AD6645: Overvoltage protection

Q

As we drive the AD6645 with a differential amplifier (AD8138), the question
comes up if it is necessary to protect the AD6645 from negative voltages at its
analog input. When supplying the op amp with a +/- supply voltage, the output
can easily swing negative to ground. How much of a problem is that for the
AD6645?

A

The absolute maximum ratings for the AD6645 allow the voltage on the input to
swing to 0V and the analog supply. In practice a negative swing of more than
approx 300mV will be enough to potentially damage the input.

If you must power the AD8138 from dual supplies (I assume your input common
mode is 0V for example), and even if the VCOM is connected to the mid-supply of
the AD6645 AVDD, it is good practice to include a schottky clamping diodes
between the input and the converter ground pin to prevent damage.