Maker Pro
Maker Pro

Which opamp?

  • Thread starter Clueless Newbie
  • Start date
C

Clueless Newbie

Jan 1, 1970
0
What's the best opamp for amplifying a sub-microvolt 300Hz to 3Khz
signal to a volt or more with the absolute minimum amount of noise?

I plan on following the amplifier stage with a bandpass filter, so
it's the noise in the 100Hz to 10Khz range that I am most concerned
about.
 
B

Ban

Jan 1, 1970
0
Clueless said:
What's the best opamp for amplifying a sub-microvolt 300Hz to 3Khz
signal to a volt or more with the absolute minimum amount of noise?

I plan on following the amplifier stage with a bandpass filter, so
it's the noise in the 100Hz to 10Khz range that I am most concerned
about.

AD797 or a solution with external dual transistors like LM394 or MAT03

ciao Ban
 
J

John Larkin

Jan 1, 1970
0
What's the best opamp for amplifying a sub-microvolt 300Hz to 3Khz
signal to a volt or more with the absolute minimum amount of noise?

I plan on following the amplifier stage with a bandpass filter, so
it's the noise in the 100Hz to 10Khz range that I am most concerned
about.

What's the signal source impedance? That's important to the choice.

Your numbers imply an opamp with a gain-bandwidth product in the tens
of GHz, which no single opamp has. This will need at least two gain
stages.

What's the application?

John
 
H

henryf

Jan 1, 1970
0
Clueless said:
What's the best opamp for amplifying a sub-microvolt 300Hz to 3Khz
signal to a volt or more with the absolute minimum amount of noise?

If you really want the ***absolute minimum amount of noise***
you may need something expensive and cryogenic. What are the
impedance and temperature of the signal source?
 
G

Guy Macon

Jan 1, 1970
0
Ban said:
AD797 or a solution with external dual transistors like LM394 or MAT03

Related questions; the AD797 has very low voltage noise,
but JFET input opamps tend to have lower current noise.
I am starting a project where I will be comparing forward
bias photoconductive and zero bias photovoltaic circuits
(see following ASCII schematics) at low light levels.

I need low noise. Photovoltaic is usually used for
low noise and photoconductive for fast response, but I
will be using a 2D PSD instead of a normal photodiode,
so I want to see for myself which circuit works best
in my application.

Would a AD797 still be a good choice for both circuits?
Or should I look into a low current noise opamp (and if
so, which one?)

(Pardon the funkyness of the little lighting bolt arrow. <smile> )



\ light REVERSE BIAS
\/\ PHOTOCONDUCTIVE
\/\ (AKA PHOTORESISTIVE)
\/
|/| resistor
VCC --| |-------/\/\/\------
|\| | |
| |\ |
photo -----| \ |
diode |- \ |
| \--------O
| /
-----|+ / opamp
| | /
--- |/
\ /
GND



\ light ZERO BIAS
\/\ PHOTOVOLTAIC
\/\
\/
|/| resistor
------| |-------/\/\/\------
| |\| | |
--- | |\ |
\ / photo -----| \ |
GND diode |- \ |
| \--------O
| /
-----|+ / opamp
| | /
--- |/
\ /
GND
 
B

Ban

Jan 1, 1970
0
Guy said:
Related questions; the AD797 has very low voltage noise,
but JFET input opamps tend to have lower current noise.
I am starting a project where I will be comparing forward
bias photoconductive and zero bias photovoltaic circuits
(see following ASCII schematics) at low light levels.

I need low noise. Photovoltaic is usually used for
low noise and photoconductive for fast response, but I
will be using a 2D PSD instead of a normal photodiode,
so I want to see for myself which circuit works best
in my application.

Would a AD797 still be a good choice for both circuits?
Or should I look into a low current noise opamp (and if
so, which one?)

Guy,
in this case the best bet is a BJT-prestage followed by a precision opamp as
described in this link.
http://users.bestweb.net/~hobbs/frontends/frontends.pdf

The AD797 is more suited for lower input impedances like dynamic microphones
and other sensors with Rin<5k.
 
R

Rene Tschaggelar

Jan 1, 1970
0
Clueless said:
What's the best opamp for amplifying a sub-microvolt 300Hz to 3Khz
signal to a volt or more with the absolute minimum amount of noise?

I plan on following the amplifier stage with a bandpass filter, so
it's the noise in the 100Hz to 10Khz range that I am most concerned
about.

For a fairly low impedance source : have a look at 2 stages with LT1128
at gain 2000, AC coupled, each.

LT1128 :
ultra low noise 35nVpp (0.1 to 10Hz) 1nV/rtHz@10Hz 0.9nV/ rtHz@1k
4.7pA/rtHz@10Hz 1pA/rtHz@1k, high speed 5V/us, BW 13MHz, 10nA bias,
20uV offset, 0.3uV/K drift, gain 1 stable, supply +-15V, precision,
SO8, 5.75$@1

If the source is high impedance, consider the first stage being a FET
Opamp.

