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I am designing a 6 wire LVDT. I need to know how to protect the signal quality of both the primary excitation input and secondary coil outputs inside the LVDT, and inside the device in which the LVDT is to be used.

The bobbin and coils are contained within a mu-metal container. The whole unit is contained within a stainless steel body.

The excitation signal has the following properties;

  1. Sinewave
  2. 500Hz
  3. 10mA rms constant current
  4. Nominal voltage of 2V rms

The secondary coil outputs have the following properties;

  1. Sinewave
  2. Same frequency as primary
  3. 2V rms at zero deflection, increases/decreases by 500mV rms at full-scale deflection.

There are two regions I need to consider; inside the mu-metal casing, and inside the stainless steel body. The external cabling is shielded and the environment should be considered noisy.

I have 3 questions;

  1. Should I twist together the coil pairs to reduce cross-talk? (I think yes as this is easy to do).
  2. Should I shield the twisted pairs? (I don't think so as they are effectively already surrounded by a Faraday cage).
  3. If so, where do I connect the shielding? (I have found much disagreement on this).

Thanks.

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(1) Should I twist together the coil pairs to reduce cross-talk? (I think yes as this is easy to do).

No, you should twist together the wires in each pair (all three pairs separately) and ideally, you might consider that the twist length for each pair is slightly different for all three pairs (in case two pairs align and induce a net emf into each other due to their respective currents).

This is common practice in cables containing multiple twisted pairs (individually screened or not).

(2) Should I shield the twisted pairs? (I don't think so as they are effectively already surrounded by a Faraday cage).

If it's a hassle then maybe not but, if the wires are in some proximity with each other and can individually vibrate or misalign with time, then screening them will help reduce anomalous error voltages appearing.

(3) If so, where do I connect the shielding? (I have found much disagreement on this).

This one is more complex to explain: -

  • Shielding is mainly to stop unwanted electric field interference from other wires. -The shield acts as a capacitor between the interference and the inner conductors and, you ground that shield at the most sensitive point of your circuit - usually receiver amplifiers.
  • Thus, in grounding at the receiver amplifier, you ensure that the voltage arising on the shield at that point is minimized at the receiver.
  • Thus, the common-mode interference that this might cause (if the shield were ungrounded or grounded somewhere else) is minimized.

But, in the presence of an interference that is magnetic in nature, there will be an induced emf along the length of the shield. And, of course, that same induced emf will be along the length of each inner conductor (whether twisted or not). In other words, the inner twisting ensures that the same induced emf appears in both wires.

So now you have to think about the "identical" CM voltage appearing at the receiver. You want it to be identical so that you can use a differential receiver amplifier and cancel out that CM interference caused by magnetic induction. And, to make it identical, you have to ensure two main things: -

  • The input impedance of your differential amplifier is impedance balanced - this is what we mean by a balanced signalling system.
  • The source-end of the signal you want to amplify is also balanced regards impedance.

If the source end doesn't have balanced impedances on both drive lines then, there is created a differential error voltage (between receiver inputs) due to the interference.

Where or when does the shield come into play with magnetic induced interference?

It's less of a problem so, certainly (and with some generality) it should be at the receiver end to reduce electric field interference (as mentioned earlier). Any specific examples may alter this advice to a certain degree so, regard this answer as a general statement.

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