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AMC7812 ADC channels multiplexing

Srinivas Jagarapu
Intellectual 510 points

Hello,

We observed when a signal is given  to ADC channel, at that instant there is an effect on the voltage read from the adjacent channels.

For example,

when voltages read from CH0(pin 33) and CH9(Pin 42), and there was no input on pin 33, 0.87V at the input of CH9 , the data out of CH9 is around 0.089V (equivalent of digital data received).It's fine, but when there was a input of 2.45V on pin 33, 0.87V at the input of CH9 ,the data out of CH9 is around 0.103V (equivalent of digital data received).

We observed similar raise in voltages in other channels when the CH0 is applied a voltage of 2.45V.

How it can be nullified/minimized?

Regards,

Srinivas J

 

 

  • 0
    TI__Genius 11750 points

    Howdy Srinivas,

    To directly answer your question, you can reduce the conversion rate of the ADC and also ensure that the active ADC inputs are driven with a low impedance source, such as a buffer.


    The reason for this channel interaction is the sharing of the internal sampling capacitor.  The internal multiplexer sequentially connects the active ADC channels to the internal sample capacitor.  When the multiplexer switches from one channel to the next, the new linked channel must be capable of driving the sample capacitor to the new voltage value by the conversion period.


    In the case you have presented above, it seems that some ADC inputs are left floating, and as such are not driving the internal sampling capacitor to a specific voltage.  As a result, the residue charge on the sampling cap is being reported during the conversion.  To create correct reads, no active ADC inputs should be left floating, and must be driven to it's 12-bit settled voltage within the conversion period.  As a result, you may need to either optimize your input driver or increase the conversion period.

    For more information on how to drive SAR ADC inputs please refer to the attached document from author Bonnie Baker.PA-001 Optimize_SAR_converter_design REV b(1).pdf

    Best Regards,

    Matt

  • 0 in reply to Matthew Sauceda
    Intellectual 510 points

    Thanks Matt.

    Changing the RC filter values fixed the issue.

    Regards,

    Srinivas J