Favourite low offset op-amp

[Keith Williams]

Hello Jim,


Or maybe he is too young. In my days choppers were those motorcycles
with extremely long handle bars, where the front wheel was way out there
and you wore a bandana and some cool tattoos. Oh, and it had to be loud.

Before that they were what my grandmother put in the glass of water by
the side of the bed at night.

 

[Jim Thompson]

Bill Sloman wrote...

We introduce the auto-zero circuit as well, pg 392.

See...

Newsgroups: alt.binaries.schematics.electronic
Subject: uV Meter with Auto Zero - AutoZeroNotes.pdf
Message-ID: <[email protected]>

Early '70's

...Jim Thompson

 

[Joerg]

Hello Keith,

Before that they were what my grandmother put in the glass of water by
the side of the bed at night.

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

 

[Joerg]

Hello Graham,

is this like the 'chopper' concept ? I'm not familiar with this - other than
conceptually.

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.

Hmmm - I ought to add that I need my input to be differential !

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

 

[Winfield Hill]

Jim Thompson wrote...

See... Newsgroups: alt.binaries.schematics.electronic
Subject: uV Meter with Auto Zero - AutoZeroNotes.pdf
Message-ID: <[email protected]>

Early '70's

My first precision autozero amplifier design was in 1965.

 

[Joerg]

Hello Spehro,

It's one of those devices that started out as a typical high
performance "$5 op-amp" and has happily slid into jellybeanitude
(genericity?) over the years, with several reliable suppliers
currently online.

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.

Some of National's 1A/3A Simple Switcher line seem to be going the
same way, over time, VERY slowly.

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

 

[Keith Williams]

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.

....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 .

 

[Jim Thompson]

Jim Thompson wrote...

My first precision autozero amplifier design was in 1965.

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

 

[[email protected]]

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.

 

[Pooh Bear]

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.

Kent Instruments ? I recall the name.


Graham

 

[John Larkin]

Kent Instruments ? I recall the name.

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

 

[Rich Grise]

Jim Thompson wrote:

LOL ! I would say 'snort' but that might come over wrong !

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

 

[Keith Williams]

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.

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. :-(

 

[Bob Stephens]

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. :-(

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

 

[Keith Williams]

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.

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.

 

[Joerg]

Hello Keith,

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). ...

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

 

[Keith Williams]

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.

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.

 

[Spehro Pefhany]

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.

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

 

[Rich Grise]

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.

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:
http://www.simlabs.arc.nasa.gov/cvsrf/images/acfs_sim.jpg
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

 

[Bob Monsen]

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:
http://www.simlabs.arc.nasa.gov/cvsrf/images/acfs_sim.jpg
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.

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:

http://www.aviationexplorer.com/707_roll_video.htm