AD7858, voltage on analog input while powered down

Question

Hello, 
 According to the absolute maximum ratings, the analog input voltage must be in the range -0.3 V to AVDD + 0.3 V. Is a positive voltage on one of the analog inputs before the AD7858 is powered up likely to cause a problem? What is the input impedance of analog inputs when the device is powered down? 
 Thank you, 
 Leo

EZAdministrator · Accepted Answer

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EZ Admin

Daryl_05 · Answer

Hi Leo, 
 Yes, it will cause a problem. The power sequence of a signal chain is the ADC should power up first before the front end amplifiers. Typically, IC has ESD diode protection that can clamp the voltage at the input pins to within 0.3V of the supplies. And these diode can only handle 10mA of current. To address your problem, you can have larger value of resistor in RC filter to limit the current or you can add two shottky diodes to lessen the current that will flow in the ESD diodes. 
 Regards, 
 Daryl

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AD7858, voltage on analog input while powered down