LTC2945-I2C not working with opto isolation

Question

Hi ADI Team, 
 For one of our designs, we are using the LTC2945 for voltage and current sensing with input voltages (VIN) of 12V and 28V, and we have used a multiplexer (MUX) between the two LTC parts. 
 We were able to detect the I2C address (via I2C scan) for the 12V parts, but for the 28V parts, the scan code does not print anything to the UART. 
 Could you please review the schematics below and provide your feedback?

Thanks, 
 Mani

mvrablic · Accepted Answer

SDAO doesn't have the same clamps as SCL and SDAI, so should not have a pull-up resistor to 27V 
 
 I wouldn't be surprised if this is damaging the SDAO pin and the reason you can't see the I2C device with this system but can with 12V. Note that 12V is also beyond the absolute maximum though and likely stressing the SDAO ESD structure too. 
 The intent is that the pull-up resistor on SDAO is referenced to the MCU power supply level, but obviously the optoisolator here will get in the way of that. INTVCC can be used for the pull-up in this case, just make sure to stay within the 10mA current limit and maybe place a ferrite bead between the pull-up and bypass capacitor to minimize the influence on the pin voltage.)

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LTC2945-I2C not working with opto isolation

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