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ADS1258-EP: MUXOUT Isolation

Part Number: ADS1258-EP
Other Parts Discussed in Thread: ADS1258

Hi,

I was looking over the ADS158 multiplexer diagram and wanted to understand the possibility of isolating the MUXOUT from any of the inputs or references. From reading the datasheet, I think the first way to approach this would be to operate the device in Auto-Scan mode but not put any of the inputs into the input list register. I wanted to see if I could get an idea if this would actually work, or if there was a better way to do this that I was missing.

  • Hi Anant Kanungo,

    I am not too sure what you are trying to do here.

    the first way to approach this would be to operate the device in Auto-Scan mode but not put any of the inputs into the input list register.

    For example, what would this accomplish? You are not reading any inputs in this case, so why do you want to isolate MUXOUT? And what do you want to isolate the MUXOUT from? Do you have components connected to MUXOUT?

    Maybe providing a quick drawing of what you want to do will help

    -Bryan

  • Hey Bryan,

    Thanks for the quick reply! I probably should have contextualized what I was doing in my post. Basically, I have a signal conditioning circuit which involves an op amp between MUXOUT and ADCIN. Because of precision requirements, I would like to periodically measure the offset voltage caused by the signal conditioning circuit and calibrate my measurements based on that. Below is a picture of the very rough circuit I have in mind.

    Basically, I want to every once in a while tie the input pins of the signal conditioning circuit together (or to ground) so I can measure just the offset voltage on the ADC pins. I can do this by using an analog switch and the GPIOs of the ADS to control the analog switch. I don't want to tie pins together on sensors though so i would like to isolate the mux output from the mux input. I know I could do this by adding two more analog switches controlled by GPIOs, but am hoping the ADS1258 has this capability to reduce part count and complexity of the circuit. I additionally am locked into using the ADS1258 for external reasons.

  • Hi Anant Kanungo,

    Do you have any unused channels on the ADS1258?

    If so you can tie this pin to ground (or maybe mid-supply, depending on the VCM range of your amplifier), and then just select this channel for both the AINP and AINN pins using fixed channel mode. This seems to be effectively the same as what you are trying to accomplish

    Let me know if this is not a solution for you

    -Bryan

  • Unfortunately I don't have any unused channels, I planned on using all as differential pairs to read 8 per ADC. If I couldn't figure out a way to do the isolation on the ADS my plan was to either add analog switches in the signal conditioning block (and use ADS GPIOs to control them) or to sacrifice a channel and figure out what to do with my unmapped sensors.

  • Hi Anant Kanungo,

    Can you not just use the OFFSET reading in the ADS1258 then?

    This will short the inputs before MUXOUT to mid-supply, which is exactly what you wanted to do, correct?

    Another option would be to use the chopping feature on the ADS1258, which can remove offset on-the-fly.

    -Bryan

  • Sorry for the late reply.... You are correct that would resolve my issue, I think I just missed it. Thanks again!

  • Hi Anant Kanungo,

    Great, glad we could help. If you need additional support with your design, please start a new thread and we will support you there.

    -Bryan