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ADS7946: Issue reading data from the ADC, continually reading the max value

Part Number: ADS7946

I'm using this part in a design and trying to read data from it. I'm using it in 16 clock frames and the SDO line is high for the first 14 bits of the 16 clock conversion frame and then low for the last 2 clocks, which yields a maximum ADC value. However, the input signal is roughly mid-scale so we should be seeing something other than the max value. My clock is only 12.5MHz (I see a note that the part prefers 40MHz) but that's not an option in this application. Could that really be the issue? I attached some waveforms showing what i'm seeing on the scope. Also, the period between samples is variable from back-to-back (meeting tacq of course) to as low as once a second (though each sample should have the same timing profile).  I have to decide reasonably quickly whether I can make this part work or I have to find a replacement. We're looking at volumes in the 100K+ for this design with a reasonably short horizon. Any help you can provide is appreciated.Waveforms.pdf0456.Schematic.pdf

  • Hello

    This is interesting, the sampling rate should not be an issue

    The first thing I would like to do is run two CS samples consecutively. This device will acquire the input and output the data on the next frame.

    Would you please run two CS cycles directly after each other with the same DC input voltage and the same channel. I would like to if the second cycle outputs the correct measurement

    Also, please check the voltages on the board are as expected, as well as the input voltage.

    Regards

    Cynthia

  • Cynthia,

    I changed the code to read two back-to-back samples and that didn't seem to make any difference.  See the attached waveform and I also measured voltages on the rails see the attached schematic.

    Dave.Wave2.pdfSchem2.pdf

  • After your previous email that you thought everything looked reasonable and that the lower clock rate shouldn't be an issue I put the board under the scope and it appears there was poor solder on the reference pin to the chip, resoldering that pin seemed to do the trick (I'm at least getting reasonable functionality for the SDO line now).  Makes sense that with the reference floating or near ground that any signal on the input would peg the output value.

    Just started looking at the actual values, but the scope response looks a lot better.

    Thanks,

    Dave.

  • This is great to hear, glad you found the source.

    The reference voltage is used to make the output measurements, thus you are correct that if this voltage is not correct the output will be directly affected.

    Glad all is working well

    Cynthia

  • Now I'm dealing with another issue, it may be compensation on the input?   I'm sending a continuous stream of pulses to the ADC (each pulse is ~2.5v tall and lasts 3us and these are ~40us apart).   I then take 500 samples with the ADC (sampling during the high portion of each pulse).  I take 500 samples.  When I plot these samples they start with ADC counts ~4000 and increase in value rapidly, slowing down as things continue till they settle around 8000 counts for the last 100 samples or so.  It looks just like a capacitive charge curve. I imagine it must be something to do with the front end compensation?  But, it seems odd (to me) that it would behave this way given the long decay period between pulses?  Any thoughts as to what might be going on?

  • Attached is a plot of the ADC counts in my FIFO from subsequent ADC reads (you can see the analog pulses next to it with the chip selects for each).  Also attached is a close-up plot of the first read and of the latter reads, we can see that the first read is stable roughly coincident with the chip select going low for the first pulse and the pulses are stable well before the chip select on the latter pulses.  Looks like we're not meeting the tacq requirement on the early pulses?

    Wave3.pdfWave4.pdf

  • Found the issue and it had to do with my control circuitry and not with the ADC.  The ADC appears to be functioning at the moment.

    Thanks.