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ADS1299 Startup

I have a question about the startup timing for the ADS1299.  We have a custom board design with multiple ADS1299 converters.  Most of the time it is working fine, but occasionally a few channels don't power up correctly.  Its roughly one time in about 50 powerup cycles, and the results are a little random.  I've seen alternate channels on one ADC affected, all the channels on one ADC affected, and other combinations.  Sometimes the output is fixed at zero, but mostly they are saturated high or low.  The input is always driven, sometimes with a few millivolt AC signal, mostly with zero (a few millivolt DC offset).  It looks to me like the PGA configuration is not correct, but I haven't investigated it too deeply and I could be wrong on that.  Powering the board down and restarting it has so far always cleared the problem.


The PowerUp Sequencing section of the data sheet (page 57) says there should be 2**16 clock cycles from power supplies up to Reset.  At the 2.048MHz clock we are using that should be about 30msec.  We are using the SPI reset command issued by a CPU on the same power supply as DVDD for the ADC, but need to wait at least 100mS before issuing the reset command or it doesn't startup correctly most of the time.  The frequency of startup errors does seem to be tied to that delay time, but its not a linear relationship.


The flow chart on the following page says to wait 1 sec before resetting.  That seems like a pretty long time.  We can tolerate the delay if necessary, but we tried it and even at that delay I had one or two times that it didn't power up correctly.

What is the required delay from powerup to reset and do you have any other ideas on how we can prevent this from happening

Tom