We are using the ADC12J4000 is our application and are seeing a Fs/4 -Fin image that appears larger than quoted in the datasheet. Specifically, a strong narrow band noise input at one frequency - but not saturating the ADC - produces also a low level Fs/4 -Fin image in the digital spectrum. We are using FG calibration including timing optimisation after the system has stabilised and the ADC cards are housed in a temperature controlled air conditioned environment so the operating temperature does not vary significantly.
Are there any special requirements for the calibration to be optimal and achieve the numbers in the datasheet? Specifically, is it necessary to mute the RF input to the device while the FG calibration is running? In our system the RF is input is fed with an active signal during the calibration, but this does not exceed any of the max ratings or even clip the ADC input range.