Favourite low offset op-amp

"Jim Thompson" schreef in bericht news: snipped-for-privacy@4ax.com...

Since he's kill-filed me, he'll never know.

Haven't we all? Win has got a couple of pages on lock-in detection in "The Art of Electronics" - it isn't exactly rocket science.

--------- Bill Sloman, Nijmegen

Well if we're going to play, "Can you top this"... my MIT BSEE Thesis (1961-1962) was done in cooperation with Harvard Med School, Peter Bent Brigham Hospital (*) EKG Laboratory, with Doctors Gorlin, Hood, Krasnow and Rolett, "Thermistors as Blood Flow Rate Transducers", and featured a discrete chopper-stabilized operational amplifier made out them thar new-fangled things called transistors ;-)

(*) Now called, I believe, Women and Children's ??

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

...and your cutters of "farwoood" . As long as we're being complete, what about them aeroplanes with wings that go roundy-round (the ones banned from Italian airspace ;).

--
  Keith

It wasn't exactly a new technique even then. My boss a George Kent (1973-6) was proud of having replaced the relay choppers in the George Kent chart recorder amplifiers with FET choppers.

The relays had - in their turn - replaced an ingenious mechanical auto-zeroing system which was even slower.

---------- Bill Sloman, Nijmegen

Pye made a pH meter with a mechanically driven varicap (like a tuning fork) and an EF37A as the input valve (tube).

The input impedance was as near open circuit as a glass insulator can be and the accuracy was totally dependent on the loop gain and the super-high-stability resistors in the DC-coupled feedback loop.

It was a brilliant piece of design and could be configured with wire links for different gains and offsets to suit various electrodes and probes.

We had one still working in the labs when I left about 7 years ago.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

Hello Keith,

And before that it described the folks that did what I just did: Chopping the firewood for next winter. Or rather "farwood" in mountain speak.

Regards, Joerg

Hello Graham,

Yes, although as said before the chopper concept also applies to certain modifications of Harley Davidsons. And according to Keith also to the third set of teeth.

That should be no big deal either. A very long time ago I needed that, too, but at several MHz bandwidth. The project was on a tight BOM budget (ain't that always so?) which is why I used a uA733. Everything else either hadn't been invented yet or was way out of budget. The uA733 was cheap and fast but didn't have any DC stability to write home about. So I clamped differentially, using a FET.

The only downside was a wee charge injection. It wasn't much because most of that went common mode. Still, I compensated for it by injecting a similar opposite charge.

Regards, Joerg

Hello Spehro,

That is one of the marketing concepts I never really understood. I can't even count anymore how often I have designed out such parts because they were initially too expensive. Once the design-out process was completed and tested the door went shut for them for good.

Switcher chips were probably the majority of chips I designed out, often replacing them with a mundane sub-10c logic chip. Much of this is still in production, likely never to be touched again.

Regards, Joerg

Kent Instruments ? I recall the name.

Graham

The old tube-type Fluke differential voltmeters used vibrator choppers and were very good. The worst part was the gas-tube voltage reference, which you had to tweak against a standard cell fairly often as it drifted around.

That reminds me, I should buy one on ebay for my collection. Nice boxes with mirror-scale meters and lots of big fat knobs.

John

When I saw that one Beatle movie where one of them puts a Coca-Cola bottle up to his nose, I was so young and naive I didn't even get the joke. :-\\

Cheers! Rich

Some of the older flight simulators I used to work on 1965 - 70 vintage still used mechanical chopper stabilized opamps in the control loading analog computers. The choppers were a little smaller than a C cell battery. I forget the chopping frequency, but you could feel it switching through the chopper's case.

Bob

In college I worked (and did the maintenance) on a pair of analog computers with chopper-stabilized op-amps. This was '70-'74, but the computers were at least a couple of decades older than that. After I graduated no one knew how to use 'em and they were scrapped. :-(

--
  Keith

1) Known reliability 2) If it works, why fix it?

I had a buddy once who was a "Simulator Engineer" at what was then Republic Airlines, in MSP. He worked 3rd shift, so he'd let friends come over and fly the DC-9 simulator. It was one of those ones in the two-story high room, on the six 6' hydraulic struts, and four HUGE TV screens outside the windows. Kind of like this one:

I found that a DC-9 doesn't respond quite as snappily to control inputs as a Cessna 150. :-) But you could do a barrel-roll in it, except it would go to about 45 degrees of bank, hit the stops, and when the math model got all the way around to about 45 degrees of bank the other way, it went WHAP! to the other side, and bonked your head against the side wall.

Landing it was really weird. When you flare, you practically have to drive it into the ground, or ground effects would float it off the end of the runway, or at least that's what it felt like.

It had a TI processor with 64K bytes of memory.

He often said that he'd like to get ahold of some fighter jet math model, but I don't know if that ever materialized.

Cheers! Rich

I guess I wasn't clear, but the analog computers (IIRC they were made by Pace) above also used mechanical choppers (about the size of a 16oz. beer can). One could hear them buzz, though I don't remember the chopping frequency. Each chassis was about 10"x6"x18" and contained a dual op amp or a servo-multiplier or servo-sine converter. One of the computers had forty op amps and a dozen multipliers and sine converters. The other was somewhat smaller.

--
  Keith

Well, each op-amp was something like a dozen tubes, so I don't think they quite had transistors down at that point (built in the '50s). IIRC the open-loop gain of the amplifiers was 1E8, or some such. The computers were designed for four-digit accuracy.

--
  Keith

There is a great story about the test pilot for the boeing 707, on the introduction to the public. Here is a video that describes it:

Early transistor thermocouple and strain-guage amplifiers used mechanical choppers too. IIRC they were more like the size of a vacuum tube, but that was a very long time ago and I were a mere wisp of a lad.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Hello Keith,

Why on earth did they use mechanical choppers? I mean, transistors had been invented by then and in a differential and thermally coupled setup the drift wasn't that bad.

Regards, Joerg