HBIST of non-linear analog building blocks in mixed- signal circuits

HBIST (Hybrid-Built-In-Selftest) is a very efficient method for testing analog parts within mixed-signal circuits by using a provided BIST (BuildIn-Selftest)-environment in the digital part of a circuit. For using HBIST of nonlinear analog circuits a new kind of signature called add-signature is proposed. The advantages and disadvantages of the add-signature are demonstrated together with simulation results. To perform HBIST without problems it is necessary following [1] to fullfill especially one condition beyond others: fig.1: Building blocks of HBIST Errors of the CUT are only visible at the output of the comparator of the AD-converter when the threshold of that comparator is exceeded. For preventing non-reproducible register sequences (signature) a minimum distance of the CUT's response to the comparator thresholds is necessary at the moment the test responses are read into the BILBO. Using linear circuits this condition might be guaranteed by adjusting the building blocks of HBIST in the right way (see fig.2). Therefore drifts might not lead to non-reproducible signatures. fig.2: Distance between test response and comparator threshold of a linear CUT If the circuit is non-linear the above mentioned condition can not be guaranteed in general, which is demonstrated in fig. 3. fig.3: Distance between test response and comparator threshold of an non-linear CUT This feature can lead to exceedings of comparator thresholds leading to the above mentioned not reproducible changes of the CUT's signature in the presence of only small drift and parameter deviations. 3. Signature analysis of non-linear CUT’s To prevent the problem of the HBIST which occurs especially for nonlinear CUT's, a "correlated" way of determining a signature in conjunction with HBIST by performing a modulo 2n addition is presented (add-signature). Therefore the AD-converted test responses are added as in equation 1. eq. (1) add signature Zi i k n − =  î    = ∑