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MCP4651 Digital Pot I2C Signals Leakage

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bbawkon
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2020/09/17 14:23:14 (permalink)
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MCP4651 Digital Pot I2C Signals Leakage

Good day,
 
I'm using the MCP4651 Dual Digital Pot inside the feedback loop of an Op-Amp to correct gain error.  The circuit works wonderfully.  The one issue I'm having, however, is that the I2C Signals are bleeding into the signal path.  Even signals that are present on the Bus but not directed at the MCP4651 are clearly visible on the output of the Op-Amp.
 
I've confirmed that the problem is the MCP4651 by removing it from the board and replacing it with fixed value resistors - Even thought The I2C bus is still active, the Op-Amp output is clean.
 
Is this a known issue with these parts, or is there something I can do to prevent this?  They layout is strong - 6 Layers, solid ground and power planes, low impedance traces, few vias, and plenty of decoupling.  I2C Frequency is 400Khz, though I've also tried 100Khz and it made no difference.  I2C pull-ups are 10KOhm.  Vdd-Vss = 5V.  There is the Master, The MCP4651 and one other I2C Device on the bus, so it's not overloaded.
 
Without the MCP4651 in circuit, noise is <1mV P-P.  With the MCP4651, noise remains <1mV P-P, but there are >50mV spikes during I2C traffic.  The spikes are large and square enough that I can actually decode the I2C bus on the Op-Amp output.
 
Any suggestions would be greatly appreciated!
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    ric
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    Re: MCP4651 Digital Pot I2C Signals Leakage 2020/09/18 17:00:33 (permalink)
    4 (1)
    Do you have a couple of spare GPIO pins?
    One solution would be to bit bang a separate I2C bus just for the MCP4651, so it wouldn't see the other I2C traffic.
     

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