I am trying to measure the input resistance of the following multiplexer:
The reason I am measuring the input resistance of this multiplexer is that I have a signal coming out of my source and upon the signal reaching the input of my multiplexer the signal is distorted. My guess is that the output resistance of the source is high compared to the input resistance of the multiplexer. I want to be able to measure the input resistance of my multiplexer but I am not sure how. I have the following ideas but not sure which is correct:
I am using a push-pull circuit to model my input of my multiplexer:
Here VDD is 3.3V and VSS is 0V. To measure the input impedance of the entire multiplexer I tried the following:
Where the signal from my function generator was 1.8 Vpp, square wave, and I tried at frequencies from 1KHz to 8 MHz. This resulted in an decrease in impedance as the frequency is increased due to MOS gate capacitance. I measured V2 with respect to ground and V1 with respect to ground (V2 and V1 were measured peak to peak). Then Iin = (v2-V1)/R (in this case R = 4.7K). Then Rin = V1/Iin. Now i am not sure if this is the input impedance of my multiplexer or not? The reason I am not sure of this is I also wanted to add in a pull-down resistor to pull down my signal to ground. So I wanted to calculate the input resistance between the ground and input. Therefore I could add in a pull-down resistor to the input of the circuit.
That way this could help bring down my signal to ground (two resistors in parallel of about the same size would decrease the resistance in half) so i wanted to measure the resistance between the input and ground and then wanted to see what the resistance would be between the input and Vss(+) if I were to use a pull-up resistor as well (for another test). For input resistance between the input and VDD(+) I used a similar set-up as mentioned above expect instead of connecting my reference to ground I connected it to VDD(+) (3.3V). I am not sure if this is correct? Then for the resistance between the input and ground I realized that this would be the exact same as what I did for the input resistance of the entire multiplexer (first scenario). Therefore, I am confused as to how to measure the total input resistance of my multiplexer, the input resistance seen between my input and ground, and my input resistance seen between my input and VDD.