I'm wondering what's the simplest way to find the value of capacitive coupling between a cable and the main power line (230Vrms @50Hz, I'm in Europe) to use afterwards in some SPICE simulations. I know that this kind of value depends strictly to the environment and the context, but this is not the point. In my case, I have two boards connected with some cables on which flow analog signals. On one side (the first board), the cable is connected to the output of an Opamp and on the other side (the second board) is connected to the input of another Opamp configured as buffer. The grounds and the power supplies are the same on both the boards. On the web I found this model for the cable (the component values are indicative)
where the PWR node represents the main power line, C3 and C4 the coupling with the cable and R3 his resistance. To find the capacitance values, I thought to use an oscilloscope and simply connect the probe to the cable under test, keeping this floating in the other end. In this way, if I know the input impedance of the oscilloscope I could find the capacitance. I made a drawing to explain the concept (hope it's clear)
I make two measure: the first with only the probe and I see on the scope the 50Hz interference which is about some mV peak-to-peak (Vo in the drawing); then I make the second with the cable connected to the probe and I obtain a greater interference of some hundreds of mV (Vt in the drawing). With these two measurements I can find Zc.
But now came my doubts:
- Is my method good for the cable's model I found? I think no, since the measurement are made on a floating cable which has a high impedance (oscilloscope) on one end and a 'open' on the other. In the real life my cable is connected to an opamp input (high impedance, ok) but the other end is connected to an opamp output (low impedance). Is my thinking correct?
- Maybe the most important question: is the model above correct?
What do you think? If you have any suggestions to find (approximately, of course) this coupling value, they are welcome.