AoE x-Chapters, High-Speed op-amps section, DRAFT

[Winfield Hill]

Read and comment. AoE x-Chapters,
High-Speed op-amps section, DRAFT.

Recall, the AoE x-Chapters are advanced material that
was meant to come after each relevant chapter, can skip
on a quick first read, go back later for detailed info.
But as main book was growing over 2000 pages, we opted to
bring out main book first, including x-Chapter cross refs,
follow with x-Chapter book. As explained in the preface.

26 pages of good stuff, from new sections 4x.5 and 4x.6
Chapter 4x is to supplement H&H AoE III, chapters 4,5,8.
(Full Chapter 4x is now 146 pages long, still growing.)

DRAFT, but getting close to being complete.

Are explanations excellent, good, OK, or confusing?
Examine tables, any of your favorite parts missing?**
Want a separate low-power table? Can we skip that?
Is the CFB op-amp scene explained well enough?
Did you like VFB scatterplots? Need some for CFB?
Have a favorite trick that should be included?
Make comments here or send to winfieldhill@yahoo.com

https://www.dropbox.com/s/aemtyly16mtj87n/Op-amps_High-speed_ch4x_DRAFT.pdf?dl=0

** For op-amps that come in multiple variants, i.e.,
single, dual, quad, we list only one part, and show
# per pkg variants in next column, so you may not see
your part, e.g., we show OPA192, not OPA2192 or OPA4192.
As you can see, we were forced to economize on space.


--
Thanks,
- Win

 

[John Larkin]

On 24 Apr 2019 04:04:21 -0700, Winfield Hill
<hill@rowland.harvard.edu> wrote:

Read and comment. AoE x-Chapters,
High-Speed op-amps section, DRAFT.

Recall, the AoE x-Chapters are advanced material that
was meant to come after each relevant chapter, can skip
on a quick first read, go back later for detailed info.
But as main book was growing over 2000 pages, we opted to
bring out main book first, including x-Chapter cross refs,
follow with x-Chapter book. As explained in the preface.

26 pages of good stuff, from new sections 4x.5 and 4x.6
Chapter 4x is to supplement H&H AoE III, chapters 4,5,8.
(Full Chapter 4x is now 146 pages long, still growing.)

DRAFT, but getting close to being complete.

Are explanations excellent, good, OK, or confusing?
Examine tables, any of your favorite parts missing?**
Want a separate low-power table? Can we skip that?
Is the CFB op-amp scene explained well enough?
Did you like VFB scatterplots? Need some for CFB?
Have a favorite trick that should be included?
Make comments here or send to winfieldhill@yahoo.com

https://www.dropbox.com/s/aemtyly16mtj87n/Op-amps_High-speed_ch4x_DRAFT.pdf?dl=0

** For op-amps that come in multiple variants, i.e.,
single, dual, quad, we list only one part, and show
# per pkg variants in next column, so you may not see
your part, e.g., we show OPA192, not OPA2192 or OPA4192.
As you can see, we were forced to economize on space.

On a quick read, I didn't see any mention of c-load stable amps. That
could be an interesting section.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Wed, 24 Apr 2019 08:16:35 -0700, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

On 24 Apr 2019 04:04:21 -0700, Winfield Hill
hill@rowland.harvard.edu> wrote:

Read and comment. AoE x-Chapters,
High-Speed op-amps section, DRAFT.

Recall, the AoE x-Chapters are advanced material that
was meant to come after each relevant chapter, can skip
on a quick first read, go back later for detailed info.
But as main book was growing over 2000 pages, we opted to
bring out main book first, including x-Chapter cross refs,
follow with x-Chapter book. As explained in the preface.

26 pages of good stuff, from new sections 4x.5 and 4x.6
Chapter 4x is to supplement H&H AoE III, chapters 4,5,8.
(Full Chapter 4x is now 146 pages long, still growing.)

DRAFT, but getting close to being complete.

Are explanations excellent, good, OK, or confusing?
Examine tables, any of your favorite parts missing?**
Want a separate low-power table? Can we skip that?
Is the CFB op-amp scene explained well enough?
Did you like VFB scatterplots? Need some for CFB?
Have a favorite trick that should be included?
Make comments here or send to winfieldhill@yahoo.com

https://www.dropbox.com/s/aemtyly16mtj87n/Op-amps_High-speed_ch4x_DRAFT.pdf?dl=0

** For op-amps that come in multiple variants, i.e.,
single, dual, quad, we list only one part, and show
# per pkg variants in next column, so you may not see
your part, e.g., we show OPA192, not OPA2192 or OPA4192.
As you can see, we were forced to economize on space.

