The AD650 is a charge balance voltage-to-frequency converter. The input signal is converted into an equivalent current by the input resistance RIN. This current is exactly balanced by an internal feedback current delivered in short, timed bursts from the switched 1 mA internal current source as shown in Fig.1. These bursts of current may be thought of as precisely defined packets of charge. The required number of charge packets, each producing one pulse of the output transistor, depends upon the amplitude of the input signal. Since the number of charge packets delivered per unit time is dependent on the input signal amplitude, a linear voltage-to-frequency transformation will be accomplished. The voltage of the integrator capacitor is shown in Fig.2. The frequency output is furnished via an open collector transistor.
Here is a tool to calculate the parameters of the AD650JNZ.