7. A Schmitt-trigger inverter is a special type of inverter with feedback that s

Question

7. A Schmitt-trigger inverter is a special type of inverter with feedback that shifts the switching threshold based on a 0 to 1 or 1 to 0 transition. In this example, when the input is initially low, the output is high. When the input increases, the output will not go low until the input gets to 2.9 V. Once the output is low, it does not transition high again until the input decreases to 2.1 V. The plot and symbol are shown below.

These types of circuits are commonly used to deal with noisy signals, such as those found in computer I/O busses or interface cables. They can also be used to convert analog signals. The following is a proposed design used to measure a current based on charging and discharging a sense capacitance voltage (CAP). Assume the lower threshold on the Schmitt trigger is set to the system VIL and the high threshold is set to VIH. Then the voltage waveforms for the CAP signal and S1 gate are plotted accordingly.

a) In this plot, what are the approximate values for VIL and VIH?

b) Write a generic expression for slope of the CAP signal (in steady state).

c) Derive an expression for the output waveform frequency in terms of I, C, VIH, and VIL

d) Suppose the circuit is implemented on a CMOS process consistent with part (a). Because of area limitations, you are limited to a 1 pF sense capacitor. Your collaborators in signal processing request that the circuit operate no faster than 500 kHz / 1 µA. Is this realistic? If not, propose an option to overcome this constraint.

Explanation / Answer

a) VIH = 3.5

VIL = 1.5 FROM THE GRAPH

B) SLOPE = VIH/VIL = 7/3

C) F= I/C(VIH-VIL)

D) YES IT IS REALISTC

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