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CCS / TMS320F28054： issue about ADCINB4 (op-amp)

Susan Yang
Genius 9955 points
Part Number: TMS320F28054

Tool/software: Code Composer Studio

Dear team

My customer is using TMS320F28054. ADCINB4 (op-amp) was used as ADC input channel.

It worked fine When used in normal temperature and high temperature. 

But the feedback temperature was still maintained at about 0 degrees when the temperature is below 0 degrees.

What is the reason for this? The program had been verified, no problem, and the sampling circuit had been verified, no problem, it seems to be only related to AD conversion.

Please help.

  • 0
    TI__Genius 10845 points

    Hi,
    This query has been assigned to the experts. Due to the US Holiday on 25/05, please expect reply by 26/05 US time.

    Thanks
    Vasudha

  • 0
    TI__Guru 62975 points

    Susan,

    Can you give us an idea of what the actual conversion value is compared to the expected value?

    Are any other PGA channels have trouble under 0 degrees?

    -Tommy

  • 0 in reply to tlee
    TI__Genius 9955 points

    1 At an actual negative temperature, the measured analog voltage input to the DSP is correct, and it will change with the temperature change, but the feedback digital value has been maintained at about 3609.

    The temperature feedback also shows that it is always around 0 ℃, as shown in the figure. As shown.

    2 There is no problem with other PGA channels.

    Regarding the reason for this, is the input voltage change too small at low temperatures?

    Or is our corresponding number too large?

  • 0 in reply to Susan Yang
    TI__Guru 62975 points

    Susan,

    The linear range of the PGA output is 0.6V to VDDA-0.6V:

    Assuming VDDA=3.3V, this translates to an ADC range of 745 to 3351.  The accuracy of conversions outside this range is not defined.

    I recommend adjusting the input circuitry to bring down the voltage so that the conversions are centered at the ADC midpoint of 2048.

    Also, if this effort is for preliminary evaluation, F28004x can also be considered.

    -Tommy