We fabricated and evaluated a second-order ΣΔ ADC with a polarity alternating feedback (PAF) comparator based on 0.4 µm InGaP/InGaAs enhancement and depletion mode high electron mobility transistors (E/D HEMT) technology. We propose a PAF technique for enhancing the sampling frequency and have applied the technique in the design of ADC circuit. The ADC has a signal-to-noise ratio (SNR) of 43 dB when operating at a differential clock frequency of 4.9 GHz, and has a power dissipation of 400 mW.