Below please see my answers:

    1. Is it better to drive the different ADC VREF pins with 4 separate OPA350 (single opamp) or 1 quad device like OPA4350?

    The answer depends on what is most important in your application: having four seperate channels isolates any interaction between them during conversion cycle but by the same token it causes dc error due to different offset of each channel.

    2. Can noise internally couple from internal devices in an OPA4350?

    At low frequency, the channels separation in OPA4350 is very high, 0.15uV/V or 136dB, and at higher frequency it is shown below:

    Therefore, it is unlikely that noise or conversion related charge injection from one channel will negatively effect another channel thru the internal coupling inside the quad package.

    3. What kind of offset difference could we expect between 4 OPA350 versus 1 OPA4350?

    There is no correlation in the voltage offset between four channels of a quad package.  Thus, there is no difference between voltage offsets inside the quad vs offsets of four individual singles - in both cases the offset of each channel may be anywhere within the Normal Gaussian distribution shown below, which has a sigma of 150uV.

    One way to eliminate the error caused by different offset of each channel would be to use a single OPA350 driving four seperate low-pass filters as shown below.  The value of Rout must be chosen in such a way as to minimize the interaction between the channels and at the same time assure fast enough recovery of VF's between each conversion cycle.