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wrong read data from ADS1278

Vincent Chang
Prodigy 160 points
Other Parts Discussed in Thread: TMS320F28335, ADS1278, THS4521

Hi,

I use McBSP of TMS320F28335 to read from ADS1278.
ADS1278 setting: Mode: 01 (High resolution)
FORMAT: 011 (Frame Sync/TDM/Dynamic)
powerdown: channels 5-8 are powered down
for testing: channels 1-4's differenial input pins are connected to GND

TMS320F28335 setting:
SYSCLK: 150MHZ

Frame sync mode
CLKGDV: 4-1=3   (CLK=150MHZ/4/4)

CLK and SCK are connected together
FPER: 512-1=511

Details of the setting and code are as attached.
0508.Doc1.pdf 
However, the readback result is not understandable. A part of the code I get is the following:

00FF8D25 00000D8F 0000627D 00002445 00FF8D0C 00000DAD

while I expect to get  all 0s.

I would be very grateful if you can help.

 

Thanks!

 

Vincent

 

 

  • 0
    TI__Guru 57296 points

    Hello Vincent,

    Thanks for your post!

    These values represent differential input voltages that are very close to 0V. What you are likely seeing the differential offset introduced by the THS4521 and the inputs of the ADS1278 together.

    Assuming that VREF is 2.5V is your system, the output codes that you show above correlate to values between -8.77mV and +7.5mV. This seems to align with the offset voltage specs in the THS4521 datasheet for a 5V supply and VOCM driven to 2.5V:

     

    The typical offset voltage for the ADS1278 is 250uV per channel (max is 2mV).

    I hope that clarifies what you are seeing now.