AD8422 is only available in the High-Performance Pinout which was introduced by
the AD8221, so a board layout change is required to switch from AD620 to
AD8422. AD8422 is pin-to-pin equivalent to AD8221, however it does have a
different gain equation, so the value of the Rg resistor must be adjusted.
The AD8221 and AD620 gain equation is: G = 1 + (49.4 kΩ)/R_G
The AD8422 gain equation is: G = 1 + (19.8 kΩ)/R_G
To put it another way, multiply your AD8221 or AD620 gain resistor value by
0.4008 in order to achieve the same gain with AD8422.