From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-01-31 17:13:11
Dear Andreas:
I know you love the idea of a driven shield and I have not tried "very" hard to talk you out of it. Take a look on my webpage at my input stage and you will see the shield drivers that I designed. This is where theory and the practical real world collide. I found that the driven shield concept does not work (for this application). I tried it both ways. The RF from theradio station and equipment switching glitches are not attenuated. The output impedance of the op amp is not low enough at these frequencies. In your case you even have a 220 ohm resistor in series to make matters even worse. The shields would actually drive noise into your groundplane under theinput resistors and right into the input stage.
I am not impressed with simulations done on a computer. The real world does not care what you simulated.
If I were to use your board, and I certainly want to, I would first rip outall the driven shield components and tie J100 pin 3 to ground. C100 and C101 are already on ground. The rest looks good. (BTW I would not use the filter section either)
Could you make these little input boards first and test them? That is by far the most important step. After that everything is much less critical.
Juergen P. (Jim) Meissner
Check out my Website at www.MeissnerResearch.com
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