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OPA4350: TMS320F28379D VREF-HI

Intellectual 350 points

Replies: 2

Views: 341

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?

2 Replies

  • 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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  • 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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