Radiated Immunity; Resistance on input of op-amp?

Nov 13, 2007 2 Replies
K

Hi



Just held an internal review. They had placed resistors on the inputs of the op-amps to increase the immunity to radiated fields (problem at



200MHz)

Commonly 10k is used and the input capacitance of say 0.2pF gives a cutoff freq of 80MHz. Is this the reason or is it the protection diodes that draw less current and thereby avoiding SCR problem with the die? Another reason?



I have never seen this before - have anyone of you guys?



I would rarther have placed a cap close to the input to ground. A



100pF ceramic would work all the way up to 1GHz.

Regards



Klaus



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B

Only the right 100pF ceramic capacitors - we went for parts designed for microwave work with a porcelain ceramic.

And you seem to have forgotten the parallel capacitance of the 10k resistor, which - IIRR - is around 0.2pF. An input capacitance of

0.2pF is also unusually low for an op amp - just the pin to earth capacitance is usually higher than that.

-- Bill Sloman, Nijmegen

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J

It does help some but mostly this is done to avoid frying the substrate diodes in case the input line is zapped with ESD. If 200MHz is still an issue you could try ferrite beads right before the opamp, or at the point where the lines enter a metallic enclosure.

That's what I did in a case where we had cell phone "intrusion" into an opamp (gets rectified and messes up out low frequency signal). JFET input opamps are less sensitive to RF but in my case they aren't low enough in noise.

Regards, Joerg http://www.analogconsultants.com/
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