Definition of amp noise

[Don Lancaster]

Ken Smith wrote:

In article <1123694243.171363.152010@g47g2000cwa.googlegroups.com>,
pmlonline@gmail.com> wrote:
[...]

Thanks Ken. I found a great pdf file on noise,
http://eesof.tm.agilent.com/docs/iccap2002/MDLGBOOK/7DEVICE_MODELING/6NOISE/NOISEdoc.pdf

In considering the JFETs as an amplifier, wouldn't I have to use
several resistors? It seems that resistors are a definite source of
noise, which would add to the 1/4 nV/sqrt(Hz) JFET.


Resistors up to a few Gohms can be bought. They introdude very little
noise current so they can be used to bias the gate. The resistor in the
drain circuit will have less effect on the noise because the signal is
already amplified at that point. The resistor in the source, should be
bypassed with a very large capacitor so that it drops out of the picture
well below the 37Hz.



You can also get that level by putting 16 LSK170s in parallel.

Yes, great idea. I was thinking about that yesterday. Although I
didn't know that it was also the sqrt() of total parallel devices.
Isn't paralleling equal to multiple sampling? Consider a computer
program that does averaging. Say we take 10 samples, sum up all 10
samples, and take average signal. I thought that 10 averages would
decrease random noise by a factor of 10.


Try this:

Take one coin call heads +1 and tails -1. Flip it a bunch of times and
take the RMS. (Or not bother because we know it comes out as one)

Take two coins and do the same adding the values.

On the average 1 time in 4 you get -2 and one time in 4 you get +2. The
other 2 times you get zero. Therefor the RMS will come out to:

sqrt ( (2^2 + 2^2 + 0^2 + 0^2)/4 )

sqrt(2) = 1.414..etc

Any time you add random stuff together you get this sort of squarerooty
thing happening.

The signal, however adds linearly so your signal to noise is improved by
N/sqrt(N) or simply sqrt(N).



Does anyone have any input on the following idea? According to the
above pdf, Inductors are considered fairly noise free. Perhaps that
pertains to well made L's. I have not verified that.


Ideal inductors are noise free. The resistances of an inductor make noise
just like resistors. Magnetic cores make a thing called Barkhausen(sp)
noise that sounds like snapping and poping on headphones. Inductors also
tend to pick up AC magnetic fields and are also magnetic.


If that is true
then a transformer should also be fairly noise free-- say a nice
toroid.


Your frequency is kind of low but take a look at Triad's or Tamura's small
audio transformers.


Why not amplify the signal to say a few hundred times with the
transformer and then use a differential amplifier?


Think more along the lines of getting up to 10 to 100 times not hundreds.
Hundreds will not be easy to do and 10 or so should be enough. The
amplifier does not need to be differential BTW.

You can make you pick up a tuned circuit to get some signal increase and
filtering right at the start.


My prediction is that the S/N ratio at the input noise terminals is

unlikely to be changed much no matter what circuitry follows it.

But fully balanced, shielded, and guarded systems are a good starting point.

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: don@tinaja.com

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

Ken Smith wrote:

You can also get that level by putting 16 LSK170s in parallel.

Yes, great idea. I was thinking about that yesterday. Although I
didn't know that it was also the sqrt() of total parallel devices.
Isn't paralleling equal to multiple sampling? Consider a computer
program that does averaging. Say we take 10 samples, sum up all 10
samples, and take average signal. I thought that 10 averages would
decrease random noise by a factor of 10.

Try this:

Take one coin call heads +1 and tails -1. Flip it a bunch of times and
take the RMS. (Or not bother because we know it comes out as one)

Take two coins and do the same adding the values.

On the average 1 time in 4 you get -2 and one time in 4 you get +2. The
other 2 times you get zero. Therefor the RMS will come out to:

sqrt ( (2^2 + 2^2 + 0^2 + 0^2)/4 )

sqrt(2) = 1.414..etc

Any time you add random stuff together you get this sort of squarerooty
thing happening.

