Why is there a DC offset on the ALPHA250 RF ADC input?

The ALPHA250 RF ADC front-end is designed to operate with a 50 Ohm termination connected to the input.

The input amplifier has bias currents that are compensated by the front-end circuit when the input is correctly terminated. Without the 50 Ohm termination, this compensation is no longer balanced and a significant DC offset can appear.

With the correct 50 Ohm termination, the residual offset is typically only a few millivolts.

Therefore, when testing the RF ADC with a high-impedance source or oscilloscope, make sure that a 50 Ohm termination is connected at the ALPHA250 input.