18.3 An Auto-Zero Stabilized Voltage Buffer with a Quiet Chopping Scheme and Constant Input Current

The readout of high-impedance sensors and sampled voltage references [1] requires amplifiers that can achieve both low offset and low input current. Recently, it has been shown that this unique combination can be achieved by an auto-zero (AZ) stabilized buffer [2]. However, its low-frequency noise density is √5 times higher than the buffer's own white-noise voltage spectral density en. Furthermore, its input current is not constant, but varies significantly with the input voltage. To overcome the first issue, a chopped AZ stabilization loop with an optimized duty-cycle is proposed to bring the low-frequency noise density close to √2·en, the fundamental limit of an AZ stabilized amplifier. The second issue is solved by replacing the transmission-gate input switches used in [2] with NMOS switches and a constant Vgs drive. This keeps their charge injection constant over a wide input voltage range, and results in a constant input current.