IDDQ - Quiescent Current
IDDQ Testing is an approach used in electronics to test CMOS integrated circuits which is based on measuring electrical current, as opposed to voltage. The technique exploits the low power drain characteristic inherent to CMOS technology, (CMOS circuits only draw current when they are changing state). A circuit will be 'conditioned' into a pre-determined logic state and then the circuit's clock is stopped. The easiest way of pre-conditioning is via the IC's scan chains, (if it is a scan based design). The theory is that the IC will drain a very low level of current in this quiescent state. Any manufacturing defect present in the circuit will likely cause the current measured to deviate from the 'norm'.

The technique is seen as a valuable supplement to other methods of circuit testing since it allows the detection of certain types of fault which might not be found using voltage-based techniques, (gate leakage within a transistor, for example).

IDDQ testing also allows certain otherwise 'untestable' faults to be detected since it eradicates one of the two concerns of testability, namely observability: Any abberent behaviour within a circuit does not need to be propagated to a device output in order to be detected - it will be evident in the quiescent current measurement made.

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