Rene
 
P

Paul Burridge

Jan 1, 1970
0
What's the best opamp for amplifying a sub-microvolt 300Hz to 3Khz
signal to a volt or more with the absolute minimum amount of noise?

I plan on following the amplifier stage with a bandpass filter, so
it's the noise in the 100Hz to 10Khz range that I am most concerned
about.

Linear's LT1024 must be a strong contender.
 
W

Winfield Hill

Jan 1, 1970
0
Paul Burridge wrote...
Linear's LT1024 must be a strong contender.

No, certainly NOT. It has low offset voltage (but not as low as
any good chopper IC) so it might be useful for DC amplification,
but the OP specified 0.3 - 3kHz. In this region it's very noisy,
14nV/Hz^1/2, so not only is it not a "contender," it's not even
an "also ran." Paul, do you know what e_n, i_n and spectral noise
density is? Go read AoE pages 428 to 455 and then report back :>)

Thanks,
- Win

whill_at_picovolt-dot-com
 
P

Paul Burridge

Jan 1, 1970
0
No, certainly NOT. It has low offset voltage (but not as low as
any good chopper IC) so it might be useful for DC amplification,
but the OP specified 0.3 - 3kHz. In this region it's very noisy,
14nV/Hz^1/2, so not only is it not a "contender," it's not even
an "also ran." Paul, do you know what e_n, i_n and spectral noise
density is? Go read AoE pages 428 to 455 and then report back :>)

Yes, m'lud. :(
 
G

Guy Macon

Jan 1, 1970
0
Ban said:
Guy,

in this case the best bet is a BJT-prestage followed by a precision
opamp as described in this link.
http://users.bestweb.net/~hobbs/frontends/frontends.pdf

The AD797 is more suited for lower input impedances like dynamic
microphones and other sensors with Rin<5k.

Thanks! that's a great link. I hadn't even considered using a
BJT cascode with bootstrapping instead of the usual single opamp
transimpedence amplifier.

After I build it and test it as described, I will try modifying
the circuit so as to put the photodiode in zero-bias photovoltaic
mode instead of reverse-bias photoconductive mode. It will be
interesting seeing how well that works.
 
C

Clueless Newbie

Jan 1, 1970
0
What's the signal source impedance? That's important to the choice.

Your numbers imply an opamp with a gain-bandwidth product in the tens
of GHz, which no single opamp has. This will need at least two gain
stages.

What's the application?
I am interfacing to a Endevco Model 22 Picomin Piezoelectric accelerometer
(http://www.bksv.com/pdf/22.pdf) It has a resistance in the G-OHMs and a
capacitance of 250P-F. I don't know enough to figure out the impedance.

My signal is nearly a microvolt when I look at it with a scope but very
noisy. Not just the usual 60Hz, but also a lot of random looking noise.

I am only going to build ten of these, and the boss is paying for the parts,
so cost isn't a big factor.

One dude wrote 'the AD797 has very low voltage noise, but JFET input opamps
tend to have lower current noise'. In my application. am I looking for low
voltage noise or low current noise? What's a good opamp for low current
noise? I want to see if I can do it with opamps only before I try something
with transistors. I find opamps easier to work with. Probly just my lack of
experience though.

Another dude wrote 'If the source is high impedance, consider the first
stage being a FET Opamp'. Sounds good to me. What's a good part number
to try? Is a FET the same as a JFET? I have only used the MOS-FET, and
I had a lot of trouble figuring it out.

Would the AD797 be good for that second stage that I will need?

I would like to read AoE pages 428 to 455. What's an AoE? ;-)

Some of the answers here are a bit over my head, but I am learning a lot.
Thanks for helping a newbie. Maybe someday I will learn enough to be the
one doing the helping!

Sorry for all of the dumb questions...
 
G

Guy Macon

Jan 1, 1970
0
Clueless Newbie said:
I would like to read AoE pages 428 to 455. What's an AoE? ;-)

_The Art of Electronics_ by by Paul Horowitz and Winfield Hil.
[ http://www.artofelectronics.com/ ].

This is the best electronics book in the universe. I wore my
first one out while making the transition from technician to
engineeer.

Seriously, you *need* this book.

Also, please take a look at this list. You will be glad that you did.
http://www.amazon.com/exec/obidos/tg/listmania/list-browse/-/34N5J7T7ZGROM/
Is a FET the same as a JFET? I have only used the MOS-FET, and
I had a lot of trouble figuring it out.

Learning how to use all of the varieties of transistor will be well
worth your while. Sometimes no opamp circuit will quite do what
you want to do. Sometimes you can't afford an opamp (like when I
was in charge of a project at Mattel that was pumping out 100,000
uC-based toys per day). Remember, if all you have is a hammer, you
tend to treat screws as if they were nails...
 
