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ADS54J66: Stuck bits in ADC data

Eric Marzec
Prodigy 20 points
Part Number: ADS54J66


Hi,

I've been seeing data from a single ADC where the bottom 5-bits are always 1s. Attached is a screenshot of data readout from two different ADCs, one ADC has this stuck-bits issue and the other is fine. You can see the sine-wave in one ADC looks very blocky due to the poor resolution, and the other 3 channels appear to have no noise because the noise doesn't show up in the 6th LSB.

The two ADCs share the same power supplies and programming. Additionally, I've read back the registers from both of the ADCs and the read-backs are identical. I've tried power-cycling and re-programming the ADC repeatedly but the issue is quite persistent. We have other ADS54J66s mounted on similar custom PCBs and programmed/readout by similar FPGA firmware and do not see this issue. So whatever is causing these stuck bits is particular to this individual ADC.

I'm hoping someone might be able to point me towards some potential causes that I might look into to resolve things, I've seen for other ADCs stuck bits can be indicative of a voltage rail being improperly decoupled. Perhaps someone can tell me if that's a likely possibility for this ADC too.

Thanks,

Eric M

  • 0
    TI__Guru* 91947 points

    Eric,

    Each ADC has 4 outputs. Are you saying the output of one channel of one ADC is bad or all four channels are bad? Are you issuing a hard reset to the ADC after power and clocks are provided? Can you power sequence the supplies per section 9.1 the attached data sheet?

    Regards,

    Jim1488.Updated data sheet.pdf

  • 0 in reply to jim s
    Prodigy 20 points

    It's all 4 channels on the ADC that are bad.

    Usually I issue a hard reset just before I configure the ADC & clock. I tried changing that so the reset is issued after programming the clock but before programming the ADC, that did not solve the issue.

    Our PCB already sequences the power supplies as suggested by that data sheet. The 3V, 1.9VA, and 1.15VD all power up simultaneously as soon as the board is powered, then the 1.15VD enables the 1.9VD. 

  • 0 in reply to Eric Marzec
    TI__Guru* 91947 points

    Can you place the bad part in test pattern mode and see if it will generate a ramp by going to ADC Page 0Fh, address 0x74 and set the data to 0x40? If this fails I am guessing you probably have a bad device or it may be soldered down improperly.