The signal, however adds linearly so your signal to noise is improved by
N/sqrt(N) or simply sqrt(N).

I'll just have to put it in the computer and let it rip.

Does anyone have any input on the following idea? According to the
above pdf, Inductors are considered fairly noise free. Perhaps that
pertains to well made L's. I have not verified that.

Ideal inductors are noise free. The resistances of an inductor make noise
just like resistors. Magnetic cores make a thing called Barkhausen(sp)
noise that sounds like snapping and poping on headphones. Inductors also
tend to pick up AC magnetic fields and are also magnetic.

Yes, I've read a lot on Barkhausen, the avalanche effect. Actually
it's fine even if the pure 37Hz sine wave is distorted, to even say a
square wave. It's the unpredictable random noise that I'm trying to
eliminate. Pure noise is always changing in phase and amplitude. I'm
sure the Barkhausen will add some 37Hz noise, but I look at it this
way. If the 37Hz signal were not present then Barkhausen noise would
be nearly zero in a toroid. If the 37Hz were present then there would
be 37Hz Barkhausen noise. Maybe the Barkhausen will help increase my
signal, lol.

Why not amplify the signal to say a few hundred times with the
transformer and then use a differential amplifier?

Think more along the lines of getting up to 10 to 100 times not hundreds.
Hundreds will not be easy to do and 10 or so should be enough. The
amplifier does not need to be differential BTW.

Wouldn't the differential help eliminate some of the terrestrial and
stray EM noise? Albeit I'm going to use a noise cancellation coil, but
it might not cancel 100%. Also, the computer and nearby equipment
would generate noise. In that case would a differential amp do
wonders?

You can make you pick up a tuned circuit to get some signal increase and
filtering right at the start.

With L's & C's? If the Q is high enough then the 37Hz signal across
the L or C would be amplified. Which method is better, LC or
transformer? Or perhaps both.

Thanks for the great tips,
Paul

 

Ken Smith wrote:

The amplifier does not need to be differential BTW.

Ken,
I hope you are right because so far I've been unable to find any
differential amplifiers even close to an op amps gain. I'm looking at
op amps with about 7 million voltage gain and 0.8 nV/Sqrt(Hz). Lets
say I have one coil that's 1-meter radius-- about 20 lbs of copper.
Next to this coil is a ferrite loopstick. The total turns on the
loopstick will be adjusted so as to cancel out any 37Hz noise in the
large coil. Although the large coil will be closer to the 37Hz signal
than the loopstick. The large coil and loopstick are properly
connected in series so that any EM noise, except the 37Hz signal, will
cancel each other out. By whatever method, the 37Hz signal is
amplified and sent to a signal meter. In such a case, do you think the
differential amp could help reduce enough noise? I'm thinking that the
amplifier itself could pick up some terrestrial noise and who knows
what else.

Paul

 

[Don Lancaster]

pmlonline@gmail.com wrote:

Ken Smith wrote:

The amplifier does not need to be differential BTW.


Ken,
I hope you are right because so far I've been unable to find any
differential amplifiers even close to an op amps gain.
Paul

Huh?

Most opamps ARE differential amplifiers!
Or can be combined by fours for even better ones.


--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: don@tinaja.com

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

Don Lancaster wrote:

pmlonline@gmail.com wrote:
Ken Smith wrote:

The amplifier does not need to be differential BTW.


Ken,
I hope you are right because so far I've been unable to find any
differential amplifiers even close to an op amps gain.
Paul


Huh?

Most opamps ARE differential amplifiers!
Or can be combined by fours for even better ones.

I was told by a EE that op-amps can be made into a differential but
they make poor differential amps. May be you know something that
people who design differentials don't know.

 

[Winfield Hill]

pmlonline@gmail.com wrote...

Don Lancaster wrote:
pmlonline@gmail.com wrote:
Ken Smith wrote:

The amplifier does not need to be differential BTW.


