### Q

What is the exact value of the differential output impedance of the ADA4938-1or ADA4938-2. Is it 2ohm or 5ohm or 20ohm?There are S parameter plots S11 nad

S22 in the datasheet but I don't know to which circuit they apply to or to

which impedance they apply to.

### A

The S11 and S22 plots include external resistors, and don’t reflect theamplifier’s actual intrinsic input and output resistances. The closed-loop

output resistance is very low, so adding external 49.9 ohm resistors makes each

output close to 50 ohms, and the differential output resistance is therefore

close to 100 ohms. The external resistors are therefore the dominant factors

that affect the output match. On the input, the driver is set up for a 50 ohm

input, again using external resistors, and the input match is based on this

configuration.