Excalibur: An Accurate, Scalable, and Low-Cost Calibration Tool for Sensing Devices

Calibration of an analog-to-digital converter is an essential step to compensate for static errors and ensure accurate digital output. In addition, ad-hoc deployments and operations require fault-tolerant IoT devices capable of adapting to unpredictable environments. In this paper, we present the design and implementation of Excalibur - a low-cost, accurate, and scalable calibration tool. Excalibur is a programmable platform, which provides linear current output and rational function voltage output with a dynamic range. The basic idea is to use a set of digital switches to connect with a parallel resistor network and program the digital switches to change the total resistance of the circuit. The total resistance and output frequency of Excalibur is controlled by a program communicating through the GPIO and (IC)-C-2 interfaces. The software provides two salient features to improve accuracy and reliability: time synchronization and self-calibration. Furthermore, Excalibur is equipped with a temperature sensor to measure the temperature before calibration, and a current sensor which enables current calibration without using a digital multimeter. We present the mathematical model and a solution to compensate for thermal and wire resistance effects and validate scalability by incorporating the concept of the Fibonacci sequence. Our extensive experimental studies show that Excalibur can significantly improve measurement accuracy. For example, for ATMega2560, the ADC error reduces from 0.2% to 0.01%, for ADS8353, the error reduces from 0.17% to 0.0014%, for INA219, the error reduces from 0.42% to 0.02%, and for MCP3208, the error reduces from 5.29% to 0.01%.