It's very hard to prevent the LO from leaking into the ADC input. Putting the filters in the right places would cause a lot of issues for the signal chain so a

It's very hard to prevent the LO from leaking into the ADC input. Putting the filters in the right places would cause a lot of issues for the signal chain so a common workaround is trying to null it with a 180 degree out of phase LO signal.