The Design and Characterization of an 8-bit ADC for 250°C Operation

Abstract

Many high temperature applications require the measurement of analog voltages. This usually requires the integration of an ADC into the design. While the temperature degradation in performance of digital circuits is well known, the effects of temperature on analog circuitry are much harder to predict. Analog design is often an iterative process in which the characterization knowledge of a fabricated design is used to improve the next iteration of the design. This paper presents the results of the most recent iteration. This paper describes how the design of an existing 8-bit ADC was optimized for the SOI process. It also presents the characterization of the ADC at various temperatures up to 250°C and shows the effects of increased leakage on the ADC parameters of linearity, accuracy, and conversion speed. The ADC discussed is a successive approximation design which uses a resistive DAC. The design was modified to take advantage of the resistive characteristics inherent in the SOI process. Specifically, the DAC resistors were formed using N-type diffusion because of their superior matching as compared to using poly. The analog circuitry in the DAC switching and in the comparator required carefully choosing where to use “A” type versus “H” type transistor geometries to prevent inadvertent SCR failures. The ADC design also included a serial interface circuit that facilitates measurements within an oven by minimizing the number of connections required for operation. The measurements were taken using a 12-bit DAC to generate the analog input voltages to the 8-bit ADC under test. The ADC digital output was compared to the digital input to the DAC. All 4096 measurement points were taken at each voltage and temperature step. The results of these measurements were post-processed to extract the characterization data. There is a discussion of the results, the effects of leakage on those results, and how these effects might be overcome to produce more accurate ADC circuits in the future.