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ADS1278 data converter output not correct

AAAA PPPP
Expert 1410 points
Other Parts Discussed in Thread: ADS1278

Hello,

I am using ADS1278 TI board that comes with EVM module. I have set switches for high resolution mode , SPI mode, discrete output format. Clkdiv is permanently shorted in the board(bug in board). I am using  27MHz external clock. Also I use the EVM module only for power supplies. The control signals and external clocks are driven by FPGA.

I am using FPGA to acquire the data values from ADC and calculate the input voltage I gave from it.I am giving DC voltage as input as of time now.

When I use all the 8 channels simulataneously , I am getting almost correct values only for small input voltage levels like 0 to 1.2 volts. Until this range the calculated input voltage has variation of upto 0.15 V. Above 1.2 V of input ie for 2.2V input and all the input voltage calculated is in range 1.7-1.9V. Such a huge error? But when I checked channels individually the input voltages calculated are  correct (with very small variation) ie give input to one channel and other channels are ground. Please make suggestions . Thanks in advance.

  • 0
    TI__Guru 66346 points
    Hello AAAA PPPP,

    It seems like you are reading data correctly and the device is working fine when you use only one channel at a time. What is the source of your DC input voltage? Perhaps it is unable to drive 8x channels? Can you confirm the input voltage across each channel with a multimeter to see if you get the expected 2.2V?

    Another thought - you are providing 2.2V differentially (AINP - AINN), but what is the absolute voltage on each pin?

    Regards,
    Ryan
  • 0 in reply to Ryan Andrews
    Expert 1410 points

    Sir,

     I am giving DC voltage from a function generator. The absolute voltages that comes from input pins are 2.2 and 0 itself. Another thing noticed is that if I give 2.34V to channels 5,6,7,8 and 0V to 1,2,3,4 the output is 2.32 with 5,6,7,8 and 0 for other channels. but if I give 2.34 for channels 1,2,3,4  and 0V to 5,6,7,8 then output is 2.2 with 1,2,3,4 and 0 for other channels. When I give same voltage to all channels none of the channel set respond correct. Also I checked the voltages going to input headers are correct, but couldn't do on IC pins with multimeter as it is very difficult to hold multimeter on IC pins.

  • 0 in reply to AAAA PPPP
    TI__Guru 66346 points
    Thanks for updated info. Is the function generator ground tied to ground on the ADS1278EVM?

    If you can, also try supplying a 2-V differential input with AINP tied to 2.5V and AINN tied to 0.5V.

    Regards,
    Ryan
  • 0 in reply to Ryan Andrews
    Expert 1410 points
    Sir,

    The grounds of the ADC board ie EVM is tied to ground of FPGA board and function generator ground.