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Original question:

ADS1256: about Differential and Single-Ended input

ADS1256: AINCOM and AGND

Sahil Goyal
Prodigy 10 points
Part Number: ADS1256

I am using ADS1256. I need help with connections. I am using 2 channels of the ADC in a single-ended configuration. I am using the WAveshare High Precision AD-DA board. The two inputs come from Analog Output of ESCON 70/10. In my case, the output voltage ranges from 0V to 4V(in the datasheet it is mentioned -4V to 4V). There is 'analog ground' pin in ESCON 70/10, which I am connecting to AINCOM. 

I want to ask, should I connect the AINCOM pin to AGND after connecting AINCOM to 'analog ground' of ESCON. What I have understood is that I don't need to connect AINCOM to AGND of ADS1256. You can correct me ,if I am wrong.

What if there is a voltage difference in the 'analog ground' of ESCON and AGND of more than 0.1V? As mentioned in the datasheet, any AIN pin cannot go below 'AGND-0.1V' with buffer off. How should I deal with this situation?

Also, if I connect 'analog ground' of ESCON to AGND, will it increase noise? My setting is PGA=1, buffer=off, sampling rate of 500Hz.

I can provide more information as you want.

Waveshare AD/DA board -- https://www.waveshare.com/wiki/High-Precision_AD/DA_Board  .

Schematic of Waveshare board -- https://files.waveshare.com/upload/2/29/High-Precision-AD-DA-board.pdf

ESCON 70/10 datasheet -- https://www.maxongroup.com/medias/sys_master/8818448400414.pdf Relevant part of datasheet - Chapter 3, Page- 24

  • 0
    TI__Mastermind 46541 points

    Hi Sahil Goyal,

    Here is some feedback for you

    Sahil Goyal said:

    I want to ask, should I connect the AINCOM pin to AGND after connecting AINCOM to 'analog ground' of ESCON. What I have understood is that I don't need to connect AINCOM to AGND of ADS1256. You can correct me ,if I am wrong.

    What if there is a voltage difference in the 'analog ground' of ESCON and AGND of more than 0.1V? As mentioned in the datasheet, any AIN pin cannot go below 'AGND-0.1V' with buffer off. How should I deal with this situation?

    You need a common reference point between all of your different signals and boards, or at least some way to bound those signals if they are floating. Typically this is accomplished by connecting the grounds of each board together. Without knowing much about your specific boards and what you are trying to accomplish, I would connect the grounds of your DAQ board and signal board together. The ADC can measure from AGND-0.1V to AVDD + 0.1V with the buffer disabled, so if the signal ground and ADC ground are at different potentials, you might violate these conditions.

    Note that the ADS1256 has a binary two's complement coding scheme, such that half the code range is positive and half is negative. Also note that this is measured differentially (AINP - AINN), and does not mean you can apply true negative signals to the ADC, just that if AINN > AINP, then the ADC code will be negative. In your case, you might want to bias AINCOM to 2.5V, which is called a pseudo differential measurement. If your input signal on AINP swings from 0 to 4V and AINN (or AINCOM in this case) = 2.5V, then you will be able to use more of the ADC code range compared to setting AINCOM = 0V

    Sahil Goyal said:
    Also, if I connect 'analog ground' of ESCON to AGND, will it increase noise? My setting is PGA=1, buffer=off, sampling rate of 500Hz.

    Why would this increase the noise?

    Also, please note that the Waveshare board is made by a third-party i.e. not TI, so we don't really offer any support on the operation or behavior of this board. You can reach out to Waveshare if you have issues with this board

    -Bryan