Keywords: level shifter, differential amplifier, ground level
Category: VLSI characterization
Challenges of characterizing digital cells start to look easy in the face of mixed signal cells with parameters that defy assumptions of digital cells. One of the assumption digital cells make is ground voltage is zero, which does not hold good for mixed signal cells like level shifter, differential amplifiers etc. How does delay measurement change with violation of this assumption is the topic of this article.
Below is an example waveform of such a cell.
Figure: multi rail wave forms of a differential amplifier
Note that this cell appears to have differential input-pair and differential output pair. Input pair switch in different direction from zero volt to 0.73 volt, whereas output pair switched from 0.30 volt
to 0.68 volt.
Measuring Arc Delay:
With changed voltages, one has to come up with new threshold voltages to find delay points. As we discussed in previous blog, delay measurement involves following 8 threshold voltages.
With these definition, we find rail swing for both input and output pins as follows:
Once swing has been determined, one can find input and output voltage levels crossing thresholds as follows:
In this article, we presented method and equations to compute delays with ground voltages other than zero. One can easily extend this method to find equation to compute output slope with ground voltages other than zero.