I have a problem concerning the AC analysis of a simple circuit, containing variable resistors (each variable resistor is a pressure sensor). I am sure this is quite simple, but I have never worked on variable networks and I cannot find answers by searching on the web.
R1, R2, R3, and R4 are the variable resistors, each one corresponding to a pressure sensor. Their resistance varies with the applied pressure, which is a stochastic process (trackinf of the hand pressure). My goal is to track in 1D the pressure along the sensor, thanks to the multiple pressure areas.
I know the circuit is bad, poor dynamics, etc. but I need a very compact and cheap circuit that can find the value of the resistors R1 to R4 by measuring output voltages via an ADC. The simplest way I have found is this one, where I put equal resistors R between the ground/power voltage and each variable resistor. By doing this way, I can measure the output voltages V0 to V4 and find the resistors by a quick circuit analysis.
As a result, I just want to measure the pressure with not so much noise to be able to conclude if the work I am doing require more investment or if this is useless to spend more time on it.
The resistors R1, R2, R3 and R4 are variable. I want to filter the output voltages (V0 to V4) as they don't see variation in voltage higher than the nyquist frequency of my ADC converter. I can sample at 50kHz, but sending my data to the computer take time and I need to filter out frequencies above 500 Hz. For this purpose, I would like to use a passive filter to remove frequencies above 500Hz and then use a digital filter to remove everything above 500Hz.
So I have two ways of doing this passive filter:
- Finding the matching R and C that will reduce the impact of the variable resistors and filter close to 500Hz in any case. I think it is problematic as R1 to R4 are time-varying and are a stochastic process;
- Finding a way to reduce at max the output impedance of each output and use a RC filter in cascade with each output. I would like to not use an Op Amp;
AC Analysis of the circuit
Can I consider the circuit as multiple two-ports networks in cascade (simple RC filter)?
Vd is my power voltage, all resistors R and capacitor C are equal.
I thought it will be possible to model the circuit like this, since the charge of the capacitor provoked the transient. Note that I didn't include V3 and V4 to clear the scheme, but I obviously should.
Do you have any idea on how I am supposed to perform the AC analysis? How can I find the bandwidth of each output Vx in response to the change in pressure at each pressure sensor Rx?
How do I tell Spice that, say, R4 changes between resistance Rlow and Rhigh in a square wave that ramps up in frequency from 10 Hz to 10 MHz? Thank you so much for your help!