Low Cost Dynamic Test Methodology for High Precision ΣΔ ADCs

Abstract

1 : In this paper, a low-cost test methodology for dynamic specifications of high precision sigma-delta (∆Σ) analog-to-digital converters (ADCs) is presented. Dynamic testing of ADCs requires an input test stimulus with total harmonic distortion (THD) and signal-to-noise ratio (SNR) about 10dB better than the ADC under test. ∆Σ ADCs are inherently high resolution converters with excellent THD and SNR due to their inherent oversampling, averaging and noise shaping properties. In the proposed test methodology, the back end digital and decimation filters of such converters are turned off and the digital pulse sequence at the output of the sigma-delta modulator is made externally observable for test purposes. It is seen that ENOB, THD and SNR of the converter can be determined with significantly increased sensitivity to device nonlinearities and noise allowing the use of less than ideal input stimulus than otherwise or significantly reduced test time. The back-end filters are then tested using traditional digital test techniques. Simulation results show the usefulness of the proposed test methodolgy.

Signature Testing and Diagnosis of High Precision S? ADC Dynamic Specifications Using Model Parameter Estimation

“…Note that ENOB is obtained from SiNAD (signal to noise and distortion) and differs from SNR. Figure 7 shows that conventional FFT test (blue x) using the coarse input stimuli fails to measure DUTs with about 11-bit or higher resolutions while our proposed technique (red +) and the technique in [6] (black •) can test all the instances with high accuracy. In addition, significant improvements of our proposed approach from the technique in [6] are 1) no need for pre-training set and 2) diagnosis capability for bad devices.…”

“…Simultaneously, dynamic specifications for 300 devices were calculated with the estimated parameters. Figure 7, the corresponding specification calculations for the 300 DUTs using our proposed approach (red +) were compared to conventional FFT test (blue x) and alternate based test (black •) proposed in [6]. Each graph shows the measured values on the y-axis and the actual values on the x-axis.…”

“…However, the out-of-band component, which is removed by the digital filter, contains valuable information about the ∆Σ ADCs nonlinearities. To prevent such useful nonlinearities information in the "outof-band" spectrum from being masked by the digital filter, the proposed methodology adopts the same DfT methodology as presented in [6]. The simple block-diagram introduced in [6] is seen in Figure 2.…”

“…This is then used to generate accurate values of the test specifications of the DUT such as SFDR. In [6], the authors present a methodology based on the alternate test approach using a low-cost test stimulus. In this approach, the bit stream obtained at the output of the sigma delta modulator is also analyzed.…”

“…In the proposed approach, the test setup proposed in [6] is adopted. However, to eliminate the use of a supervised learner and to provide additional diagnostic information, we propose a signature test based approach for go/no-go testing and diagnosis of the key dynamic specifications of the ADC (ENOB, SNR, and THD) using nonlinear optimization driven model parameter estimation.…”