Testing & characterization of data converters

Modern system-on-chips (SoCs) are mixed-signal in nature, comprising a combination of purely analog and mixed-signal circuits. Analog circuits are characterized in the analog domain, and the test setup typically uses signal generators for stimulus, while the output is measured directly on spectrum analyzers, oscilloscopes, etc. Mixed-signal systems like analog-to-digital converters (ADCs) & digital-to-analog converters (DACs), on the other hand, entail complexities not only in design but also in testing, owing to their mixed-signal nature. In particular, since the output of ADCs is digital bits, characterization of ADCs involves careful post-processing of this digital data. This post-processing is required to be done in the time domain or in the frequency domain, depending on the parameter being measured. This demands that the designer or test engineer be aware of the different parameters that are of interest in an ADC and the relevance of these parameters for each application. For example, static linearity of ADCs is measured in the time domain, while dynamic linearity requires careful measurement in the frequency domain using the discrete Fourier transform (DFT). Besides, even the analog stimulus for ADCs must be carefully chosen and applied so as to capture the true performance of the ADC. As a result, understanding basic signal processing techniques and tools is essential for anyone working with ADCs. Such working knowledge not only helps during testing but also aids in making design choices that make the design amenable to easier testing (like having a crude digital-to-analog converter (DAC) on-chip for easier testing of high-speed ADCs). --- This tutorial aims to comprehensively explain the basic signal processing knowledge required for testing. In addition, it will discuss all other important aspects while testing, such as how to choose the input signal frequency, the minimum no. of FFT points, etc. Besides, the challenges in testing both discrete-time and high-speed continuous-time ADCs and DACs will be explained in detail with example results.