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ADS8688: Need help designing a 48 channel ADC with with multiple ADS8688 and an MSP432P401R Launchpad

Spencer Zanardi
Prodigy 110 points
Part Number: ADS8688

Hi,

I am interested in designing a daisy chain of ADS8688's to measure the voltages 48 batteries, which are wired in 8 parallel columns of 6 series batteries each, to power some motors. The batteries are roughly 1.5 V, so each column will be 9 V. The batteries voltages will drop as the experiment runs (roughly 4 minutes). I like the ADS8688 because it can measure up to +/- 10.24 V and it can measure differential inputs. My research team and I have also experienced battery polarity reversal on some batteries, so being able to measure negative inputs is extremely valuable, which the ADS8688 can also do. Also, the sampling rate is unimportant as we are just looking for anything at this point. I just have some basic questions that I was hoping I could get answered before I go ahead and begin building this device:

1. I mentioned that my measurements may experience negative voltages. Will the MSP432 be able to measure a possible negative voltage of say -1.5 V? I am not interested in inverting that result, so if the MSP432 would not work, do you have something else in mind that could help me? My internet research is not proving to be helpful.

2. Is it possible to connect 12 ADS8688's to an MSP432 and write a code to extract the data we need? My knowledge of the MSP432 is limited, but good enough that I think I could figure out the code; I just want to know that it will work before I try. If there aren't enough pins, would you suggest using a multiplexer, and, if so, do you have one in mind?

3. I know that I can use to MSP423 to provide 5 V to the ADS8688's. Do you think I would run into problems with drawing to much current from the MSP432, or is that something I do not need to worry about?

4. I am worried about the accuracy of my results. In the measurements, we never look past the thousandths decimal place. Do you think with 16-bit resolution at 5 V, we should be able to safely measure at least 3 decimal points?

Thank you in advance for any help you can give and me please let me know if you have any questions for me or need something explained in more detail.

-Spencer

  • 0
    TI__Mastermind 35770 points

    Hello Spencer,

    Thanks for your query on E2E.

    Firstly ADS8688 can only support single-ended bipolar or unipolar input signal, and it can not support differential input signal and can not handle the common-mode voltage, the voltage on the AIN_xGND pins should be restricted to -0.3V~0.3V comparing to the ground. You can use each input channel of ADS8688 to measure the ABSOLUATE voltage for battery to the ground, however, you can not use one input channel of this ADC to directly measure the voltage difference on each battery. There are two solutions to solve this limitation to your application:

    1.  Use two channels to measure one differential signal for each battery, for example, AIN_0P and AIN_1P (AIN_0GND and AIN_1GND should be connected to the ground), then your software can calculate the voltage difference on the battery.

    2. Use an instrumentation amplifier to cooperate with ADS8688 to measure, please check the circuit:   High common-mode differential input voltage to ±10-V ADC input circuit

    Regarding your questions:

    1. I'm not an expert for MSP432, you may go to the Processor Forum to check. However, based on my experience, the MSP43x controller can not support bipolar input directly, you will have to use complex external circuit including amplifier for signal conditioning. Our direct bipolar input ADC can support it directly.

    2. Four ADS8688s are recommended to use Daisy-chain mode with a same SPI interface, otherwise, the loading increases on the shared output which may lead to digital timing errors. If you are going to use more SPI interfaces, you may have to check the resource on MSP432 or go to the Processor Forum to ask.

    3. I'm not sure what you mean "use MSP432 to provide 5V to the ADS8688", are you taking about the 5V power supply on ADS8688?

    4. I think the best way is to check with your batteries and an ADS8688EVM before you design your own circuit board.

    Please let me know if you have any further question about these information.

    Thanks&regards

    Dale

  • 0 in reply to Dale Li
    TI__Mastermind 35770 points

    Hello Spencer,

    I did not get your response, I'm going to close this thread, please let me know if you need any further support. 

    Thanks&Regards

    Dale