On a quick read, I didn't see any mention of c-load stable amps. That
could be an interesting section.

I could scribble up a half page or so on the subject.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Winfield Hill]

John Larkin wrote...

On a quick read, I didn't see any mention of c-load
stable amps. That could be an interesting section.

What p/n would you include for that?


--
Thanks,
- Win

 

[John Larkin]

On 24 Apr 2019 10:01:00 -0700, Winfield Hill
<hill@rowland.harvard.edu> wrote:

John Larkin wrote...

On a quick read, I didn't see any mention of c-load
stable amps. That could be an interesting section.

What p/n would you include for that?

There are only a few declared c-load stable opamps. We use a lot of
LM8261s. In reality, many rro amps are c-load stable, because adding a
capacitive load reduces the rolloff corner frequency but doesn't add a
pole.

LTC has a dozen or so c-load opamps, LT1200 and such, but LTC tends to
be expensive.

We often distribute a heavily-bypassed reference rail, and it's
convenient to drive that with a c-load-stable amp.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Winfield Hill]

John Larkin wrote...

On a quick read, I didn't see any mention of c-load
stable amps. That could be an interesting section.

I could scribble up a half page or so on the subject.

Hey, go for it!

In the VFB bold-letter Comments notes, we have AB.
"AB. stable with unlim cap-load." Four types are
marked with the AB comment.

There aren't very many C-load opamps, and my memory
is most of them have fT under 10MHz, which would be
too low for these tables. But a separate section
labelled C-Load, would not have that restriction.


--
Thanks,
- Win

 

[John Larkin]

On 24 Apr 2019 11:34:28 -0700, Winfield Hill
<hill@rowland.harvard.edu> wrote:

John Larkin wrote...

On a quick read, I didn't see any mention of c-load
stable amps. That could be an interesting section.

I could scribble up a half page or so on the subject.

Hey, go for it!

In the VFB bold-letter Comments notes, we have AB.
"AB. stable with unlim cap-load." Four types are
marked with the AB comment.

Oops, missed that on a quick read.

Some opamps are unstable over a fairly narrow range of cap loads, but
are happy with big caps. Use the Jim Williams technique: a big enough
capacitor will stabilize anything.

There aren't very many C-load opamps, and my memory
is most of them have fT under 10MHz, which would be
too low for these tables. But a separate section
labelled C-Load, would not have that restriction.

With a big c load, one might not expect much bandwidth!

We developed a fancy I/Q modulator box to simulate a blade-tip sensor
in a jet engine. It looked great in Spice but it oscillates big-time
in real life. We used some really fast current-mode amps, AD8009s. I
sent my guy your opamp tables and he'll use that to pick some slower
(roughly 250-300 MHz) voltage mode amps, which we hope will tame
things.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[George Herold]

On Wednesday, April 24, 2019 at 2:34:42 PM UTC-4, Winfield Hill wrote:

John Larkin wrote...

On a quick read, I didn't see any mention of c-load
stable amps. That could be an interesting section.

I could scribble up a half page or so on the subject.

Hey, go for it!

In the VFB bold-letter Comments notes, we have AB.
"AB. stable with unlim cap-load." Four types are
marked with the AB comment.

There aren't very many C-load opamps, and my memory
is most of them have fT under 10MHz, which would be
too low for these tables. But a separate section
labelled C-Load, would not have that restriction.


--
Thanks,
- Win

I went looking for the LT1200 that JL mentioned. I found this,

https://www.analog.com/media/en/reference-design-documentation/design-notes/dn107f.pdf

George H.

 

[Joerg]

On 2019-04-24 04:04, Winfield Hill wrote:

Read and comment. AoE x-Chapters,
High-Speed op-amps section, DRAFT.

Recall, the AoE x-Chapters are advanced material that
was meant to come after each relevant chapter, can skip
on a quick first read, go back later for detailed info.
But as main book was growing over 2000 pages, we opted to
bring out main book first, including x-Chapter cross refs,
follow with x-Chapter book. As explained in the preface.

26 pages of good stuff, from new sections 4x.5 and 4x.6
Chapter 4x is to supplement H&H AoE III, chapters 4,5,8.
(Full Chapter 4x is now 146 pages long, still growing.)

DRAFT, but getting close to being complete.

