Favourite low offset op-amp

[Jim Thompson]

Varicap parametric bridges.

John

What did they call the big-ass ones.

I vaguely remember using some kind of varactor to triple me up to the
2 meter band. (5W

...Jim Thompson

 

[Rich Grise]

Is that term in the Dictionary of Electronics?

It's an onomatopoeia, so probably not.

I think "bang-bang" might be, however. ;-)

Cheers!
Rich

 

[Rich Grise]

Bill? Did I miss an actual technical contribution by Sloman ?

Jim, are you mellowing out in your old age? ;-)

I love auto-zeroing.

Once upon a time, I even built a micro-voltmeter, using LM324's, to
measure voltage drop on PCB tracks, to locate shorts.

Zero, measure, zero, measure....

You built stuff? I thought all you did was ideate and simulate. ;-)
Oh, right - at our age, we don't have time for all that trimming and
crap, right? ;-)

Cheers!
Rich

 

[John Larkin]

What did they call the big-ass ones.

I vaguely remember using some kind of varactor to triple me up to the
2 meter band. (5W

...Jim Thompson

Burr-Brown did a very impressive potted-brick opamp using modulated
varicaps in the front end; CMRR was about infinite, as the entire
input circuit was transformer-coupled. I think Bob Pease did an
article on it. I seem to recall that TI did a monolithic varicap opamp
at some point... I may have a datasheet in The Dungeon.

Paramps are cool. Somebody did a paper on using ceramics caps as
active gain elements, and I have a paper on making nonlinear
transmission lines (shock lines) using the nonlinearity of ceramic
caps.

Just because I know about this stuff doesn't mean I'm old enough to
have experienced it all first-hand.

John

 

[Joerg]

Hello Jim,

You're showing your age, John. And they weren't all that wonderful...
CMOS is MUCH better.

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.

Regards, Joerg

 

[Jim Thompson]

On Tue, 17 May 2005 16:52:36 -0700, John Larkin

[snip]

Paramps are cool. Somebody did a paper on using ceramics caps as
active gain elements, and I have a paper on making nonlinear
transmission lines (shock lines) using the nonlinearity of ceramic
caps.

Just because I know about this stuff doesn't mean I'm old enough to
have experienced it all first-hand.

John

Yes it do ;-)

...Jim Thompson

 

[Jim Thompson]

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.

Regards, Joerg

Reminds me when we first met the daughter-in-law's family.

My son, bride-to-be and her mother show up in the bride's Honda Civic.

They say that father is on the way.

All of a sudden the house is shaking, and this crazy, hippy-looking
guy, with a bandanna, comes up the hill on a Harley chopper... so loud
that all the neighbors pour out of their houses.

It's the father-of-the-bride. Turns out he's a big-time dealer in
Harley parts in LA (now moved to Palm Springs).

Then he took off the bandanna... it had a velcro closure. I gave him
a ration of shit for lack of authenticity ;-)

...Jim Thompson

 

[John Larkin]

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.

I've seen that sort of thing on The Discovery Channel.

John

 

[Rock]

AD8551, prob. overkill for your application, 1uV offset, sold as a zero
drift op amp. Just used these as a thermopile amplifier, they are
simply amazing. At $2.30 in singles from Digi Key they are a wonder!

Rocky

 

[Pooh Bear]

Pooh Bear wrote...

If you wish, but it'd be better to discuss your application and
circuit, to see why you need 0.1mV, and if there's a workaround
that'll allow you to use cheaper opamps.

The application is to 'dc servo' part of an amplifer stage that by design
isn't capable of high DC precision. Basically need to counter some Vbe
mismatches.

Graham

 

[Pooh Bear]

Spehro Pefhany wrote...

TI's OP07DD, 60uV typ, 32.5 cents qty 2500, not bad, good call Spef!

Certainly best bet so far. I'd forgotten it. I'm sure I used one about 20 yrs
ago as a photodiode amplifier.


Graham

 

[Pooh Bear]

AD8551, prob. overkill for your application, 1uV offset, sold as a zero
drift op amp. Just used these as a thermopile amplifier, they are
simply amazing. At $2.30 in singles from Digi Key they are a wonder!

Astonishingly low offset.

Hmmm only 5V single rail though.

Graham

 

[Pooh Bear]

Hello Graham,

If you can afford to do auto-zero as Bill suggested and your opamp
budget really is 15 cents for a dual: Why not use ye olde LM324 and
clamp to zero? Ok, you'd have to spring another 5c or so for a BSS123 to
do the clamping but a quad LM324 can be had for around 10c.

Hi Joerg,

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

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

Graham

 

[Pooh Bear]

Reminds me when we first met the daughter-in-law's family.

My son, bride-to-be and her mother show up in the bride's Honda Civic.

They say that father is on the way.

All of a sudden the house is shaking, and this crazy, hippy-looking
guy, with a bandanna, comes up the hill on a Harley chopper... so loud
that all the neighbors pour out of their houses.

It's the father-of-the-bride. Turns out he's a big-time dealer in
Harley parts in LA (now moved to Palm Springs).

Then he took off the bandanna... it had a velcro closure. I gave him
a ration of shit for lack of authenticity ;-)

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


Graham

 

[[email protected]]

The AD8551 has an absolute maximum supply voltage of 6V, which doesn't
fit too well with Pooh Bear's +/-17V rails.

That's why I suggested the LTC1150 - which can take +/-16V rails.

It's always a good idea to read the whole posting before responding - I
didn't notice the +/-17V on the first pass either, and had to rewrite
my response in consequence.

 

[Bill Sloman]

Bill? Did I miss an actual technical contribution by Sloman ?

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

I love auto-zeroing.

Once upon a time, I even built a micro-voltmeter, using LM324's, to
measure voltage drop on PCB tracks, to locate shorts.

Zero, measure, zero, measure....

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.

 

[Winfield Hill]

Bill Sloman wrote...

Jim Thompson wrote...

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.

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

 

[Spehro Pefhany]

Certainly best bet so far. I'd forgotten it. I'm sure I used one about 20 yrs
ago as a photodiode amplifier.


Graham

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.

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


Best regards,
Spehro Pefhany

 

[Adrian Tuddenham]

Burr-Brown did a very impressive potted-brick opamp using modulated
varicaps in the front end; CMRR was about infinite, as the entire
input circuit was transformer-coupled. I think Bob Pease did an
article on it. I seem to recall that TI did a monolithic varicap opamp
at some point... I may have a datasheet in The Dungeon.

Paramps are cool. Somebody did a paper on using ceramics caps as
active gain elements, and I have a paper on making nonlinear
transmission lines (shock lines) using the nonlinearity of ceramic
caps.

Just because I know about this stuff doesn't mean I'm old enough to
have experienced it all first-hand.

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.

 

[Jim Thompson]

Bill Sloman wrote...

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

But I was doing it before you were born ;-)

Actually, IIRC, I ran across those notes here at the house, so they're
not buried in the archival storage. I'll post if I can re-find.

...Jim Thompson