A 140 dB-CMRR Current-Feedback Instrumentation Amplifier Employing Ping-Pong Auto-Zeroing and Chopping

This paper presents a precision general-purpose current-feedback instrumentation amplifier (CFIA) that employs a combination of ping-pong auto-zeroing and chopping to cancel its offset and 1/f noise. A comparison of offset-cancellation techniques shows that neither chopping nor auto-zeroing is an ideal solution for general-purpose CFIAs, since chopping results in output ripple, and auto-zeroing is associated with increased low-frequency noise. The presented CFIA mitigates these unintended side effects through a combination of these techniques. A ping-pong auto-zeroed input stage with slow-settling offset-nulling loops is applied to limit the bandwidth of the increased noise to less than half of the auto-zeroing frequency. This noise is then modulated away from DC by chopping the input stage at half the auto-zeroing frequency, reducing the low-frequency noise to the 27 nV/ white-noise level, without introducing extra output ripple. The auto-zeroing is augmented with settling phases to further reduce output transients. The CFIA was realized in a 0.5 μm analog CMOS process and achieves a typical offset of 2.8 μV and a CMRR of 140 dB in a common-mode voltage range that includes the negative supply.