Are explanations excellent, good, OK, or confusing?
Examine tables, any of your favorite parts missing?**
Want a separate low-power table? Can we skip that?
Is the CFB op-amp scene explained well enough?
Did you like VFB scatterplots? Need some for CFB?
Have a favorite trick that should be included?
Make comments here or send to winfieldhill@yahoo.com

https://www.dropbox.com/s/aemtyly16mtj87n/Op-amps_High-speed_ch4x_DRAFT.pdf?dl=0

** For op-amps that come in multiple variants, i.e.,
single, dual, quad, we list only one part, and show
# per pkg variants in next column, so you may not see
your part, e.g., we show OPA192, not OPA2192 or OPA4192.
As you can see, we were forced to economize on space.

Pretty good.

Two suggested additions:


1. EMI behavior of opamps. This is generally not understood at all by
engineers and (still!) not taught at universities from what young EEs
told me. A bipolar input stage will rectify RF at the first BE junction,
even stuff at cell phone frequencies. This rectification or demodulation
is very inefficient but since that is inside the loop any resultying
baseband AM will hit at full tilt because it happens at "open loop".

This has brought me several consulting assigments so maybe don't add it.
OTOH I am trying to retire, so maybe put it in, so I can cycle more,
bike more and brew more beer.

No kidding, two weeks ago I explained that to two engineers at a client
and they went "WHAT?!". Many don't believe it until demonstrated. GSM
phones are good for doing that, when they connect to a new tower.
Switching to a CMOS opamp usually makes that problem go away 100%. When
I did that for the first time I almost achieved guru-status at that
client because nobody had ever fixed an EMC issue without a serious
re-layout there.


2. Back-to-back input protection diodes between IN+ and IN-. Very often
overlooked. In datasheets they are sometimes only mentioned in a
footnote under the abs max table but often there is only a +/-0.3V diff
limit. Aside from pouring gasoline on the above mentioned EMI issue
these diodes can really throw people a curve when using opamps in an
unorthodox way or as a comparator. Thou shalt not do that but ...

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Johann Klammer]

On 04/24/2019 01:04 PM, Winfield Hill wrote:

Read and comment. AoE x-Chapters,
High-Speed op-amps section, DRAFT.
AD8011 is available in DIP8

 

On Wednesday, April 24, 2019 at 2:34:42 PM UTC-4, Winfield Hill wrote:

John Larkin wrote...

On a quick read, I didn't see any mention of c-load
stable amps. That could be an interesting section.

I could scribble up a half page or so on the subject.

Hey, go for it!

In the VFB bold-letter Comments notes, we have AB.
"AB. stable with unlim cap-load." Four types are
marked with the AB comment.

There aren't very many C-load opamps, and my memory
is most of them have fT under 10MHz, which would be
too low for these tables. But a separate section
labelled C-Load, would not have that restriction.

I've been using the National (is that now TI?) LM8272 for some years now. It's 15 MHz so I guess not much different. Works a treat for audio work.

--

Rick C.

- Get a 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[Winfield Hill]

Joerg wrote...

Pretty good.
Two suggested additions:

1. EMI behavior of opamps. This is generally not understood at all by
engineers and (still!) not taught at universities from what young EEs
told me. A bipolar input stage will rectify RF at the first BE junction,
even stuff at cell phone frequencies. This rectification or demodulation
is very inefficient but since that is inside the loop any resultying
baseband AM will hit at full tilt because it happens at "open loop".

This has brought me several consulting assigments so maybe don't add it.
OTOH I am trying to retire, so maybe put it in, so I can cycle more,
bike more and brew more beer.

No kidding, two weeks ago I explained that to two engineers at a client
and they went "WHAT?!". Many don't believe it until demonstrated. GSM
phones are good for doing that, when they connect to a new tower.
Switching to a CMOS opamp usually makes that problem go away 100%. When
I did that for the first time I almost achieved guru-status at that
client because nobody had ever fixed an EMC issue without a serious
re-layout there.


2. Back-to-back input protection diodes between IN+ and IN-. Very often
overlooked. In datasheets they are sometimes only mentioned in a
footnote under the abs max table but often there is only a +/-0.3V diff
limit. Aside from pouring gasoline on the above mentioned EMI issue
these diodes can really throw people a curve when using opamps in an
unorthodox way or as a comparator. Thou shalt not do that but ...

Thanks, Joerg, very good suggestions. I've used the CMOS trick
before, people should know about it. I'll go through all the
datasheets, and flag the ones that have input diodes.


--
Thanks,
- Win

 

[Winfield Hill]

Johann Klammer wrote...

