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ADS1256: Buffer: off operation

Prabhav Manchanda
Prodigy 10 points
Part Number: ADS1256

Tool/software:

I am using ads1256 with the buffer off. When the PGA gain is set to 64, and the input pins are left open, the output was appearing to be clipped to the maximum allowed level (0.0781 V).  It. Appears that there is a voltage of around 1.5 V on the pins with the buffer is disabled. I suspected that it could be coming from some internal biasing circuitry ( I have a board with some series resistance at the input). The issue seems to go away when using the ADC in differential mode as both the pins are now at the same potential. 

1)Has someone else observed this behaviour as well? 

2) Can the ADC work in a true differential mode or even in the differential mode setting the MUX is switching between the inputs? ( making it sort of pseudo differential). I observed some switching noise at higher data rates, which seem to go way at 2 SPS. 

3) does an increase in Vref also scale my max input voltage range ? 

Regards

  • 0
    TI__Mastermind 46491 points

    Hi Prabhav Manchanda,

    Answers to your questions:

    Prabhav Manchanda said:
    Has someone else observed this behaviour as well?

    When you leave the input pins floating, they will generally float to one of the supplies. So the values you are seeing make sense, and does not mean there is anything wrong with the ADC

    Prabhav Manchanda said:
    Can the ADC work in a true differential mode or even in the differential mode setting the MUX is switching between the inputs? ( making it sort of pseudo differential). I observed some switching noise at higher data rates, which seem to go way at 2 SPS. 

    I am not sure what this question is asking. All delta-sigma ADCs are true differential input, such that the common-mode range does not have to be a fixed value. The ADS1256 can accept a true differential input, a pseudo differential input, or a single-ended input (with the buffer disabled).

    Prabhav Manchanda said:
    does an increase in Vref also scale my max input voltage range

    An increase in VREF changes the maximum differential signal, but not the absolute input voltage range. The absolute range is mostly limited by the ADC analog supply, though in the case of the ADS1256 there are further limitations when the buffer is enabled

    -Bryan