A 6.7nV/√Hz Sub-mHz-1/f-corner 14b analog-to-digital interface for rail-to-rail precision voltage sensing

Many sensors demand energy-efficient precision voltage sensing interfaces with low noise performance down to very low frequencies. Examples include micro Kelvin resolution temperature sensors for wafer stepper temperature stabilization and others. In many such applications, analog-to-digital conversion is inevitable, which mandates sampling. So far, sensor interfaces have demonstrated low offset drift using chopping/auto-zeroing while preserving the intrinsic SNR of the transducer by anti-aliasing filtering before sampling. Integration with a boxcar window, or boxcar sampling, is a particularly convenient way to realize anti-aliasing filtering in the sampling process. However, implementations of the technique, due to their use of just a voltage-to-current converter whose output current is integrated over a time window, e.g. in a sigma-delta modulator, remain susceptible to gain drift resulting from either transcon-ductance or time window drift. Meanwhile, related work suggests that embedding the boxcar sampler in a feedback loop effectively removes this limitation. Whereas the loop in is closed electromechanically, the design presented here overcomes the same limitations by using a purely electrical feedback loop to establish a well-defined gain that is insensitive to boxcar sampler parameter drift.