On-chip test signal generation for acoustic and ultrasound microelectronic interfaces

Low-cost testing of Analogue and Mixed-Signal (AMS) cores requires avoiding the use of expensive AMS testers. Testing AMS cores using a digital tester can be achieved by generating on-chip, from digital seeds, the analogue test signals for the core cells and by producing output digital signatures from the cells response. In this paper, we will focus on the on-chip analogue test signal generation. In our study, we are limited to the case of cells working from a few tens of Hz up to a few tens of MHz since we plan to design a generic AMS core for interfacing acoustic and ultrasound sensors. Note that testing audio circuits for deep sub-micron technologies is expected to be a major challenge when they are combined with large numbers of noisy digital circuits [1]. At this stage, we have focused our attention towards the generation of DC and single tone analogue signals. We will first review previous work before detailing the actual realization of the on-chip test generation strategy. The strategy is next evaluated by simulation and a first version of a CAD tool useful for defining the digital test patterns required is illustrated. Finally, we will conclude with a summary of our current and future work. 1. Previous work The strategy that we use is based on the approach proposed in [2]. As shown in Figure 1, to generate an analogue signal, a bit-stream is first loaded into a shift-register during phase T1. The loaded bit-stream is then periodically reproduced during phase T2 at the output of the shift-register, by connecting the output of the register to its input. The actual bit-stream (a test pattern or seed for the shift-register) must be previously obtained by simulation by means of a Σ∆ modulator that encodes the desired analogue test signal (in this example, a single tone signal with frequency f sampled at frequency fs). The bit-stream at the output of the shiftregister clocked at frequency fs is filtered on-chip to recover the actual encoded analogue signal (in this figure a low-pass filter with a cut-off frequency of fc=f). It is well known that by repeating a sample set, the actual signal will contain a limited number of coherent frequencies [3] given by : 2 ,...., 3 , 2 , 1 , 0 N M f N M f s = = [1]