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Problem with ADS5500 output

Shrinivas Gotur
Prodigy 30 points
Other Parts Discussed in Thread: ADS5500

I am using ADS5500 14-bit parallel configuration adc in my board interfaced to virtex-4 FPGA,The pin no 40 (DFS) of ADS5500 decides the output format whether straight binary or 2's complement .so in design it is set to 2's complement format now as per the datasheet of ADS5500 the output for 2's complemnt format should be in the range of 2000 to 1FFF and for straight binary it is 0000 to 3FFF.so in my case it should be around 1FFF for positive drive and 2000 for negative drive. but output of adc is coming around 3FFF.I've checked the DFS pin it is giving 1.24V which is set for 2's complement format .Please assist me Thanking you in advance

  • 0
    TI__Genius 17320 points

    Hi Shrinivas,

    Are you applying any signal to the ADS5500?

    Without applying a signal to the ADS5500, I would expect an output equal to mid-scale (2000 in offset binary or 0000 in 2's compliment). Of course there is going to be some input offset voltage at the ADC input, so you would expect the "no-input" voltage to drift +/- a few LSB. If the offset voltage was 1 LSB negative then the output code in 2's compliment would be 3FFF. So I think this is as expected.

    If you apply a +1.15 V differential voltage to the input pins, you should get a value of 1FFF at the output. If you apply a -1.15 V differential voltage you should get 2000 at the output. Based on the typical offset voltage spec (+/- 1.5 mV), without any input applied you could have a midscale output code between 3FF4 to 000B in 2's compliment.

    Regards,
    Matt Guibord 

  • 0 in reply to Matt Guibord
    Prodigy 30 points

    thank you for reply Matt,

    yes i am getting 3fff when i  am not applying the input .my board is having 4 db of gain so when i apply 700mv (peak to peak )sine wave  i am getting 0140(hex) value for positive peak at the output of adc is it correct ?? can you tell me how to convert adc values to voltage ??

  • 0 in reply to Shrinivas Gotur
    TI__Genius 17320 points

    Hi Shrinivas,

    The voltage at the input will be equal to (x / 2^n) * Vfs - Vfs / 2

    Where x is the output code in offset binary, n is the number of bits, and Vfs is the full scale voltage of the ADC. If the output is coded as 2's compliment, then simply invert the MSB to convert to offset binary for use in the equation above.

    Regards,
    Matt Guibord