In my last job, we had a problem with the 200Hz TDMA noise from the
2.4GHz transmitter getting into the audio system. We went down the rad-hard path but the answer was CMOS opamps. It took a while to convince managment to spend the money (though I use some really cheap CMOS opamps that are available now).SOIC with the pins bent down? ;-)
At one client switching to a CMOS opamp fixed a problem they had scratched their head about for months. It took about 15 minutes. That wasn't the lowest cost consulting gig though because it required almost a whole day of travel on my part. But it did result in some dropped jaws.
:-)
Spehro had the right smeller and suggested the TLV27 series. It looks like that will fit the problem nicely. Strangely, when I looked for opamp + CMOS +>15V that one never came up anywhere.
The other option is dropping VCC to 12V or so which opens the door for more amps but that would require other changes along with it.
Yeah, this problem came up suddenly. After the product had been in production for a year, production stopped. It took a while to figure out that it was a new reel of opamps. The junk they were using was from NJR (cheap) and evidently they changed the process without telling anyone, increasing the Ft above 2.4GHz.
Including CMOS R-R opamps.
It usually isn't the Ft. Any old bipolar process can present that problem. RF sneaks in through the wires, leadframe, whatever. Then a wee bit of it gets rectified at the BE junction of the first pair, slightly asymmetrical. Since this hits at full open loop gain it can show up as a fat signal at the output.
All it takes is changing the layout a bit and things can become worse than they ever were. GSM is very prone to create the effect because for whatever reason it begins negotiating with the cell tower at max power.
Many of those are out because they aren't properly abs maxed for safe
12V operation. Can't go that low.
Sure, the 200Hz TDMA'd 2.4GHz being rectified into a 200Hz audio signal. A lower Ft device is going to be a crappier rectifier at frequency.
No changes, other than a more recent reel of opamps. They were attacking the problem everywhere except the opamp, including some expensive shielding (that I made use of when they couldn't pass ESD tests).
Many, sure, but not all. Microchip has a reasonably nice (and
*cheap*) one that will do 12V.
My usual approach to ST was via the Farnell/Newark/element 14 web-site.
If a broad-line distributor doesn't stock the part, there's not a lot of point in negotiating the manufacturer's webs-site, and Farnell does usually link you to the data sheet if they stock the part.
Their supply voltage selection didn't work, but this part is rated for +18V max
Out here in the Wild West we tend to check Digikey and Mouser, then a few high volume distributors such as Arrow and Avnet. Digikey has the only search engine that really works for my kind of job so that makes it my preference.
Thanks. Looks like a 2nd source to the TI part above. However, the datasheet isn't very good. For example, no hint about the output low level compromise versus CM voltage which this device normally should also have. Also, AFAICT no mention of the functionally guaranteed input CM range in the tables. That stuff is important. It'll probably be ok but I prefer parts where there is published hard data (in the tables, not typical values in graphs).
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