On 04/24/2019 01:04 PM, Winfield Hill wrote:
Read and comment. AoE x-Chapters,
High-Speed op-amps section, DRAFT.

AD8011 is available in DIP8

Good to know, thanks, added to fixes.


--
Thanks,
- Win

 

[Gerhard Hoffmann]

Am 24.04.19 um 21:35 schrieb Joerg:

1. EMI behavior of opamps. This is generally not understood at all by
engineers and (still!) not taught at universities from what young EEs
told me. A bipolar input stage will rectify RF at the first BE junction,
even stuff at cell phone frequencies. This rectification or demodulation
is very inefficient but since that is inside the loop any resultying
baseband AM will hit at full tilt because it happens at "open loop".

It's not that FETs are any worse at demodulating than BJTs, it's just
that they need a higher source impedance for the same dBms. Say, a
different cable transformation.
At the same time their higher Cs make it harder for that match to exist.
The loop gain is the same.

It's much like in HP's app note "Every Schottky diode is a zero bias
detector".

2. Back-to-back input protection diodes between IN+ and IN-. Very often
overlooked. In datasheets they are sometimes only mentioned in a
footnote under the abs max table but often there is only a +/-0.3V diff
limit. Aside from pouring gasoline on the above mentioned EMI issue
these diodes can really throw people a curve when using opamps in an
unorthodox way or as a comparator. Thou shalt not do that but ...

These diodes are anti-parallel, not an efficient rectifier. It's a very
good thing that they are there. A low noise op amp with zenered BE
junctions at the input is no longer a low noise op amp. And an op amp
with 100 dB open loop gain and a Volt between its inputs is a design
error.
Using an op amp as a comparator is not unorthodox, it's wrong.
The diodes are there to protect other stuff from dying.


BTW an ADA4898 can draw a _lot_ of supply current just when one of its
inputs is open circuit. It can get too hot to touch.

cheers,
Gerhard

 

[John Larkin]

On Wed, 24 Apr 2019 11:51:42 -0700, John Larkin
<jjlarkin@highland_snip_technology.com> wrote:

On 24 Apr 2019 11:34:28 -0700, Winfield Hill
hill@rowland.harvard.edu> wrote:

John Larkin wrote...

On a quick read, I didn't see any mention of c-load
stable amps. That could be an interesting section.

I could scribble up a half page or so on the subject.

Hey, go for it!

In the VFB bold-letter Comments notes, we have AB.
"AB. stable with unlim cap-load." Four types are
marked with the AB comment.

Oops, missed that on a quick read.

Some opamps are unstable over a fairly narrow range of cap loads, but
are happy with big caps. Use the Jim Williams technique: a big enough
capacitor will stabilize anything.


There aren't very many C-load opamps, and my memory
is most of them have fT under 10MHz, which would be
too low for these tables. But a separate section
labelled C-Load, would not have that restriction.

With a big c load, one might not expect much bandwidth!

We developed a fancy I/Q modulator box to simulate a blade-tip sensor
in a jet engine. It looked great in Spice but it oscillates big-time
in real life. We used some really fast current-mode amps, AD8009s. I
sent my guy your opamp tables and he'll use that to pick some slower
(roughly 250-300 MHz) voltage mode amps, which we hope will tame
things.

The misbehaving circuit is an all-pass phase shifter

https://www.dropbox.com/s/nhevg979w7gtxib/90_degree.JPG?dl=0

(from the Williams book) which tops out about 30 MHz. The AD8009s
simulate fine here but oscillate wildly in real life. We just tried
some LT1818s, 400 MHz VFB amps, and they behave.

This all-pass doesn't like current feedback amps.



--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Joerg]

On 2019-04-24 12:49, Winfield Hill wrote:

Joerg wrote...

Pretty good.
Two suggested additions:

1. EMI behavior of opamps. This is generally not understood at all by
engineers and (still!) not taught at universities from what young EEs
told me. A bipolar input stage will rectify RF at the first BE junction,
even stuff at cell phone frequencies. This rectification or demodulation
is very inefficient but since that is inside the loop any resultying
baseband AM will hit at full tilt because it happens at "open loop".

This has brought me several consulting assigments so maybe don't add it.
OTOH I am trying to retire, so maybe put it in, so I can cycle more,
bike more and brew more beer.

No kidding, two weeks ago I explained that to two engineers at a client
and they went "WHAT?!". Many don't believe it until demonstrated. GSM
phones are good for doing that, when they connect to a new tower.
Switching to a CMOS opamp usually makes that problem go away 100%. When
I did that for the first time I almost achieved guru-status at that
client because nobody had ever fixed an EMC issue without a serious
re-layout there.


