Testing High Resolution SD SD ADC’s by using the Noise Transfer Function Daniela De Venuto Dipartimento di Elettrotecnica ed Elettronica Politecnico di Bari, Italy d.devenuto@poliba.it Andrew Richardson Centre for Microsystems Engineering University of Lancaster, UK Abstract A new solution to improve the testability of high resolution SD Analogue to Digital Converters (SD ADC’s) using the quantizer input as test node is described. The theoretical basis for the technique is discussed and results from high level simulations for a 16 bit, 4 th order, audio ADC are presented. The analysis demonstrates the potential to reduce the computational effort associated with test response analysis versus conventional techniques. 1. Introduction Testing high-resolution mixed signal interfaces in production requires high accuracy testers, large data storage resources and processor intensive response analysis. Test time, infrastructure and cost per device are therefore currently too high for this class of function. Much effort has been put into addressing this problem for ADC’s and DAC’s [1-8]. Commonly accepted Automatic Test Equipment (ATE) based testing of ADCs is based on the verification of a subset of converter performance parameters that include offset, gain, signal-to-noise ratio, total harmonic distortion, INL, DNL and power consumption. Dynamic performance parameters are extracted from the output bit stream by using FFT analysis of the converter’s response to a sinusoidal test stimulus. Static performance parameters can be determined from measured code transition edges (feedback loop test) or from the number of code occurrences in response to a periodic signal (histogram testing). The accuracy of such parameter measurements requires a large number of samples (approx. 16536 for 90dB THD and SNR measurements) and up to 10s of test time for a single channel 16-bit audio ADC. Various Built-In Self-Test (BIST) techniques for analog and mixed signal circuitry have been published in [1-9] that target test time reduction. All these techniques address the measurement of just one or of only a limited number of specification parameters using conventional techniques. Moreover, the circuitry introduced by most of these methods tends to require significant die area and dedicated additional test patterns for verification before use. In this work the focus is on how to simplify and accelerate the testing of high order sigma-delta converters for audio applications without degrading test quality. The method presented in this paper utilises the noise transfer function (NTF) of the converter as figure of quality. In practice extracting this function requires the injection of a sine-wave into the quantizer input and performing a spectral analysis at the ADC output. The NTF contains crucial information related to the dynamic behaviour of the converter. The comparison of a measured response with a reference can in itself be a powerful test and diagnostic tool. The information in this curve can also be used to extract three important specifications: gain, noise, and distortion. Furthermore, by using the NTF together with the techniques described in section 3, these parameters can be extracted through a lower number of samples than required for traditional FFT evaluation. The technique preserves the accuracy of the measurement without the need for complex design modification or Design-for-Testability (DfT) structures. The paper is organised as follows. In section 2, a brief overview of the theory of the sigma-delta converters will demonstrate the importance of NTF characteristic on the quality of the converter. Section 3 will discuss the noise