Is the AD8019 output high impedance when in power down mode?
No. ~30 ohms at 100kHz and 125 ohms at 1MHz
The power down feature on the AD8019 is designed only to power down the part.
It reduces quiescent current, removes bias levels from all or part of the
circuit. It does not set the output of the opamp into a high impedance mode.
The input impedance of the AD8019 will be a non-linear function of the voltage