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ADS5277: data read in Not correct and stable

Hong Sun
Intellectual 300 points

Part Number: ADS5277

Hello there,

we have designed a waveform capture circuit using ADS5277 and an OP-Amp + single end to differential convertor with FPGA connecting the output of ADS5277.  However the reading is not stable (incorrect neither) even though the readings using the ADS5277's test mode are correct.  To simplify the problem, we feed DC into the system (our DC is about -0.875V when entering the single to differential convertor), but the reading is around 0x275 and the variation can go up to 0x390.   after further reviewing our schematic, we did find several issues but problem exists after we fixing the issue.

And one more sympton:

Vcm reading =0.25V instead of 1.45V as specified in the datasheet.

Please see the enclosed file for our schematic and changes we made.

thanks for your help!

HongADS5277 issue.docx

  • 0
    TI__Mastermind 44680 points
    Hi Hong
    The incorrect and unstable converter output values are most likely due to the incorrect common mode voltage of the applied signal. That must be corrected to ensure proper operation and performance.
    To ensure the ADC is still able to correctly generate the internal Vcm voltage, temporarily disconnect the Vcm output pin from the Vocm pins of the amplifiers. Check that approximately 1.45V is output. If this is correct, then you will need to investigate why the Vocm pins of the amplifiers are loading the Vcm voltage and preventing proper operation when Vcm is connected to the Vocm pins.
    Keep in mind that the Vcm output is only capable of supplying a total of 2mA to all loads. You may need to buffer the Vcm signal in some way.
    You will also need to ensure that the amplifiers you are using will work OK with supplies at +5V and -5V and with the Vocm and output common mode voltages set to 1.45V.
    Best regards,
    Jim B