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ADS127L18: What is the analog input range in Hardware Programming

Ton Kuijper
Prodigy 90 points
Part Number: ADS127L18


Tool/software:

Hello,

I went through the datasheet over and over and am still confused about the analog input range.

I want to use hardware programming.

I understand in Hardware mode the ADC is forced into High Reference Range.

Somewhere else I read that in High Reference range, the ADC is forced to +/-VREF (so +/-2VREF is not possible).

So in Hardware Programming mode, the ADC is ALWAYS in High Reference Range and +/-VREF range.

I found in fig-2-1 of the EVM Manual the lower right curve, single ended 5V pp at 2.5V offset. But not if that is for +/-REF or +/-2VREF.

I want a span of -2.048 V to +2.048 V (4.096 V pp) around +2.5V.

Do I need a VREF+ of 2.048V or 4.096 V??

(VREF- is connected to AVSS)

  • 0
    TI__Mastermind 47630 points

    Hello Ton,

    The reference range only applies to the values of the reference used.  In high reference range, the reference voltage can be in the range of 1V up to the full analog supply range.  The value of the reference determines the full scale differential input range of the ADC, or the difference in voltage between the AINP and AINN inputs.  The specified range is +/-Vref, which means the peak-to-peak input range will be 2*Vref.

    If you were using SPI mode and set the input range to +/-2Vref, your peak-to-peak input range would be 4*Vref.

    In your case, you want a span of -2.048V to +2.048V, or 4.096Vpp, so Vref should be equal to 2.048V in this case.

    If you are using a fully differential amplifier (FDA), similar to the EVM, then your single-ended input of +/-2.048V around +2.5V will be directly converted to a fully differential output by the FDA.  The positive input of the FDA is your input signal, and the negative input should be set to +2.5V.

    Regards,
    Keith Nicholas
    Precision ADC Applications