SaM - Driven Shields or Tri-Axial Cable

Remeber:

To reduce electrical noise we can shield our circuit, using a coaxial cable However this does not solve the problem of having a long cable. To solve both problems we need a tri-axial cable:

• In this cables we have:
  • An internal shield called guard that surrounds the wire.
  • An external shield which is usually grounded.
  • In this types of cables the guard is kept at the same voltage as the wire.
• The various coupling capacitances of the tri-axial cables are:
  • $C_G$ : Guard capacitance between wire and internal shield.
    Tho since wire and internal shields are at the same voltage, this capacitance is empty of charge, so the special property of tri-axial cables is that:$C_G=0F$
  • $C_2$ : Second shield capacitance, this has charge, so its value is different from $0$.

Let’s draw better the circuit:

• The sensor will see a capacitance $Z_i=C_{eq}$ equal to:$C_{eq}=\frac{C_G}{A+1}$
• I will need an amplifier which has a large gain.
  But remember that now we are operating at AC obviously, so it has to use large frequency because of the small value of the capacitance of the sensor ($C_S$).
  Usually we need to operate, around the operating range (depending on the value of the sensor) of $100\text{kHz}÷2\text{MHz}$.
  ⇒ So we need this open loop gain to be $\sim 100$ or $1000$, at around $1\text{MHz}$.
• So in conjunction to tri-axial cables we need wideband operational amplifiers.
• The last thing we need, since we have a variable signal, is a filter to remove the drifts at low frequency.

I write here the equivalent we find at the end for the tri-axial cable system:

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Memory Card

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So if we need a long cable, we need to use tri-axial cables:

• In this cables we have:
  • An internal shield called guard that surrounds the wire.
  • An external shield which is usually grounded.
• In this types of cables the guard is kept at the same voltage as the wire.
• The various coupling capacitances of the tri-axial cables are:
  • $C_G$ : Guard Capacitance between wire and internal shield.
    Tho since wire and internal shields are at the same voltage, this capacitance is empty of charge, so the special property of tri-axial cables is that:$C_G=0F$
  • $C_2$ : Second Shield Capacitance, this has charge, so its value is different from $0$.

Let’s draw better the circuit:

• $C_G$ is the guard capacitance.
• $C_2$ is the outer shield capacitance.
• So the idea is to have changed this input resistance here ($Z_i$), so to have a reduced the effect of the capacitance of the cable.
• You see that the feedback, keeps the two terminal $+$ and $-$ at a very very close voltage with respect of one to the other, so almost a short circuit, no current flowing there.
  So we don’t have any effect in this case of loading and reduced sensitivity on the voltage divider in this part.

So:

• $C_{eq}$ is the capacitance seen by the sensor.
• I will need an amplifier which has a large gain.
  But remember that now we are operating at AC obviously, so it has to use large frequency because of the small value of the capacitance of the sensor ($C_S$).
  Usually we need to operate, around the operating range (depending on the value of the sensor) of $100\text{kHz}÷2\text{MHz}$.
  ⇒ So we need this open loop gain to be, I don’t know, $100$ or $1000$, at around $1\text{MHz}$.

So in conjunction to tri-axial cables we need wideband operational amplifiers.

The last thing we need, since we have a variable signal, is a filter to remove the drifts at low frequency. There are still other problem, but at least we can the sensor even if we have a long cable.

I write here the equivalent we find at the end for the tri-axial cable system: