Hardening a simple op-amp circuit against EMI

Russell Warren · 2009-03-17T15:52:33+00:00

I've got a simple op-amp circuit that is misbehaving in an EMIenvironment. It looks like below (hopefully the ascii drawings workout)... ___10k_______ | | GND--------|\ 20k | \____|___--100----------|+/ |/The numbers are resistor values and the + input comes from a unitygain buffer.On first inspection my first obvious thought was to limit thebandwidth to avoid EMI issues (circuit only actually needs kHz'ishbandwidth) so I put a simple RC filter on the input as follows: ___10k_______ | | GND--------|\ 20k | \____|___--100----------|+/ | |/ 1u = | GNDBut this still had EMI issues and was probably bad because hanging a 1uF cap off the input is a bad idea? Not that I have any fundamentalbasis for making that statement.Consulting an EMC design book which has been *extremely* helpful inthe past (EMC for Product Designers by Tim Williams), one of thethings it recommends is putting low value resistors at each of theinputs, right at the pins. It also indicates that the values shouldbe no larger than a few hundred Ohms. ie: adding 200 Ohms to each pinof the circuit above: ___10k_______ | | GND----200-|\ 20k | \____|___--100------200-|+/ | |/ 1u = | GNDI'd like to understand what is going on here. My questions are:1. Is hanging a large capacitance on the input of an op-amp a badidea? If so, why? I saw some rumblings somewhere about CMRR issues,but don't get it.2. With the op-amp having a high input impedance, what is the reasonfor limiting the resistors at the input pins to "up to a few hundredOhms"?I'm trying to simulate the impact of it all in Switchercad, but I'm ahack and my attempts have not been that illuminating. I've spent sometime trying to work out how to simulate and understand when anamplifier circuit is stable or not, but haven't been able...

Mar 17, 2009 88 Replies

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Clifford Heath 17 years ago

I use a nice little foxhunt sniffer made by VK3YNG which is remarkable for the incredible dynamic range in its front-end (quite a sensitive lo-noise receiver, but you can still DF when you're standing on 100W of RF transmitter, it doesn't swamp).

Anyhow, he found out the hard way that a commonly recommended decoupling method, namely a small cap closely paralleled with a larger one, often causes major problems when the small C resonates with the inductance of the larger one.

... just one case in point.

If anyone has an idea how to build a front-end like these have now that the dual-gate GaAs FET is unobtainium, I'd like to hear it.

Clifford Heath.

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JosephKK 17 years ago

Speaking for yourself i see. .

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bill.sloman 17 years ago

Sure. I don't know a lot of things that Jim Thompson believes to be true. The ignorance of the real world invoved is entirely Jim's.

-- Bill Sloman, Nijmegen

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Phil Hobbs 17 years ago

Colman's mustard tins. I made an arc lamp out of one once.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net

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