Low noise opamps without input diodes?

[Joerg]

["Followup-To:" header set to sci.electronics.design.]
Joerg wrote:

Neat! Thanks. Comes only in duals but heck, can't beat that price. And
it is 2nd sourced, found it also at ST and ON-Semi. This one ought to be
really popular with high end audio guys yet for some reason I have never
heard them talk about it.

It's just too cheap.

Ah, that's the solution: Gold-plate its pins and top it off with a glass
dome that shows the part number in pseudo-3D. Maybe with a blue LED in
there.

 

[Fred Bartoli]

Werty a écrit :

2 kinds of Op Amps ,

Mosfet and

Bipolar .

Bipolar has almost no change in input current.
Thus the voltage drop across the input R's\
is no problem . Accurate .

MOSFETs have a extreme TEMPCO , thus
their input Z is meaningless . It may as well
be 50K ohm , to swamp out the extreme
input current changes .
They are 4 times higher B.W. .

Noise is very low in Bipolar OpAmps .
But low noise is also very low B.W.

Have you noticed the old Bipolar Op Amps
are still very useful ? MOSFETs did not obsolete
Bipolars ..

I can see you have very deep understanding of the subject.
Sort of.

 

[martin griffith]

On Wed, 21 Mar 2007 22:09:33 +0100, in sci.electronics.design Fred
Bartoli

I can see you have very deep understanding of the subject.
Sort of.

ROFL, thanks Fred


martin

 

[colin]

I am going to check out Phil's suggestion, the LM833. Best of all, that
one is multi-sourced.

Yes I think I remember looking at that one once, but you talk of high-Z
nodes,
wont the bias current be a problem ?

If its not then there are a few more with not so low bias current on the TI
list.

Colin =^.^=

 

[Joerg]

Yes I think I remember looking at that one once, but you talk of high-Z
nodes,
wont the bias current be a problem ?

Well, it doesn't exactly help, of course, but I may be able to live with
<1uA in this case..

If its not then there are a few more with not so low bias current on the TI
list.

I'll have to go through it again. Any particular ones that come to mind?
I rarely use low noise opamps since I design most of my stuff in
discretes. Mainly to maintain true 2nd-sourcing for everything. Opamps
are mostly relegated to more mundane jobs in my designs.

 

[colin]

.....


I'll have to go through it again. Any particular ones that come to mind? I
rarely use low noise opamps since I design most of my stuff in discretes.
Mainly to maintain true 2nd-sourcing for everything. Opamps are mostly
relegated to more mundane jobs in my designs.

nop soz not without looking again, seem to remember the old ne5534... stil
being there
and making sure there are no hidden input diodes is a bit of a pain
sometimes.


There are some improved lm358 types but these tend to have diodes.
Ive got a couple of such types of op amps wich were usless for what I needed
at the time
becuase of input diodes but would otherwise do quite well for what you need.

Colin =^.^=

 

[Joerg]

nop soz not without looking again, seem to remember the old ne5534... stil
being there
and making sure there are no hidden input diodes is a bit of a pain
sometimes.

That one does have the diodes :-(

http://focus.ti.com/lit/ds/symlink/ne5534a.pdf

There are some improved lm358 types but these tend to have diodes.
Ive got a couple of such types of op amps wich were usless for what I needed
at the time
becuase of input diodes but would otherwise do quite well for what you need.

Diodes unfortunately disqualify the parts except for clean amplifier
apps. But there I can easily design a low noise amp with discretes anyway.

 

[colin]

......

Diodes unfortunately disqualify the parts except for clean amplifier apps.
But there I can easily design a low noise amp with discretes anyway.

this one ...
http://www.farnell.com/datasheets/65378.pdf ?

Colin =^.^=

 

[Joerg]

.....



this one ...
http://www.farnell.com/datasheets/65378.pdf ?

That one would be good as well. Has diodes but these are zeners and one
can usually live with the 10V differential they allow. However, the
price is many times higher than for example the LM833.

 

[Phil Allison]

"Joerg"

That one would be good as well. Has diodes but these are zeners and one
can usually live with the 10V differential they allow. However, the price
is many times higher than for example the LM833.

** You after DIL or SMD pack ?

The one above is SMD only.



......... Phil

 

[John Larkin]

The OPA series seems hard to buy these days and I've heard rumors of
some difficulties. So, I am looking for non-OPA varieties:

<5nV/rtHz would be nice. +/-15V supplies. No back-to-back diodes on the
inputs like the AD8674 has, need to use some of them as comparators and
trackers. Can't easily buffer and they must track some high-Z nodes. I
don't need the full VEE to VDD swing but at least 5V differential or so.

Maxim makes a *comparator* with back-to-back input diodes. Talk about
unclear on the concept!

John

 

[[email protected]]

Neat! Thanks. Comes only in duals but heck, can't beat that price. And
it is 2nd sourced, found it also at ST and ON-Semi. This one ought to be
really popular with high end audio guys yet for some reason I have never
heard them talk about it.

You might want to verify that the offset voltage won't be altered as
you put extreme differential voltage on the inputs. Generally, that is
why you put the protection diodes on the dif pair.

