Electronic – Effect of cable capacitance to low frequency analog voltage signal transmission

cablescapacitorsignal integrity

Regarding sending a low freq. (<150Hz) analog voltage signal with CAT6 STP cable I have heard that: if the frequencies down the cable will be low, there may be an issue with it driving a capacitive load (the cable inner to screen capacitance) in which case a buffer amplifier may be needed at the sensor end.

1-) Why might the capacitance of the cable be a problem for low freq. analog voltage signal transmission? Can this be explained theoretically or with a circuit model?

2-) How come a buffer can fix this?

Best Answer

The lower the frequency you go, generally there are fewer problems but, there is one notable exception; that being when a cable is used for two-way telephony (more later).

A cable is a transmission line and it has four notable parameters: -

  • Capacitance per unit length
  • Inductance per unit length
  • Resistance per unit length
  • Conductance per unit length

These four parameters are used in t-line analysis to predict the characterisic impedance of the cable: -

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Picture source.

This formula is then modified for RF by assuming that jwL is much greater than R and that jwC is much greater than G: -

$$Z_0 = \sqrt{\dfrac{L}{C}}$$

So typically for 250 nH per metre and 100 pF per metre, Z0 is 50 ohms (do the math!).

As frequency drops the characteristic impedance takes on a new form: -

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Picture source.

At mid audio the two parameters that dominate are R and C hence Z0 becomes: -

$$\sqrt{\dfrac{R}{j\omega C}}$$

Notice the dotted line on the graph - this is at 600 ohms and it is the nominal impedance used by telephones to obtain what is known as minimal sidetone. Sidetone is not wanted - it's the audio you can hear in the earpiece when you speak into the microphone - this is needed to be low in telephony else it can become an annoyance and some signalling properties are reduced (DTMF dialling tones can be misinterpreted for instance).

if the frequencies down the cable will be low, there may be an issue with it driving a capacitive load

Not usually unless your application is telephony but, of course, if your driver is weak you should use a buffer. The important thing with signalling over cable is that it has good resilience to noise and you generally use STP cable when you have an impedance-balanced drive signal.

So, if your driver is "weak" its impedance may be unpredictable and your whole system becomes susceptible to external noise.

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