A Decorrelating Design-for-Digital-Testability Scheme for $\Sigma {-}\Delta$ Modulators

This paper presents a novel decorrelating design-for-digital-testability (D³T) scheme for Sigma-Delta modulators to enhance the test accuracy of using digital stimuli. The input switched-capacitor network of the modulator under test is reconfigured as two or more subdigital-to-charge converters in the test mode. By properly designing the digital stimuli, the shaped noise power of the digital stimulus can be effectively attenuated. As a result, the shaped noise correlation as well as the modulator overload issues are alleviated, thus improving the test accuracy. A second-order Sigma-Delta modulator design is used as an example to demonstrate the effectiveness of the proposed scheme. The behavioral simulation results showed that, when the signal level of the stimulus tone is less than -5 dBFS, the signal-to-noise ratios obtained by the digital stimuli are inferior to those obtained by their analog counterparts of no more than 1.8 dB. Circuit-simulation results also demonstrated that the D³T scheme has the potential to test moderate nonlinearity. The proposed D³T scheme has the advantages of achieving high test accuracy, low circuit overhead, high fault observability, and the capability of conducting at-speed tests.