I assume you will check for phase reversal.

 

[Joerg]

You might want to verify that the offset voltage won't be altered as
you put extreme differential voltage on the inputs. Generally, that is
why you put the protection diodes on the dif pair.

I assume you will check for phase reversal.

It won't be more than 3-4V.

 

[Eeyore]

You might want to verify that the offset voltage won't be altered as
you put extreme differential voltage on the inputs. Generally, that is
why you put the protection diodes on the dif pair.

Well.... exactly.

The b-e junction will break down (avalanche) at about 6V reverse anyway so what
happens when you apply say 12V differential to an unprotected input ?

Graham

 

[Joerg]

"Joerg"






** You after DIL or SMD pack ?

The one above is SMD only.

Actually SMD is perfect. Digikey also has them in DIP (from ST) if you
ever need that. The DIP version is Digikey P/N 497-1598-5-ND and it's
currently in stock. There is also a DIP version from National but
Digikey is out of stock on that one.

I was thinking of getting a few DIPs as well because I we did the
prototype in thru-hole. However, that'll be an ugly hack because it was
done with quad opamps in sockets. It'll look like little Eiffel towers ;-)

 

[Joerg]

Maxim makes a *comparator* with back-to-back input diodes. Talk about
unclear on the concept!

ROFL! Luckily I have already had my morning coffee, else I'd be wiping
off the monitor now.

 

[[email protected]]

Well.... exactly.

The b-e junction will break down (avalanche) at about 6V reverse anyway so what
happens when you apply say 12V differential to an unprotected input ?

Graham

Breakdown is kind of mushy. [The closer you look, the sooner you think
it broke down. This is because the physics are logarithmic, and you
are looking with linear eyes.] You can measure changes in the offset
at less that breakdown. Perhaps some localized part ot the junction
gets stressed. I just don't recall how significant this is, well, as I
always put in the diodes and only designed ASICs, where the op amp
specs are often not as important as opposed to designing an op amp for
op amp's sake. ;-) Of course, with good instrumentation, you can
measure all sorts of small changes that may not be indicative of a
problem, .

I'm not really sure I'd get the warm and fuzzy feeling putting a
device on the market that whacked the unprotected dif pair. I suppose
it depends on the application.

 

[Eeyore]

Well.... exactly.

The b-e junction will break down (avalanche) at about 6V reverse anyway so > what

happens when you apply say 12V differential to an unprotected input ?


Breakdown is kind of mushy. [The closer you look, the sooner you think
it broke down. This is because the physics are logarithmic, and you
are looking with linear eyes.] You can measure changes in the offset
at less that breakdown. Perhaps some localized part ot the junction
gets stressed. I just don't recall how significant this is, well, as I
always put in the diodes and only designed ASICs, where the op amp
specs are often not as important as opposed to designing an op amp for
op amp's sake. ;-) Of course, with good instrumentation, you can
measure all sorts of small changes that may not be indicative of a
problem, .

I'm not really sure I'd get the warm and fuzzy feeling putting a
device on the market that whacked the unprotected dif pair. I suppose
it depends on the application.

Well..... the SSM2135 that Phil Allison mentioned has back to back 9V zeners across
the inputs. First time I've seen that integrated on a chip as opposed to normal
diodes.

Graham

 

[Joerg]

[email protected] wrote:


You might want to verify that the offset voltage won't be altered as
you put extreme differential voltage on the inputs. Generally, that is
why you put the protection diodes on the dif pair.

Well.... exactly.

The b-e junction will break down (avalanche) at about 6V reverse anyway so > what

happens when you apply say 12V differential to an unprotected input ?


Breakdown is kind of mushy. [The closer you look, the sooner you think
it broke down. This is because the physics are logarithmic, and you
are looking with linear eyes.] You can measure changes in the offset
at less that breakdown. Perhaps some localized part ot the junction
gets stressed. I just don't recall how significant this is, well, as I
always put in the diodes and only designed ASICs, where the op amp
specs are often not as important as opposed to designing an op amp for
op amp's sake. ;-) Of course, with good instrumentation, you can
measure all sorts of small changes that may not be indicative of a
problem, .

I'm not really sure I'd get the warm and fuzzy feeling putting a
device on the market that whacked the unprotected dif pair. I suppose
it depends on the application.

That's how comparators work where offset is often also a decisive factor
in part selection. I have never noticed a measurable offset increase
because of differential voltage application. Heck, some of the older
gear here in the lab contains such circuits and they are still within
specs after 20+ years. Most of the times I do not rely on offset specs
though but make sure it gets clamped away towards the end of the chain.

Well..... the SSM2135 that Phil Allison mentioned has back to back 9V zeners across
the inputs. First time I've seen that integrated on a chip as opposed to normal
diodes.

That is indeed unusual. Typically you see that in MOSFETs and dual-gate
FETs for TV tuners.

 

[Phil Allison]

"Eeysore Fucking Idiot "

Well..... the SSM2135 that Phil Allison mentioned..

** Bloody hell - not again.

My name is not "colin " - you ass.




......... Phil