Hardening a simple op-amp circuit against EMI

This is not a useful amplifier. It's fully open-loop. Did you really mean this?

That would be a gain of three amplifier. Certainly much better behaved.

Jeroen Belleman

The ASCII art doesn't seem to have worked as it should have. I can only magine that you meant

GND -10k---+---20k-----+ | | +----|\\ | | \\____|___ --100----------|+/ |/

Which is a straight-forward follower with a gain of three.

It should have worked if the high-frequency interference was coming from the output of your unity gain buffer, always assuming that the unity gain buffer was up to driving a 1uF load; most of them won't like it.

It shouldn't be problem for the op amp y0u are looking at, but may present a problem for the device driving that input.

The input impedance of an op amp can be seem as a high value resistor in parallel with a couple of pF of capacitance to ground. Your extra resistors adds a delay around the feedback loop. If you limt the resistance to a few hundred ohms, the extra delay is less than a nanosecond, which is negligible. Higher resistance mean more delay, which can wreck the stability of the negative feedback loop.

Your problem is that you need to understand how your electromagnetic interference is getting into your op amp and producing the "misbehaviour" you are complaining aobut on the out of the amplifier.

It would help if you told us what the misbehaviour looks like, and what the electromagnetic interference you are complaining about consists of.

I've seen the audio signal from local radio stations on op amp outputs ...

The usual rules of thumb are to keep the connections to the inputs of an op amp as short as possible, and to route them over a solid ground plane if this is at all possible.

You have to keep in mind that noise on the power supplies to an op amp tends to appear on the output - more at high frequencies than at low frequencies. The op amp data sheet should include a plot of the Power Supply Rejection Ratio against frequency for both the positive and the negative power pins. I've put low pass filters on both power pins of fast op amps to deal with this problem; in one case I tried to get away with just a ferrite bead and a capacitor, and managed to set up a resonant circuit, which I ended up critically damping with a small resistor in series with the bead - ferrite beads are supposed to be lossy inductors, but mine wasn't anywhere near lossy enough at the frequency in question.

-- Bill Sloman, Nijmegen

Is is conducted noise, capacitively coupled noise, RF pickup noise or magnetic induction noise?

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

Did you know you can place a (small) bypass capacitor between the + and - inputs of an OpAmp... without much, if any, effect on the GBW? Now you do ;-)

Discovered that probably 30+ years ago while debugging a noise pick-up problem PCB at Interface, Inc.

...Jim Thompson

Let's say it's magnetic. I think using the lowest component impedances possible helps if it's magnetic induction noise (I think it's called that.) Example: IF the op amp is next to a leaky transformer.

An induced noise current will yield a noise voltage of I(Z).

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

Why say that?

...Jim Thompson

Ok, Russell, I won't be able to see your posts and replies because you use gmail (maybe use a better address?). But I want to alert you to something that is rarely known in the trade:

Opamps with bipolar transistors in the input can cause RF to be rectified. A cell phone transmitting in the GHz range can cause that trouble even with an opamp that has only 10MHz or so GBW. This is because the BE junctions at IN+ and IN- rectify RF and that rectified signal shows up in your signal. With GSM you'd have pretty hard "rat-tat-tat" style bursts when the phone connects to a new tower or checks in once in a while. CMOS opamps suffer much less from this behavior because there is no BE junction that is slightly conducting, there are usually only substrate diodes that are sufficiently reverse biased (as long as your input swings stay withing the rails).

With strong EMI none of the external methods such as ferrite beads and capacitors help much. I had one such case where the EMI went straight through the SO-8 plastic smack dab into the chip. So I recommended a CMOS opamp to that client or, as an alternative, shielding. We then tried both and both fixed it. Since they really liked the BJT-based amp for audio noise level reasons they opted for shielding.

"Small" relative to ... ???

Jim

I had that on my truck's radio with my old sprint cell phone.

Magnetic induction is just a possibility. The OP didn't mention what this circuit is next to.

For all the circuits I've dreamed up, power supply noise or crappy mixed mode PCB layout was the problem.

Fixable with filters and/or better routing.

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

Try sizes until you get what you want ;-) Say 100pF.

It CAN be calculated. This is left as an exercise for the student ;-)

...Jim Thompson

Try a different opamp maybe. Jfet and CMOS opamps are usually much less sensitive to rfi pickup than bipolars. The bipolar diff input pair tends to rectify.

I recently saw an ad for an opamp that's specifically EMI hard, but I can't recall whose. LTC maybe?

John

Top-Post Repaired On Tue, 17 Mar 2009 11:39:46 -0700, RST Engineering (jw) wrote: Lack of Attribution Repaired:

Large?

And don't top post, and leave your victim's "Xxxx wrote:" line in.

Rich

The only BJT-input opamps I remember being quite EMI-proof were those in metal TO-cans. But that has kind of gone out of fashion ;-)

Back then I needed that, since about 1-2 feet from the respective gear was my kilowatt-size RF amp, often without the lid and then a Pringles can size inductor would be looking at you. So I used uA709 in TO cans since in our neck of the woods the uA741 only came in plastic dip. This was a few decades ago when copper foil was unobtanium.

The DIY stained-glass-supplies shops have all sorts of cool stuff, like solderable sticky-back copper foil of various widths, and some amazingly klutzy soldering irons.

John

in

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me

You can express this more formally, by pointing that the long-tailed pair at the input of a bipolar op amp goes non-linear if the voltage between the inputs goes above about 26mV.

At low frequencies - within the op amps bandwidth - negative feedback stop this from happening, but at higher frequencies, there isn't any feedback.

The long-tailed pair on the input of a FET- or MOSFET-input op amp goes non-linear in the same way, but only at signal levels around a volt.

It explains the rectifier action ...

-- Bill Sloman, Nijmegen

You could have issues with the type of OP-AMP you're using.. bipolar type op-amps in some environments will actually act as R.F. detectors. If the EMI is surrounding the chip boundary, chances are you won't get rid of it. If you can test this circuit with the input disconnected from the remote source and shunted to common with an R to verify if this is really happening or not, it would be a great help.

Using a MosFet type of Op-Amp on the front end many times alleviates the problem.

Have you tried shielding around the circuit and employed some bypass caps?

In Europe they were called Tiffany lamp shops. But the pricing in those places was far beyond reach for someone in high school. But grandpa gave me a big stash of copper and I thought I'd be in heaven. From then on I could make shielded enclosures without having to convince mom that we all "needed" a can of butter cookies. Because then she make fresh ones, no can.

Happens much earlier. Some hundred microvolts from a cell phone and your low noise pick-up circuit is toast. Rectification happens the instant RF hits a forward biased diode path such as a BE junction of a properly biased BJT amp stage, whether diff or not. Ye olde LM324 and his brethren can't regulate that away via feedback.