A microwave-chip-based coherent multi-frequency RF driver is developed for a channel-interleaved photonic analog-to-digital converter (PADC) system, which comprises a multi-class optical demultiplexer and supports a sampling speed of 40 GSa/s. The generated signals from the RF driver are adjustable in both amplitude and phase. We analyze the relationship between the characteristics of the generated RF driver signals and the demultiplexing performance in theory based on the optical signal-to-distortion ratio (OSDR). It is the most effective parameter to evaluate the performance of the demultiplexer in a PADC system without an electronic analog-to-digital converter. By precisely adjusting the amplitude and phase of signals, the OSDR is optimized. The results verify the compatibility between the RF driver and the PADC system.