Hardening a simple op-amp circuit against EMI

It shouldn't be. Much below 26mV and the input stage is linear, so what the collector current gains on the highs should be exactly cancelled by what it loses on the lows. Once you get above the 26mV region, whichever of the two transistors is turned on is pretty much fully turned on, and the other one is close to off, and - for NPN transistors - the input starts following the upper envelope of the incoming RF, rather than its average. If you've managed to set up a resonant circuit for the cell-phone frequency around the op amp inputs, you may see higher voltages at the inputs than you'd expect.

Seems unlikely.

Absolutely not. Cell phones used to use an 800MHz carrier - I think that this has now gone up to a few GHz. Very few op amps have any gain left at these frequencies.

--
Bill Sloman, Nijmegen

Sure. They tin easily and they're Curiously Strong(tm). Clabber Girl baking powder cans too.

Cheers, James Arthur

It has nothing at all to do with gain. What happens is this:

RF gets in, GHz range. The amount of RF current present in the IN+ region is not 100% the same phase/amplitude as in the IN- region. Now some of that gets rectified in the BE junctions. This rectified amount is what the opamp sees as a real input signal because, in the case of GSM and other cell phone system, the bursts are spectrally very low in frequency. RF on, RF off, RF on, and so forth. Once this stuff is in there, there ain't nothing you can do to get rid of it if it's in your band of interest.

--
Regards, Joerg

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Watch your stomach when eating all those Altoids. They are delicious but some of them seem quite strong. I liked the Fishermen's Friends cans but that stuff is really potent. I remember someone who took one and then fell asleep on the couch. He was so tired that he woke up the next morning. One cheek was completely numb, just like the Novocaine from the dentist would do.

--
Regards, Joerg

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Do you use stake pins as feed-throughs ?:-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

Horrors! Don't you know that stuff is mostly cornstarch?

(Dan sez hi; he's in the conference room digging through old submetering drawings.)

John

Argh. You are absolutely correct... that is a consequence of a last minute edit to the drawings that I obviously screwed up. Sorry about that and thanks for clarifying.

Since I completely messed up the drawing (in more ways than that), let me do it more completely and accurately...

GND -10k---+------20k------+ | | ------ +--------|\\ | | | |\\ | \\____|___ | DAC|-----|+\\__100_______100_________|+/ | | +--| / | |/ | | | |/ | TS921ID | | | OP727 | |____| |____________|

There were several informative comments about BJT op-amps being more sensitive to RFI. The OP727 is listed as having a bipolar pnp input stage and CMOS output stage, and the TS921ID is listed as BiCMOS. Where is the "bi" split in BiCMOS? Is that the same as the OP7272? ie: BJT input stage and CMOS output stage?

The info on reasons for RFI affecting the op-amp were particularly useful. I still have some confusion as to the impact of RFI pickup at the op-amp inputs, but I'll save that for another post.

Regarding the capacitive load on the OP727 follower which I previously said was a unity gain buffer... the data sheet says it can drive up to

500 pF loads. I always hear that the 100 Ohm load in the feedback path helps drive capacitive loads (but again don't fully understand why). With the addition of the RC filter in the circuit it actually looks like below.

GND -10k---+------20k------+ | | ------ +--------|\\ | | | |\\ | \\____|___ | DAC|-----|+\\__100_______100_________|+/ | | +--| / | | |/ | | | |/ | 1uF = TS921ID | | | > It should have worked if the high-frequency interference was coming

Is the 1 uF load still a problem >> 2. With the op-amp having a high input impedance, what is the reason

Interesting... thanks. When you say the resistor is causing a delay, do you mean the phase change resulting from the filter created by the serial resistor and the input capacitance?

Also - if it is delay in the feedback loop which is of concern, what would the reason for limiting the resistor value directly on the + input pin?

I've had trouble f> > I'm trying to simulate the impact of it all in Switchercad, but I'm a

The device is in a temperature chamber and I see problems at certain points in temperature cycling. I expect it is EMI kicking out of the compressor when it is on.

The nature of the bad behaviour is hard to describe because we are picking it up very indirectly and subtly in another measurement. It is not a huge pickup, it just affects our signal of interest (a low bandwidth LC reading elsewhere) by < 1%, but this is is quite significant in the application. I do not see it directly in that op- amp because it is hard to get to and measure in the setup. However - we do know the root of the problem is in the circuit above (by elimination). We have not yet reproduced the problem in a setup where it is easier to both induce and measure the problem.

