The On-Chip Calibration Benefits of New Simultaneous SAR Analog-to-Digital Converters

This article evaluates the impact of external resistors used in front of resistive analog-to-digital converters (ADCs). These families of simultaneous sampling ADCs include a high input impedance resistive programmable gain amplifier (PGA) that drives the ADC and scales the input signal, allowing direct sensor interface. However, there are several reasons why a design would end up adding external resistors in front of the analog input. The following sections provide a theoretical explanation of the gain error expected, as a function of the resistor sizes, and different ways to minimize these errors. This article also examines resistor tolerance and the ADC’s input impedance effects of the different calibration options. Beyond the theoretical study, bench measurements compare several devices to prove the excellent accuracy reached with on-chip gain calibration features. The gain calibration feature enables a system error of less than 0.05%, across a wide range of front-end resistor values, without having to perform any calibration routine, but just by writing a single register per channel.

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