Ken,
I hope you are right because so far I've been unable to find
any differential amplifiers even close to an op amps gain.
Paul

Huh?

Most opamps ARE differential amplifiers!
Or can be combined by fours for even better ones.

I was told by a EE that op-amps can be made into a differential
but they make poor differential amps. May be you know something
that people who design differentials don't know.

Don Lancaster is an EE, as am I. We're sorry to see you confused.

Opamps are in fact high-quality differential amplifiers, with high
common-mode rejection and with very high gain at DC, approaching
infinity in the ideal case, and dropping at 20dB/decade until the
unity-gain frequency is reached, which is typically 5 to 200MHz.
These high-performance amplifiers are used inside feedback loops
to make various kinds of circuits. Perhaps you are interested
in difference or instrumentation amplifiers rather than opamps?
These are sometimes called differential amplifiers, which is an
incomplete description. Analog devices offers a free book about
these, "A Designer's Guide to Instrumentation Amplifiers," that
you can use to enhance your knowledge. Order it on this page,
http://www.analog.com/en/content/0,2886,759%255F842%255F65746,00.html

Burr-Brown (under the name TI), NSC, and Linear Technology are
other companies that make similar high-performance amplifiers.


--
Thanks,
- Win

 

[Winfield Hill]

pmlonline@gmail.com wrote...

Don Lancaster wrote:
By a conservative estimate, the manmade and terrestrial noise at this
frequency will be something like 170 decibels ABOVE the ktb noise.

It does not matter in the least which device he uses for his input stage.

Don, after further studying, I believe that I can cancel out nearly all
the terrestrial noise with a second nearby coil since the 37Hz signal
would be at a local small area. The second coil, that would be away
from the 37 Hz signal, would be the terrestrial noise cancellation
coil. I would expect the terrestrial noise to cancel out fairly well
since the RF wavelength at 37 Hz is over 8 million meters.

But if Don's 170dB estimate (a factor of 316 x 10^6) is correct, you'll
certainly not be able to imply cancel out the offending noise.

... a transformer should also be fairly noise free-- ...

Correct, it's low-frequency noise is given by its dc resistance.
But that won't abrogate the reality of ambient E-M signal noise.


--
Thanks,
- Win

 

[Ken Smith]

In article <1123780715.211012.308010@g44g2000cwa.googlegroups.com>,
<pmlonline@gmail.com> wrote:
[...]

Yes, I've read a lot on Barkhausen, the avalanche effect. Actually
it's fine even if the pure 37Hz sine wave is distorted, to even say a
square wave. It's the unpredictable random noise that I'm trying to
eliminate. Pure noise is always changing in phase and amplitude. I'm
sure the Barkhausen will add some 37Hz noise, but I look at it this
way. If the 37Hz signal were not present then Barkhausen noise would
be nearly zero in a toroid. If the 37Hz were present then there would
be 37Hz Barkhausen noise. Maybe the Barkhausen will help increase my
signal, lol.

The earth's field going through the core and any mechanical stress will
cause noise. Also a slowly changing field will make noise.

Think more along the lines of getting up to 10 to 100 times not hundreds.
Hundreds will not be easy to do and 10 or so should be enough. The
amplifier does not need to be differential BTW.

Wouldn't the differential help eliminate some of the terrestrial and
stray EM noise? Albeit I'm going to use a noise cancellation coil, but
it might not cancel 100%. Also, the computer and nearby equipment
would generate noise. In that case would a differential amp do
wonders?

You are going to use a transformer, I assume. The primary connects to the
signal and the secondary to the amplifier. The primary is balanced and
the secondary need not be.

With L's & C's? If the Q is high enough then the 37Hz signal across
the L or C would be amplified. Which method is better, LC or
transformer? Or perhaps both.

Tune the transformer if nothing else.

