The invention measures the frequency of a heterodyne laser radar (LADAR) system signal in the input cell of a focal plane array (FPA). Embodiments amplify the return signal, and drive it into a counter for a fixed period of time. The frequency is the number of counts divided by the count time. An example design amplifier amplifies the return of a single photon response of an avalanche photodiode with a gain of 100 into a digital signal level at a 200 MHz rate with only 84 μW, demonstrating the feasibility of the approach.