2. Back-to-back input protection diodes between IN+ and IN-. Very often
overlooked. In datasheets they are sometimes only mentioned in a
footnote under the abs max table but often there is only a +/-0.3V diff
limit. Aside from pouring gasoline on the above mentioned EMI issue
these diodes can really throw people a curve when using opamps in an
unorthodox way or as a comparator. Thou shalt not do that but ...

Thanks, Joerg, very good suggestions. I've used the CMOS trick
before, people should know about it. I'll go through all the
datasheets, and flag the ones that have input diodes.

Main thing is to explain the mechanism, why it happens. IME people
really do not know this, often even after decades in the industry. Then
they engage in expensive and time-consuming system level fixes to muffle
a susceptibility problem where a simple module level remedy (a switch to
CMOS) would have done the trick.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

On 2019-04-24 14:17, Gerhard Hoffmann wrote:

Am 24.04.19 um 21:35 schrieb Joerg:

1. EMI behavior of opamps. This is generally not understood at all by
engineers and (still!) not taught at universities from what young EEs
told me. A bipolar input stage will rectify RF at the first BE
junction, even stuff at cell phone frequencies. This rectification or
demodulation is very inefficient but since that is inside the loop any
resultying baseband AM will hit at full tilt because it happens at
"open loop".

It's not that FETs are any worse at demodulating than BJTs, it's just
that they need a higher source impedance for the same dBms. Say, a
different cable transformation.

MOSFETs don't demodulate at all because there is no conducting diode
path. I have found cases with hardcore EM susceptibility to the point
where a person outside the concrete walls of the building could upset a
circuit operating at single-digit kHz range, just by turning on their
cell phone. GSM ones were especially bad. After switching to a CMOS
opamp ... nad, zilch, not even when holding a cell phone right above the
open circuit and then turning it on.

A downside is that one can often not achieve the same low noise
performance with CMOS opamps.

At the same time their higher Cs make it harder for that match to exist.
The loop gain is the same.

For RF in, say, the cell phone range there is no or hardly any loop gain
unless you have a super-fast opamp.

It's much like in HP's app note "Every Schottky diode is a zero bias
detector".

Yep

Or in American "Every opportunity has its tax".

2. Back-to-back input protection diodes between IN+ and IN-. Very
often overlooked. In datasheets they are sometimes only mentioned in a
footnote under the abs max table but often there is only a +/-0.3V
diff limit. Aside from pouring gasoline on the above mentioned EMI
issue these diodes can really throw people a curve when using opamps
in an unorthodox way or as a comparator. Thou shalt not do that but ...

These diodes are anti-parallel, not an efficient rectifier.

Can be enough. There is always an offset.

... It's a very
good thing that they are there. A low noise op amp with zenered BE
junctions at the input is no longer a low noise op amp. And an op amp
with 100 dB open loop gain and a Volt between its inputs is a design
error.

Not if you know what you are doing and the mfg has blessed railing the
output.

Using an op amp as a comparator is not unorthodox, it's wrong.

Nah, BTDT. You have to know the limits and not exceed reverse Vbe
anywhere. Which can be a mere -2V in RF stuff.

The diodes are there to protect other stuff from dying.

Real hot stuff doesn't have diodes. For example the SD5400 quad FET
array, a marvelous IC. One of my circuits almost drove a tech insane
because every one of them he soldered in died. Until I showed him how to
avoid every last bit of ESD. Once inside the circuit everything was safe.

BTW an ADA4898 can draw a _lot_ of supply current just when one of its
inputs is open circuit. It can get too hot to touch.

They should explain that in the datasheet.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[John Larkin]

On Wed, 24 Apr 2019 12:35:55 -0700, Joerg <news@analogconsultants.com>
wrote:

On 2019-04-24 04:04, Winfield Hill wrote:
Read and comment. AoE x-Chapters,
High-Speed op-amps section, DRAFT.

Recall, the AoE x-Chapters are advanced material that
was meant to come after each relevant chapter, can skip
on a quick first read, go back later for detailed info.
But as main book was growing over 2000 pages, we opted to
bring out main book first, including x-Chapter cross refs,
follow with x-Chapter book. As explained in the preface.

26 pages of good stuff, from new sections 4x.5 and 4x.6
Chapter 4x is to supplement H&H AoE III, chapters 4,5,8.
(Full Chapter 4x is now 146 pages long, still growing.)

