PROJECT TITLE :
Digital Offset Cancellation for Long Time-Constant Subthreshold OTA-C Integrators
Systems using integrators with very very long time constants can place a severe constraint on allowable amplifier offset. Postfabrication cancellation of those offsets is a demand in these applications. This brief describes the look, simulation, and measurement of a subthreshold operational transconductance amplifier (OTA) with digital calibration and its strong calibration algorithm. Statistical simulations show the theme's ability to scale back the output offset by a issue of 40. Measurements on several prototype OTAs validate the technique's ability to bring the standard deviation of integrated output offset to less than ten mV in most cases, or regarding thirty input-referred offsets. A secant-based mostly algorithm is proposed that finds the optimum digital calibration code for each OTA in an exceedingly small range of search steps that minimizes the integrated output offset. This algorithm is robust to nonmonotonic tuning characteristics, permitting the additional circuitry to be minimally sized. For low-frequency applications below 5 Hz, the scheme has nearly no web impact on die space in processes permitting circuitry below the mixing capacitor.
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