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ADS8472EVM parallel data output

Other Parts Discussed in Thread: ADS8472, ADS8471, ADS8482

I am using the ADS8472EVM and am running into several issues. First, it looks like the ADC is outputting its parallel data in binary twos complement coding according to Table 1 (pg. 24) of ADS8472 datasheet. If this is true, the HEX CODE(s) to the right of the BINARY CODE seems incorrect. Please confirm. Second, my setup consists of 1MHz conversion starts in Auto Read mode and am polling its data at 2MHz. When grounding the inputs, it seems I get a few readings where the most significant nibble is "random". Obviously, noise can be a factor but I would expect values close to bipolar zero.

  • Hi Roland,

    Can you describe what you mean when you say that you believe the output of the ADS8472 is 'incorrect'?  Do you have a spread sheet or raw data file that compares a specific analog input to a digital output code that we can take a look at?  If you can let us know the code values you receive with the shorted input test, that would be great!  That will go a long way in helping us understand what the problem that you are seeing might be. 

  • Hi Tom,

    When I said the output of the ADS8472 seems incorrect, I meant that the digital binary output code doesn't correlate to its hex code.  For example looking at Table 1 on pg. 24 of the datasheet, it states:

    Zero scale BINARY CODE = 1000 0000 0000 0000 and its HEX CODE = 2000

    Shouldn't the HEX CODE be 8000?  I connected a negative voltage on the analog input channel on the evaluation board and recorded its results using a high speed usb digital interface card.  I've attached the raw data file as well as an image showing its histogram results.  You can disregard the columns to the right of the data ("expected") in the text file as it is irrelevant.   

    6327.ThroughDatNegative1.38V.txt

    I just need confirmation of what my data is showing as my results for a negative analog input isn't correlating to the HEX CODE value.

    As for the shorted input test, my issue might be premature as I don't see this issue with a battery test.  It's possible the AGND I'm connecting to the shorted input is noisy due to using my test bench DC power supply and the ADC is picking it up.  I've designed a prototype using the ADS8471 that has all power generated on board with proper filtering techniques and will test it with the same parameters.  Note that the Table 1 of this datasheet is what I expected. 

  • Thanks Roland,

    Now I believe I see the problem.  The ADS8472 is a spin of the ADS8482 (an 18-bit device) and it would appear as though there is a typo in Table 1 on page 24.  The trailing zeros were removed but the 'HEX CODE' column is not correct.  It should read 7FFF, 0000, FFFF and 8000 so we'll get that fix initiated ASAP.