OPA4350: TMS320F28379D VREF-HI

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Dany Bourget

Replies: 2

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Part Number: OPA4350

On the F2837x controlCARD devboard, the OPA4350 is used to drive the ADC VREFHI. Actually on my board I used the OPA378 and the OPA625. Could I used the OPA625 to drive the VREFHI of the TMS320F28379D or I must use the OPA4350. What are the critical characteristics to select the rigth opamp for this aplication?

• OPA4350
• OPA625
• TMS320F28379D
• OPA378

Intellectual 350 points

Dany Bourget

• Mar 9, 2017 1:56 PM
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2 Replies

• Guru 57625 points

  

  Devin Cottier

  

  • Mar 14, 2017 7:29 PM

  Hi Dany,

  The main things to consider here are:

  • Op-amp offset error will affect the total gain error specification (along with whatever IC or other source you choose to provide the reference voltage, and the ADC itself).

  OPA320 => 150uV = 0.2LSBs (12-bit mode, 3.0V reference) = 3.3LSBs (16-bit mode, 3.0V reference)

  OPA350 => 500uV = 0.7LSBs (12-bit mode, 3.0V reference) = 10.9 LSBs (16-bit mode, 3.0V reference)

  OPA378 => 50uV = 0.1LSBs (12-bit mode, 3.0V reference) = 1.1 LSBs (16-bit mode, 3.0V reference)

  OPA625 => 3mV = 4.1LSBs (12-bit mode, 3.0V reference) = 65.5LSBs (16-bit mode, 3.0V reference)

  (I just pulled the first max Vos number I saw out of the datasheets, you would also want to add in or ensure this comprehends the operating temperature range of your application)

  Note: A chopper-corrected op-amp, while having very low offset error, may not be a good choice for the ADC reference driver because you may see some tones centered at the chopping frequency (and they are usually lower BW).    

  • Op-amp bandwidth.  In addition to the large capacitor on the pin, you are going to want a high bandwidth op-amp to help settle out the high current transients on the VREFHI pin.  There is somewhat of a trade-off you can make between the VREFHI capacitor size and the op-amp BW.  We haven't evaluated this thoroughly, but I would recommend the BW be at least 10MHz. 

  OPA320 => 20MHz

  OPA350 => 38MHz

  OPA378 => 900kHz (Note: probably not appropriate for driving the ADC reference)

  OPA625 =>120MHz

  • Op-amp stability when driving a large capacitor.  You may have noticed in our recommended VREFHI driving circuits we have OPA350 + 22uF capacitor + 100mOhm resistor and OPA320 + 2.2uF capacitor + 560mOhm resistor.  The resistor is to ensure the op-amp remains stable when driving the capacitive load.  A better op-amp for ADC reference driving will be able to handle a larger capacitor while still using a small damping resistor.  The only way I know how to check this is to simulate the phase margin using TINA-TI (I can explain how to do this in more detail if needed).
  • Op-amp output noise.  Note: the capacitor on the VREFHI pin will also help attenuate this noise a little bit.   

  OPA320 => 7nV / sqrt(Hz) = roughly 0.1 LSBs of peak-to-peak noise (12-bit, 3.0V range, 1MHz BW) or 1.2 LSBs of peak-to-peak noise  (16-bit, 3.0V range, 1MHz BW)

  OPA350 => 7nv / sqrt(Hz) 

  OPA378 => 20nv / sqrt(Hz) = roughly 0.2 LSBs of peak-to-peak noise (12-bit, 3.0V range, 1MHz BW) or 3.5 LSBs of peak-to-peak noise  (16-bit, 3.0V range, 1MHz BW)

  OPA625 => 2.5nV / sqrt(Hz) = roughly 0.0 LSBs of peak-to-peak noise (12-bit, 3.0V range, 1MHz BW) or 0.4 LSBs of peak-to-peak noise  (16-bit, 3.0V range, 1MHz BW)

  (These are all more than capable for 12-bit operation and the OPA378 is maybe borderline for 16-bit operation, but would probably be OK).  

  • And you obviously want to consider other factors like package size, cost, current consumption, supply range, temperature rating, automotive qualification, etc.

  

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• Intellectual 350 points

  

  Dany Bourget

  

  • Mar 15, 2017 2:13 PM

  In reply to Devin Cottier:

  Hi Devin,
  
  Thank you very much for your detailed explanation. I will use the OPA4350 for my design.
  
  It will be great if you could explain how to simulate the phase margin with Tina.
  
  Best Regards

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