Input Currents of CMOS SAR ADC in Microcontroller, their Measurement and Behavior Model from Users Point of View

Abstract

The article introduces possible issue of internal CMOS SAR microcontroller's ADC input currents. It describes a nature of the ADC input current and its dependence on a sampling frequency. Methods for measurement of parameters such as input charge, residual voltage, and equivalent capacitance of sample-hold circuits are presented and used in the proposed simplified model of ADC input behavior. These parameters, which are not listed in datasheets, if not respected in data acquisition designs, can adversely affect how the ADC is used when measuring signal sources with nonnegligible internal resistance. The article also explains how to solve a voltage measurement on a source with high internal resistance even in a situation where, according to datasheets, it should already have limitations. The proposed simplified CMOS SAR ADC input behavior model in an MCU can help users in MCU-based data acquisition system design.