AD5274 shutdown mode


I have a question about AD5274 in shutdown mode. From my understanding, the shutdown mode would provide infinity resistance between port A and port W. Am I correct?

I'm using this chip to set the gain of an amplifier. I put the chip to shutdown mode to set to unity gain. (The amplifier is unity gain stable.) However, amplifier output has a high frequency component when I put AD5274 into shutdown mode.

The channel 3 (purple) from the attachment is the output from my amplifier, which should be the inverted version of channel 4 (green). But the half cycle of the output isn't correct.

any idea?