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Old 10th September 2008, 02:20 AM   #1
azone is offline azone  United States
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Default What type cable to use for double shielded driven guard?

I have an application where I have an audio input coming into a non-inverting amp with high gain adn high input impedance.

There is some capacitively coupled noise making it's way in and after being amplfied the signal-to-noise just takes a beating,

you can hear some noise/buzzing.

I have eliminated this noise by 99% by using a driven guard, where the shield is connected to the inverting input of the the

op-amp (which is ideally a the same potential as the non-inverting input). The shield at the other side of the cable, the audio

input, is left open. This works very good and is all fine and dandy but performance is improved even more if I run a second

shield and connect it to system ground at the audio input side (leave it open at the op amp input side).

My question is, as far as I know there is no such cable that has one (or more) conductors and two independent shields?

I've considered using a twisted pair cable with shield and using one of the pair as a shield but this is not ideal. IS there

such a cable that has two isolated shields?
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Old 10th September 2008, 07:12 AM   #2
Calvin is offline Calvin  Germany
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Location: close to Basel

www.sommercable.com offers under ´professional studio equipment´ such a cable, the SC-Galileo 238PLUS ,order-nr: 200-0271, 3.32€/m.
Under the name ´SC-Polaris Triax 8´and ´Triax 11´ they offer a video cable with insulated double shielding (no: 600-0313, 601-0353, 601-0353, 601-0303), ~7€-11€/m.
They offer excellent cables with very affordable prices.

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