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PCM1865: Gain error

Mochi
Genius 4165 points
Part Number: PCM1865

Hello,

Our customer is facing strange behavior of PGA function on PCM1865.

 

Using PPC3 GUI and feed I2C to their ES board from PCM1865EVM.

Change SW gain from 25.5dB to 26dB at SE connection. Output data is suddenly disappeared as attached data.

Another specific point is from 31.5dB to 32dB setting at differential connection, output data is 6bit shifted -36dB reduction.

Two assembled bords has same issue, but PCM1865EVM standalone configuration PGA function works correctly.

 

Have you seen such a unique problem. Could you give us any idea how can we solve it?

  

Regards,

Mochizuki

PCM1865 gain.pdf

  • 0
    Genius 4165 points

    Hi,

    We get new finding.

    On the customer schematic, there is 100Kohm input resistor between input pin to GND directly.

    And gain error is disappeared if remove this resistor.

    It seems internal bias current is leaking and PGA function made strange behavior.

    What is limitation of this resistor value? To avoid ESD damage they had placed it.

     

    Regards,

    Mochizuki

  • 0 in reply to Mochi
    TI__Intellectual 2620 points

    Hi Mochizuki,

    When they put a series resistor it will create a voltage divider with the input impedance of the PCM1865. The driving impedance should generally be much lower than the input impedance to avoid attenuation.

  • 0 in reply to Brian Donoghue
    Genius 4165 points

    Hi Brian,

    Thank you for the comment.

    In this case, the resister is located between GND and input pin, it is not series connection.

    Input pin might be biased to internal Vcom, we expect the resister will cause of leakage current to bring PGA malfunction.

    So, is there any specification the external resister value?  1Mohm is fine or no acceptance any external resisters?

     

    Regards,

    Mochizuki

  • 0 in reply to Mochi
    TI__Intellectual 2620 points

    Mochi,

    Thanks for the explanation. It sounds like the customer is using DC coupling, is this true? DC coupling is not recommended. See this detail from the datasheet. We can review the customer schematic if further investigation is needed. 

    Brian

  • 0 in reply to Brian Donoghue
    Genius 4165 points

    Hi Brian,

    Thank you for prompt reply.

    Yes, that is DC coupling. The reason is that ADC PCB and Frontend pre amp PCB are separated.

    DC blocking capacitor is in Frontend PCB and ADC PCB input pin is directly connect to a connector.

    To avoid ESD event to High impedance input pin, they put 100kohm resistor.

    We will recommend to move DC blocking cap to ADC input PCB side to eliminate DC coupling resistor.

     

    Regards,

    Mochizuki