This circuit charges and discharges sampling capacitor Cs by alternately switching it between input VIN and the inverting terminal of the operational amplifier configured as an integrator. Frequency input is provided by digital inputs VA and VB, which are out-of-phase non-overlapping digital clocks. Initially Cs is charged by VIN, which is connected to Cs through the analog switch and enabled by VA(CLOSED). Next, VIN is disconnected from Cs when VA disables (open) the analog switch. VB then enable Cs to be switched across the input terminals of the integrating amplifier. VOUT is determined by the amount of charge on Cs transferred across to the integration capacitor Cf. It is important to select an analog switch that has very low charge injection specifications, such as the ALD4201. Switching by analog switch introduces charge injection that adds or subtracts extraneous charge to Cs and introduces errors to the signal charge stored on it.Clocks VA and VB required to drive a quad analog switch such as t