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Old 14th July 2007, 10:49 AM   #1
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Default Input Noise Guarding of OPAmps

What's the best way to make Input Noise Guarding of OPAmp input circuits ??
Many uses Ground Plane, but BURR-BROWN recomends this:
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Old 14th July 2007, 01:10 PM   #2
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That kind of guarding is usually for those that use cheap boards and want high impedance inputs (Teflon boards cost about $1 a square inch last time I checked).

I had to use a guard ring on a low frequency spectrum analyzer integrator cap, otherwise the charge would bleed off too fast.
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Old 14th July 2007, 01:15 PM   #3
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Jan, this technique usually has nothing to do with noise reduction as applied to audio. Basically, the idea of the guard is to prevent leakage currents on the PCB from upsetting the DC performance of the op-amp. The guard ring is tied to a low source impedance which is at the same potential as the inputs. This can be accomplished by buffering the feedback voltage divider with a second opamp. The guard ring intercepts any leakage currents arising from delta voltages elswhere on the pcb .

Note that or this echnique to work, a double ided PCB is required (if using more than 2 layers, GR is required on every layer).

Might reduce very low frequency leagage noice in an mc amp, but I only have (good) experience using this technique on thermocouple amps.
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Old 14th July 2007, 01:16 PM   #4
Bonsai is offline Bonsai  Taiwan
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good point from djk - giard ring is absolute must for integrator applications - usually in dual slope A-D applications
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Old 14th July 2007, 03:08 PM   #5
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Thanks all

I was just wondering....
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Old 14th July 2007, 04:09 PM   #6
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to prevent leakage currents I bend the input-pins and wire them free air.
But thats not audio
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Old 14th July 2007, 11:02 PM   #7
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Quote:
Originally posted by juergenk
to prevent leakage currents I bend the input-pins and wire them free air.
But thats not audio
regards
called "dead bug" -- popular in low power ham radio applications as well.
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Old 15th July 2007, 03:58 AM   #8
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I try to make use of sockets as much as possible
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Old 15th July 2007, 04:37 AM   #9
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Hi ACD,
If you need to go to that extent, air currents can generate much more noise also. One of those things. Thermal drift is also noise, and once again air currents can really mess things up.

-Chris

Edit: Hi juergenk,
Sockets tend to make everything much worse. From capacitance to thermocouple effects. Many more paths for leakage too!
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Old 15th July 2007, 05:02 AM   #10
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Hi Chris,
thats not an audio circuit, the (+)input-pin is bend up to preserve the GOhm input impedance and avoid leakage currents.
The socket doesn't hurt in this special application.
Guess the hints in the BB datasheet weren't meant for audio circuits too.
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