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ADS9226: ADS9226 sometimes outputs zero data on startup

Murray Smith
Prodigy 10 points
Part Number: ADS9226
Other Parts Discussed in Thread: THP210

We have been using ADS9226 for some time now, and we occassionally see cases where the part will output zero data (i.e. all data lines = '0') after a power cycle.  Sometimes this affects both channels, other times only one channel.  The data sheet does not recommend any particular power rail sequence, but we find that it prefers to have the analog 5V rail up first followed by the digital 3V3 rail.  The only way to recover correct operation is to completely power cycle the part.   This is our schematic:  

image.png

and our power sequencing looks like this:    

image.png

Blue = AVDD (5V) Green = DVDD (3V3) Yellow = REFIN (2V5) all measured at the device pins.  

Is this a known issue?   Do you have any recommendations for how we can make the startup more reliable?   

  • 0
    TI__Expert 4780 points

    Hello Murray, 

    Apologies for the delay, I was out of office. 

    Could you please share the schematic including the ADC drivers used? I see the capacitors of the input RC are not populated, is there a reason for this? 

    Could you also measure what the input common mode is? (AINP-AINM)/2, or at least the voltage at AINM? 

    What external reference is being used? does is share the supply with the ADC?  is it able to drive the 1kΩ and 10uF? it also seems to be taking some time to reach the 2.5V 

    Best regards, 

    Yolanda 

  • 0 in reply to Yolanda Gonzalez
    Prodigy 10 points

    Hi Yolanda

    Thanks for the reply.

    The ADC inputs are driven by THP210 fully-differential opamps which are powered from the same 5V rail used to power the ADC AVDD pin:

    We had issues with code jumps around zero when the ADC input RC capacitors were fitted.  Removing them resolved this.  

    Common mode input voltage is 2.5V.  This is defined by the COM pin of the THP210 driver amp which is connected to the same external 2.5V reference used for the ADC.

    External reference is LTC6655LN.  It is supplied from a separate regulator.  The slow rise time is due to the RC filter time constant, and also the iternal filter time constant of the reference.  Here is the power sequence using a slower timebase:

    Blue = AVDD (5V) Green = DVDD (3V3) Yellow = REFIN (2V5) all measured at the device pins.