I'm interfacing a dsPIC33EP512MC806 with a set of analog systems. The analog systems manage a battery and have vregs for the digital systems. The analog systems measure the voltage and current consumption of the battery providing feed back to the micro. The current is measured with a low quiecent current IC (ZXCT1010). The voltage is measured with a low quiecent current opAmp in a divide by four configuration.
It is the opAmp that concerns me. The digital side will get shut down by disconnecting power from the analog side. The analog side will remain powered on. Because the analog side remains powered on the opAmp still applies about 2.1 volts to the pin of the processor. When the processor is powered on there is no current or at least less than the meter can measure. With the processor turned off there is about 300uA of current from the opAmp into the pin. I believe that an ESD diode is being forward biased. There is a 1k resistor and a 0.1uf capacitor serving as a low pass filter. Also there is another 1k resistor serving as input protection to the pin. This makes for 2k of resistnace between the op amp and the pin of the micro controller.
I'm having a hard time finding any information on what the ESD diode can tolerate. Can the opAmp remain connected to the micro controller indefinitely if the micro controller is shut off?