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ADS1258 input noise

Charlie55711
Prodigy 90 points
Other Parts Discussed in Thread: ADS1258

When we have chop mode enabled we see a slight bump at the input pin of the ADS1258 - about .2 V pulse 1.5 ms wide (one channel conversion time). We were told by colleagues that TI said this is due to the chop mode. But we are only seeing it on channels 1 thru 3. We are also getting undesirable results for these three channels - converted values that are only within .030 % of applied value. On all other channels we get great results - within .004 % of applied value. Is this bump at the channel input of the converter the cause of the reduced accuracy that we are seeing?

I have attached a schematic of the input processing circuit that feeds each converter input pin. Not shown on the drawing at the input of the ADS1258 are two reverse biased Schottky diodes for ESD protection - one to 5V and the other to Analog Common.

ADS1258 voltage levels arre as follows: AVDD = 5VD        DVDD = 3.3V        AGND = DGND = Analog Common        VREF = 5V to Analog Common

 

  • TI__Mastermind 43915 points

    Hi Charlie, 

    Do you mind taking a scope picture of the ''bump'' you are seeing so I can understand what it looks like? Thank you. 

    Are all the channels include the same drive circuitry that you attached? Try applying the same signal to each of the channels to see if the outputs results vary. If they do, then it may be a layout issue. 

    Also, you can try adding a small capacitor (1-2nF) at the input of the ADC. It usually helps with charging the sampling capacitor in the modulator as each sample is taken. 

    Regards,

    Tony Calabria

  • in reply to Tony Calabria
    Prodigy 90 points

    Trace 1 is the channel 3 input pin, Trace 2 is the DRDY.

     

  • in reply to Charlie55711
    TI__Mastermind 43915 points

    Hi Charlie, 

    Looks like it has to do with the charging of the internal sampling cap. Add a 1-2nF capacitor at the ADC input. Add a cap, it will help with filtering as well as help charge the modulator's internal sampling cap. Also, the bandwidth of your amplifier looks like it is on the low end for a delta sigma. Have you tried using a faster op amp? 

    Regards,

    Tony