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ADS1235: Can multiple ADS1235 be in AC bridge mode and share the same excitation?

Part Number: ADS1235

I have several bridges in my system. I would like to share the excitation among several of the bridges - to simplify system wiring. I also would like to use ac excitation (ref figure 82 in the data sheet). So, I will have several ADS1235. Can I synchronize the ADC's so one ADC does the excitation control, but all work properly in AC mode?

  • Hi Mark Fredette,

    You should be able to use the START pin or START command to synchronize the start of conversion for multiple ADCs. If you are going to run in pulse convert mode where you take one sample, then wait for some time, pull start high / send the START command, take another sample, etc., I think this will work okay. If you want to run in continuous conversion mode, it is possible that the ADCs could get out of sync over time and the bridge chopping functionality might not work for all ADCs. I would therefore issue the START command or toggle the START pin occasionally throughout your process to ensure all of the ADCs are resynchronized

    You would also need to make sure all of the ADCs are in the bridge chop mode because this mode automatically swaps the reference inputs internally to the ADC. You could also perform this switching manually using another set of switches per VREF input pin

    Also, please note that we have not tried this setup to see if it actually works. It theoretically should, but if you find any additional "gotchas" during your testing, please feel free to post them here.

    -Bryan

  • thanks, Bryan,

    I guess pulse convert mode is the best way to accomplish what I want. Is the pulse convert mode compatible with AC bridge operation - I mean will the 1235 know to properly change sample signs and sum the correct sample pairs?

    AC mode and continuous sampling must allow for signal settling between polarity switches. If in pulse convert mode, how would this timing be managed?

    I plan to have a uP on board and maybe 4 to 6 full bridge strain gauges, each with it's own ADS1235. I want 20 Sa/s for each output.

    Also, can you elaborate on how to do the option of manually switching with the VREF pin?

    mark

  • Hi Mark Fredette,

    Yes, the pulse convert mode will work with AC excitation mode. The delay is applied before each conversion in the AC excitation cycle. So, the total time for 1x output in pulse convert mode = [delay + conversion latency] + [delay + conversion latency]. I confirmed this on the EVM

    Also, can you elaborate on how to do the option of manually switching with the VREF pin?

    You would need another set of switches per VREF input pair in order to ensure that whenever the bridge voltage is swapped the differential VREF voltage is always positive, assuming your bridges are connected as shown below. When the 5V is applied to the top of the bridge and GND is connected to the bottom, the VREF voltage is positive. In the second phase, the VREF voltage is negative. Internally, the ADC swaps these pins to ensure the voltage the actual modulator sees is positive. So you must accommodate this switching externally if you are not using the AC excitation modes on the other ADCs

    -Bryan

  • got it, thanks Bryan 

  • You are welcome, Mark!