Digitally-programmable gain amplifier with direct-charge transfer and offset cancellation

A Programmable-Gain Amplifier (PGA) has a digital value that programmably adjusts the gain of the analog amplifier. A variable capacitor has several switched sub-capacitors that are enabled by the digital value. Enabled sub-capacitors are switched between a sampled input and a virtual ground on one terminal, and connect to a summing node on the other terminal. The summing node connects to the inverting input of an op amp either through a switch or through a double-sampling capacitor that stores an offset. A feedback capacitor is in parallel with a sampling capacitor during a second clock phase when direct-charge transfer occurs, reducing power consumption of the amplifier. The feedback capacitor samples the sampled input during the first clock phase. The PGA gain is proportional to the sum of capacitances of enabled sub-capacitors. The gain can be adjusted for sensor inputs to an Analog Front-End (AFE), such as for an electro-cardiogram (ECG).