J

Jim Thompson

Jan 1, 1970
0
//www.guymacon.com> wrote:

[snip]
Sometimes you can't afford an opamp (like when I
was in charge of a project at Mattel that was pumping out 100,000
uC-based toys per day).
[snip]

Did you know Jack Ryan?

He tried to hire me in 1968, but the Ruth Handler wouldn't approve the
financial package I needed to endure "living" in Los Angeles.

Her obit is a real laugher, claiming she invented everything ;-)

...Jim Thompson
 
J

John Larkin

Jan 1, 1970
0
I am interfacing to a Endevco Model 22 Picomin Piezoelectric accelerometer
(http://www.bksv.com/pdf/22.pdf) It has a resistance in the G-OHMs and a
capacitance of 250P-F. I don't know enough to figure out the impedance.

Piezos are definitely high impedance. The first stage is usually a
"charge amp", an opamp with mostly capacitive feedback, which
equalizes the piezo output into nice g's per volt or something.

My signal is nearly a microvolt when I look at it with a scope but very
noisy. Not just the usual 60Hz, but also a lot of random looking noise.

I am only going to build ten of these, and the boss is paying for the parts,
so cost isn't a big factor.

One dude wrote 'the AD797 has very low voltage noise, but JFET input opamps
tend to have lower current noise'. In my application. am I looking for low
voltage noise or low current noise? What's a good opamp for low current
noise? I want to see if I can do it with opamps only before I try something
with transistors. I find opamps easier to work with. Probly just my lack of
experience though.

You need really low current noise here, so a jfet opamp would be a
good choice.

This is not simple enough to adequately describe here: you'll need a
low-noise jfet opamp, the proper feedback network, and superb
isolation/decoupling/shielding.

Google "charge amplifier" and "piezo accelerometer signal conditioner"
and stuff like that.
Another dude wrote 'If the source is high impedance, consider the first
stage being a FET Opamp'. Sounds good to me. What's a good part number
to try? Is a FET the same as a JFET? I have only used the MOS-FET, and
I had a lot of trouble figuring it out.

Would the AD797 be good for that second stage that I will need?

The second stage shouldn't be critical, maybe an OP-27 or something
like that.
I would like to read AoE pages 428 to 455. What's an AoE? ;-)

Horowitz and Hill's book "The Art Of Electronics." Tell your boss you
need it immediately.


John
 
G

Guy Macon

Jan 1, 1970
0
Jim Thompson said:
Did you know Jack Ryan?

Before my time. I came in right after Jill Barad, the worst CEO of
any company ever, left in disgrace. She paid $3.6 billion to
purchase the Learning company, promising that it would bring in
$50 million per year.

Stop for a moment and think about that rate of return. Compare it
to putting the money in the lowest-rate investment that you know of...

The bad news was that instead of making $50 million per year, it lost
$206 million the first year. Buy the time they dumped it, they were
looking for someone who was willing to buy a company that was losing
$50 million a month. The selling price? Zero dollars, with a promise
to pay Mattel a small percentage if it ever returned to profitability.

Jill's reward for this performance? Severance pay of $50 million.
For two years work losing the comapny over $2.5 billion dollars.

See:
http://www.findarticles.com/cf_dls/m5072/19_22/62534744/p1/article.jhtml
http://www.findarticles.com/cf_dls/m5072/19_22/62534744/p2/article.jhtml
He tried to hire me in 1968, but the Ruth Handler wouldn't approve the
financial package I needed to endure "living" in Los Angeles.

Was it as good as Jill's? :)
 
J

Jim Thompson

Jan 1, 1970
0
Jim Thompson said:
Did you know Jack Ryan? [snip]
He tried to hire me in 1968, but the Ruth Handler wouldn't approve the
financial package I needed to endure "living" in Los Angeles.

Was it as good as Jill's? :)

Not hardly. All I wanted was Mattel to pay the down payment of a
house in Palos Verdes. Handler balked, so I went back to Arizona and
bought a 3000 sq.ft. house on an acre, for $37,500 ;-)

...Jim Thompson
 
G

Guy Macon

Jan 1, 1970
0
Guy Macon said:
Thanks! that's a great link. I hadn't even considered using a
BJT cascode with bootstrapping instead of the usual single opamp
transimpedence amplifier.

I would also like to experiment with an opamp solution.
What's a good part to use when you need low current noise
rather than low voltage noise, and wish to avoid adding
external transistors?
 
C

Clueless Newbie

Jan 1, 1970
0
Guy Macon wrote
What's a good part to use when you need low current noise
rather than low voltage noise, and wish to avoid adding
external transistors?

That would be that JFET op amp that you were talking about, right? And that's
what I need to work best with my high impedance sensor, right? I would also
like to know a good part# to try.
 
Top