Feedback lab 3 (part1)

- Measuring input capacitance: so far, two groups were looking at the output signal of the inverter to determine input capacitance. As the name suggests: this is simply wrong. Instead, you need to observe voltage and current at the input. There is several methods you can use for this and you have found and used many of those quite correctly. For example introducing a series resistance and measure the time constant tau=RC of the input (!) signal. You can also source a voltage and measure the current and use I=C dV/dt or integrate over the current used from 0 to Vdd (which is the total charge) and use Q=C Delta V. Or vice versa, source a current an measure the voltage. One ingenious group also measured the propagation delay of the inverter with another idential inverter conected at its output and once again with the large 50pF capacitance at the output, correctly concluding that the ratio of the delays is proportional to the ratio of capacitances. Thus they  could quite nicely estimate the node capacitance when an second inverter is the load.

One difficulty could be the Miller effect that distorts the charging curve of the input when the output switches. it might introduce some error (no cause for point deduction), or one could take its effect into account and get an accurate result....