Radiated Immunity; Resistance on input of op-amp?

[Klaus Kragelund]

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

 

[[email protected]]

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.

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.

 

[Joerg]

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?

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.

I would rarther have placed a cap close to the input to ground. A
100pF ceramic would work all the way up to 1GHz.

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.