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Reading Variation of ADS1256 if four channel use

Yashpal Patel49892
Expert 1835 points
Other Parts Discussed in Thread: ADS1256, TLC271

Hi,

We are using ADS1256 in our projects.

If we use four channel in differential mode.

AN0 - 0 to 5V

AN1 - fixed 2.5V

AN2 - 0 to 5V0842.PKH-035C.pdf

AN3 - fixed 2.5V

AN4 - 0 to 5V

AN5 - fixed 2.5V

AN6 - 0 to 5V

AN7 - fixed 2.5V

If only one channel is use and other three channel connect with ADC then it is working fine but if all four channel use then reading of ADC unbalance (not steady or variation in reading).

For your ref, we are attaching herewith schematic of our card..

Pls help.

Regards,

Dharmendra

  • 0
    TI__Mastermind 43915 points

    Hi Yashpal,

    The attenuation in the crosstalk in this part is very high which makes me think that the source is the reference voltage. Looks like you have a fair amount of capacitive load hanging directly off of the output of the TLC271 drive amplifier without an isolation resistor. That could be a source of instability of the reference line which would cause the variation. If that is the case, I would expect you to have a fair amount of code variation on all your reads.

    Do you have the internal buffer enabled or disabled for these tests? You will want to make sure that you have the buffer disabled for these conversions in order to stay within the input limitation specs.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Expert 1835 points

    Hi Tony,

    we already Internal buffer disable.

    we remove the Capacitor from output of TLC271 and added 100E resistor in series, but same problem occur.

    Please do the needful.

    Regards,

    Dhamendra

  • 0 in reply to Yashpal Patel49892
    TI__Mastermind 43915 points

    Hi Dhamendra,

    Disabling the internal buffer requires that you have the circuitry to drive the input yourself. The amplifier that you chose looks like it has the speed needed to drive the input. I would change the input RC to about 100 ohms and 2.2nF for optimal results. If the variation is showing up on the converted results of all channels, I would look at the reference as being the source. For debugging, you can try enabling the internal buffers and inputting a fixed DC signal (possibly 2V) within the input limits of the ADC to test to see if the problem is still there. This should, at least, tell you if the problem is the drive circuitry or something else. Remember to follow the cycling procedure shown in Figure 19 to ensure that you are resetting the digital filter before each conversion.

    Regards,

    Tony Calabria