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ADS117L11: REF and Vcm pin usage

Will Flanagan
Prodigy 10 points
Part Number: ADS117L11
Other Parts Discussed in Thread: INA240

Hello,

I have a single-ended signal (0-5V, 2.5V referenced. Coming from an INA240 current sensor). Reading the ADS117L11 datasheet initially, I thought the output of the ADC would correspond to the differential inputs level-shifted by the differential REF pins:

Vout = (AINP - AINN) - (REFP - REFN)

Reading it again, however, this does not seem to be the case. I now think that the REF pins are just used to set the range of inputs:

Vout = (AINP - AINN); -(REFP - REFN) < Vout < +(REFP - REFN)

I understand this would be for the 1x input range setting of this chip. Taking this second track, to read my signal correctly my connections would be:

AINP: OUT of INA240

AINN: 2.5V reference level

REFP: 2.5V reference level

REFN: GND

This would give me an output signal between -2.5V and +2.5V? Is this the correct interpretation of the datasheet?

If these connections would work, would there be any issues with using the Vcm pin on the ADC as the 2.5V reference level? I understand this is mid-supply voltage and not exactly 2.5V but it would simplify the PCB a lot.

Thanks,

Will

  • +1
    TI__Mastermind 46130 points

    Hello Will,

    Welcome to the TI E2E community!

    Yes, your understanding is correct.  The full scale input range is +/-Vref, where Vref=(REFP-REFN).  In this configuration, with 2.5V connected to AINN, you will get negative full scale code with 0V (or equivalent of -2.5V differential) and positive full scale code with +5V (or equivalent of +2.5V differential).

    The Vcm pin is not the best option to connect to the AINN input pin.  Since you need 2.5V on this pin, I suggest connecting the reference voltage to this input and enabling the internal pre-charge amplifiers for both the AINP and AINN inputs.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 10 points

    Thank you Keith!

    Just out of curiosity, why isn't Vcm a good option? Is it because any shifts in supply voltage would affect Vcm or is there another reason?

  • 0 in reply to Will Flanagan
    TI__Mastermind 46130 points

    Hello Will,

    The Vcm output is a low power, low bandwidth amplifier.  It is intended to set the common mode input level when using an external fully differential amplifier.  The ADC inputs have a dynamic current requirement that would cause settling time issues when using the VCM output.  These effects would be reduced by using the internal pre-charge buffers, but would still be present.

    Regards,
    Keith