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Old 17th August 2015, 02:44 PM   #1
JAY X is offline JAY X  Spain
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Default Opamp Bandwidth limiting and RFI

Hi All!

Is there a relationship between opamp bandwidth and RFI susceptibility?

I try to limit the bandwidth of my opamp circuits to 100khz max 300khz. As with low impedance design this means to place a quite high value feedback capacitor (470pf) across low feedback resistors 1k or 2k2 or 3k3 in some circuits, (transformer buffer) for example, even if the original schematic uses a lower size capacitor.

I use normal bipolar opamps: NE5532, Lm833, or fet models like OPA2134.

Thank you very much!

JAY X
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Old 17th August 2015, 03:08 PM   #2
udok is offline udok  Austria
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The opamp can not amplify RF (at least if you do not choose a super duper fast one).
Even if the opamp would amplify RF it would not matter as you can not hear it :-)

What happens is:
At RF frequencies the opamp is a pile of nonlinear PN junctions.
RF is demodulated at these nonlinear junctions and down-mixed to audio frequencies.
The same principle applies to AM radio detectors or in the IMD distortion test.

Now the down-mixed RF is treated as a normal audio signal which is amplified as designed by your circuit.
So, opamp bandwidth has little influence (at least for RF > 1MHz).
A better strategy is to place an RF LC-filter on each input and output line.
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Old 17th August 2015, 03:16 PM   #3
jcx is offline jcx  United States
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rectification/detection in the input stage is most often cited as a problem
jfet input are often recommended as relatively RF immune
higher bias current linearizes bjt diff pair even for RF signals
large junction C will short out RF too

Bob Pease letters had someone claiming a audio power amp demodulated the nearby AM transmitter antenna farm's signal when powered off

Quote:
Originally Posted by jcx View Post
one place I worked you could see a Raytheon plant's radar dishes pointing at us, the target tower shacks were ~ 50 yards from our 2nd story wood frame building's electronic lab

some days it wasn't worth trying to debug anything - one afternoon a sub MHz low power CMOS op amp integrator was spontaneously resetting every minute - turning up the 300 MHz 'scope sweep to max the "reset" trace was still vertical
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Old 18th August 2015, 08:31 AM   #4
JAY X is offline JAY X  Spain
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Hi!


Now i see more clear! ok, so it is more important proper input /output filtering than the closed loop bandwidth... ok.


Another question that worries me, is about trace length in between opamp stages. Due to my layout, (summing mixer) i use to place the circuits near the related connectors (diff amps, line drivers, summing amps), and then connect each stage thru traces.... that become a bit long (15 to 20cm).

My PCB is 300mm length by 145mm height. The ground plane (two layer board), is UNCUT. So, maybe with LC filters at the opamp output or a resistor (47-100R), trace length will be less susceptible to RFi?? (apart of using good shielding techniques, pin1 to chassis etc..)

Thank you very much for your help!
JAY X
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