Twin-T Filtering

Active Twin-T

Noise Reduction Through Driven Shield

This trick comes from this stackexchange answer. Very interesting, although it’s bending my brain to understand.

Basically if you have a weak differential signal you’ll use a differential amp to get rid of any common mode noise. Also, generally when working with weak/sensitive signals you’ll oftentimes have the actual circuitry at some distance from the sensors for various reasons. This creates a problem: how to get the signal to the rest of the circuitry. Shielding your signal wires helps prevent the long sensor leads from picking up common mode noise, but what if the common mode noise is present at the sensors? Adding a long grounded shield just creates a large capacitive load.

The solution to this is to drive the shield with the common mode noise component instead of grounding it. When you do this you get a “solid” shield connection that is riding the common mode noise, which leaves the weak differential signals free of the heavy capacitive loading that a grounded shield creates.

This schematic comes from this thesis paper (in German, saved locally for posterity). The fun stuff is in Chapter 8. A good place to pick up this common mode component is apparently the “middle” of the gain setting resistor of the instrumentation op-amp.

This technique is used in medical applications, and you can extend this technique of driving the shield with the common mode noise to driving the patient. EDN has an article on this. (also saved here for posterity). Example connection:

The LMC6001 is an opamp designed specifically for super low current applications (25fA max.) Bet it’s not cheap.

twint.png (13 KB) Andrew Kohlsmith, 04/25/2013 08:09 AM

twin2graph.png (12.5 KB) Andrew Kohlsmith, 04/25/2013 08:09 AM

picture456-1.png (48 KB) Andrew Kohlsmith, 04/25/2013 09:55 AM

98510984x.pdf - German paper detailing driven shield cabling (Chapter 8) (7.43 MB) Andrew Kohlsmith, 04/25/2013 10:04 AM

driven-right-leg.pdf - EDN article on driven right leg signal acquisition (339 KB) Andrew Kohlsmith, 04/25/2013 10:08 AM

drl.png (58.9 KB) Andrew Kohlsmith, 04/25/2013 10:10 AM

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