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TMS320F28377D VREFLOx

Tjarco Boerkoel
Expert 1725 points

Other Parts Discussed in Thread: LMP7709, REF3225, REF3230, OPA2227, OPA4376, LMP7704

Hi All,

Could someone give me more info about the VREFLO pins on the TMS320F28337? What are the voltage limits when using a ratiomatric reference? Could I just supply a 'differential reference' with a common mode offset (within maximum specs)? Does it have a S/H circuit inside (being a pulsed load)? Are there recommendations for an external reference? I see the LMP7709 being used with a 2.2UF capacitor, but isn't this capacitive load outside the recommended specs of the operational amplifier?

Best regards,


Tjarco Boerkoel

  • 0
    Expert 1725 points

    Is no one able to tell more about the VREF design in detail? It's not desired designing a reference without understanding the internal circuit.

  • 0 in reply to Tjarco Boerkoel
    TI__Genius 17121 points

    Tjarco,

      Unfortunately our expert is out of the office until next week, I'll try to provide some details later today.

    Regards,

    Joe

  • 0 in reply to Joe Prushing
    TI__Genius 17121 points

    Tjarco,

      It is common practice to simply tie REFLO to VSSA, it does not have to be driven from a differential reference, though as you suggest, as long as the REFHI and REFLO stay within the respective voltage ranges that should work too.  Of course the voltage being sampled must also be between the reference voltages else the digital code will saturate.

      I realize we currently do not spec the VREFLO voltage range and that is because more characterization is required before we publish those details.  To give you an idea though, the maximum voltage for REFLO will probably be spec'd somewhere around 0.5V.

      There is no S/H on the reference circuits.

      We do provide some recommended references in the TRM, such as the REF3230 or REF3225 (see figure 9-15) but do not yet have a specific recommendation for the buffer.  As you point out, the control card uses LMP7709 and we have some internal development boards using OPA2227.

      I reviewed the LMP7709 datasheet and did not see an explicit recommendation for load capacitance.  Can you please clarify where you saw the limitation?  In the meantime I will follow up with the folks that designed the control card for that part of your question as well.

    Regards,

    Joe

  • 0 in reply to Joe Prushing
    Expert 1725 points

    Hi Joe,

    Thank you for the information. Indeed, this was not specified for this amplifier. I think I had another datasheet open and switched them.

    Best regards

  • 0 in reply to Tjarco Boerkoel
    TI__Genius 17121 points

    OK thanks for the clarification Tjarco.

    Regards,

    Joe

  • 0 in reply to Joe Prushing
    TI__Guru 57255 points

    Hi Tjarco,

    In future revisions of this kit we will be switching to the OPA4376AIPW as the unity-gain buffer for each VREFHI input (the LMP7704 was another good option, but we decided on the OPA4376 partially due to cost)

    The LMP7709 is designed to be unity-gain unstable which was not ideal for this application.  It may be worth noting that we didn't seen issues when we used this opamp on the cCARD.


    Thank you,
    Brett