DRAFT, but getting close to being complete.

Are explanations excellent, good, OK, or confusing?
Examine tables, any of your favorite parts missing?**
Want a separate low-power table? Can we skip that?
Is the CFB op-amp scene explained well enough?
Did you like VFB scatterplots? Need some for CFB?
Have a favorite trick that should be included?
Make comments here or send to winfieldhill@yahoo.com

https://www.dropbox.com/s/aemtyly16mtj87n/Op-amps_High-speed_ch4x_DRAFT.pdf?dl=0

** For op-amps that come in multiple variants, i.e.,
single, dual, quad, we list only one part, and show
# per pkg variants in next column, so you may not see
your part, e.g., we show OPA192, not OPA2192 or OPA4192.
As you can see, we were forced to economize on space.


Pretty good.

Two suggested additions:


1. EMI behavior of opamps. This is generally not understood at all by
engineers and (still!) not taught at universities from what young EEs
told me. A bipolar input stage will rectify RF at the first BE junction,
even stuff at cell phone frequencies. This rectification or demodulation
is very inefficient but since that is inside the loop any resultying
baseband AM will hit at full tilt because it happens at "open loop".

This has brought me several consulting assigments so maybe don't add it.
OTOH I am trying to retire, so maybe put it in, so I can cycle more,
bike more and brew more beer.

No kidding, two weeks ago I explained that to two engineers at a client
and they went "WHAT?!". Many don't believe it until demonstrated. GSM
phones are good for doing that, when they connect to a new tower.
Switching to a CMOS opamp usually makes that problem go away 100%. When
I did that for the first time I almost achieved guru-status at that
client because nobody had ever fixed an EMC issue without a serious
re-layout there.

There are a few specifically EMI hardened zero-drift opamps around
now, like ADA4522. They seem ideal, but they are complex inside and
have some quirks.

JFET opamps probably rectify a lot less than bipolars.

2. Back-to-back input protection diodes between IN+ and IN-. Very often
overlooked. In datasheets they are sometimes only mentioned in a
footnote under the abs max table but often there is only a +/-0.3V diff
limit. Aside from pouring gasoline on the above mentioned EMI issue
these diodes can really throw people a curve when using opamps in an
unorthodox way or as a comparator. Thou shalt not do that but ...

Maxim makes a *comparator* with back-back input diodes!



--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Larkin]

On Wed, 24 Apr 2019 23:17:57 +0200, Gerhard Hoffmann <dk4xp@arcor.de>
wrote:

Am 24.04.19 um 21:35 schrieb Joerg:

1. EMI behavior of opamps. This is generally not understood at all by
engineers and (still!) not taught at universities from what young EEs
told me. A bipolar input stage will rectify RF at the first BE junction,
even stuff at cell phone frequencies. This rectification or demodulation
is very inefficient but since that is inside the loop any resultying
baseband AM will hit at full tilt because it happens at "open loop".

It's not that FETs are any worse at demodulating than BJTs, it's just
that they need a higher source impedance for the same dBms. Say, a
different cable transformation.
At the same time their higher Cs make it harder for that match to exist.
The loop gain is the same.

It's much like in HP's app note "Every Schottky diode is a zero bias
detector".



2. Back-to-back input protection diodes between IN+ and IN-. Very often
overlooked. In datasheets they are sometimes only mentioned in a
footnote under the abs max table but often there is only a +/-0.3V diff
limit. Aside from pouring gasoline on the above mentioned EMI issue
these diodes can really throw people a curve when using opamps in an
unorthodox way or as a comparator. Thou shalt not do that but ...

These diodes are anti-parallel, not an efficient rectifier. It's a very
good thing that they are there. A low noise op amp with zenered BE
junctions at the input is no longer a low noise op amp. And an op amp
with 100 dB open loop gain and a Volt between its inputs is a design
error.
Using an op amp as a comparator is not unorthodox, it's wrong.
The diodes are there to protect other stuff from dying.

Some opamps are pretty good comparators, just the right speed for some
applications. AD8033, OPA171.

BTW an ADA4898 can draw a _lot_ of supply current just when one of its
inputs is open circuit. It can get too hot to touch.

cheers,
Gerhard

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Miles, KE5FX]

On Wednesday, April 24, 2019 at 3:41:06 PM UTC-7, John Larkin wrote:

This all-pass doesn't like current feedback amps.

You're not using 10K feedback resistors, are you? That could lead to
odd behavior with CFB amps.

-- john, KE5FX