Thanks... I'm certainly looking at better decoupling on the supply rails and what you say is good advice to keep in mind for the future.

Now - from lab testing yesterday (where I was rather than responding to the thread) the following circuit change eliminates the observed problem on a few units.

GND -10k---+------20k------+ | | ------ +--------|\\ | | | |\\ | \\____|___ | DAC|-----|+\\__100_____12k7_____100__|+/ | | +--| / | | |/ | | | |/ | 10nF = TS921ID | | | OP727 | | |____| |____________| GND

The 12.7k + 10 nF RC filter was added to add some bandwidth limiting to more what the circuit actually cares about, and the 100 Ohm was added to the TS921ID input pin (pin lifted and stuffed under) based on the general RFI recommendations, filtering direct-to-pin pickup with the input capacitance of the op-amp.

I know that although it fixed the observed problem, it was hardly a full EMC test, so it isn't where I'll leave the circuit, but that was all I did due to time constraints and the joy of hand soldering 0402 SMT components. The circuit still needs more investigation and testing.

eg: I still have to determine if the 10 nF will be a problem for the OP727. It seems like not based on performance, but it is obviously greater than the 500 pF OP727 spec and I don't know if the 100 Ohm feedback resistor actually fixes it or why, as stated above. Although it performs ok, is it surfing the edge of stability?

Thanks to all for your help. The goal of my post wasn't to have my problem solved for me, but more to get a better understanding of what the heck I'm doing and why. All the posts are certainly helping and are much appreciated.

Russ

On late-night PBS here, there's a show about hobbyist glassworking, and one of the show's sponsors is Weller. ;-)

Cheers! Rich

Oh not to worry--I don't EAT them. My Mom does & gives me the cans. Win-win.

That's a timely tip. I broke a molar Thursday, crowned yesterday.

Grins, James Arthur

The final crown? That's pretty fast. My last one was around $800 without root canal :-(

--
Regards, Joerg

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I just sort of make up whatever's handy and appropriate--

- 1/4w resistor leads thru teflon tubing for routine stuff, usually soldered/anchored to a PCB within, - add ferrite bead and 2 bypass caps to ground for others, - a 1/4" chunk of hardline coax makes a nice teflon-insulated feedthru, - actual, official feed-thru caps when you really need them, - good old turret ground terminals for ground connections, - or official SMA rf connectors if need be.

The casing's soft, so you can just punch through it with an awl.

There's something neat about an enclosure that's all solderable ground plane, 100% shielded, pops open easily for access, and smells good when you solder it.

Cheers, James Arthur

Gaaack! _Now_ he tells me!

Hi Dan!

James

No, a temp, and $1,400. $1k for the crown, and $400 for the prep complicated by the lost cusps. She screwed a couple Ti pins into the tooth--cool. I didn't know they did that.

James Arthur

$1400? Yikes! I assume that includes the final crown but it's still very expensive. Probably there comes a time when it would make sense to travel to some foreign country, have major dental work done there, and use the savings to tack on a nice vacation there.

The screws may have been a dowel, that would be pretty much standard procedure if the crown needs it for support.

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It's the first time I needed Ti. As far as the $1k for the crown, sounds high to me too, but that's the standard quote.

That's another side-effect of government payment schedules--it coordinates the price of all the providers.

James Arthur

Hmm, since your address is Verizon I assume you are in the US. AFAIK dentist charges are not regulated by the government. All I can tell you is that it's a whole lot less expensive out here (east of Sacramento), or at least was when I got my last one about a couple years ago. Still painful though because as a self-employed I must pay 100% out of pocket. Luckily we have HSA now so it's pre-tax. But post SE-tax :-(

--
Regards, Joerg

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You are correct. Even Medicare does not cover dental work.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

The charges aren't regulated, just the payments.

If companies do this, it's called 'collusion' or 'price fixing,' and it's illegal.

Last time I shopped I got three quotes, all identical.

C'est la vie

Cheers, James Arthur

Ok, I don't know about Medi-Cal but once when a new dentist moved into town we went there and requests pricing for certain common treatments. He was more than 15% higher than our current dentist so we did not switch.

Now, with propane companies, that's a whole 'nother matter. Last time I compared they were "miraculously" all pretty much the same.

--
Regards, Joerg

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