--
--
kensmith@rahul.net forging knowledge

 

[Pooh Bear]

pmlonline@gmail.com wrote:

Winfield Hill wrote:
pmlonline@gmail.com wrote...

Could someone so kindly tell me exactly what 1.3nV / Sqrt(Hz) @ 1KHz
means on a differential amp spec? What would the noise be between
37.00 Hz and 37.01 Hz? Would it be 1.3nV / Sqrt(37.00 - 37.01) or
1.3nV / Sqrt(37)

That's 1.3nV times sqrt 0.01Hz.

Thanks, it makes sense. So at 0.01 Hz bandwidth it comes to 130pV.
Where does the noise come from? Is it purely thermal or just a
byproduct of the amplification process?

Here's my problem. I am trying to get a rough guestimate of the values
to see if it is even plausible. I am estimating that a signal will be
picking up ~ 1pV of RF at 37Hz on a coil.

37Hz isn't an RF frequency. It's in the audio band !

Graham

 

[Joel Kolstad]

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:42FC227D.87ACCBCF@hotmail.com...

37Hz isn't an RF frequency. It's in the audio band !

Are you just arguing semantics? Obviously there's plenty of 50/60Hz EM
radiation floating around, and I expect that various subharmonics and
switching produce plenty of detectable radiation at 37Hz.

 

[Spehro Pefhany]

On Fri, 12 Aug 2005 05:15:57 +0100, the renowned Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote:

pmlonline@gmail.com wrote:

Winfield Hill wrote:
pmlonline@gmail.com wrote...

Could someone so kindly tell me exactly what 1.3nV / Sqrt(Hz) @ 1KHz
means on a differential amp spec? What would the noise be between
37.00 Hz and 37.01 Hz? Would it be 1.3nV / Sqrt(37.00 - 37.01) or
1.3nV / Sqrt(37)

That's 1.3nV times sqrt 0.01Hz.

Thanks, it makes sense. So at 0.01 Hz bandwidth it comes to 130pV.
Where does the noise come from? Is it purely thermal or just a
byproduct of the amplification process?

Here's my problem. I am trying to get a rough guestimate of the values
to see if it is even plausible. I am estimating that a signal will be
picking up ~ 1pV of RF at 37Hz on a coil.

37Hz isn't an RF frequency. It's in the audio band !

Graham

As are the 76 and 82Hz ELF/SLF frequencies known to be used by the US
and Russian navies respectively to communicate with submerged nuclear
attack submarines.

 

[Ken Smith]

In article <1123782360.794502.45890@z14g2000cwz.googlegroups.com>,
<pmlonline@gmail.com> wrote:

Ken Smith wrote:
The amplifier does not need to be differential BTW.

Ken,
I hope you are right because so far I've been unable to find any
differential amplifiers even close to an op amps gain.

ASCII-ART

GND
!
--- C1A
--- T1 = Triad SP-4?
! C3A
from--[RF bead]-+---+----!!-- ------ OP-amp input
coil ! !. !
--- ).(
---C2 ).(
! ).(
-----[RF bead]-+---+----!!-- . !
! C3B ! ------ GND near op-amp
! SHIELD
!
--- C1B
---
!
GND


The beads and the C1A and C1B serve to keep the local radio stations out
of the system. C2, tunes the receiver coil. C3A and C3B are fairly
optional. The tune the transformer. The transformer is a step up by
about 10:1.

--
--
kensmith@rahul.net forging knowledge

 

[Robert Latest]

On 2005-08-09, pmlonline@gmail.com <pmlonline@gmail.com> wrote:

noise. Hopefully some clever math technique, possibly taking advantage
of the known signal phase, can bring the detection time under a minute.

The fact that the signal phase is known suggests that you have
access to the generator of the signal, which would make your
problem a piece of cake for a commercial lock-in amplifier.

robert

 

Ken Smith wrote:

ASCII-ART

GND
!
--- C1A
--- T1 = Triad SP-4?
! C3A
from--[RF bead]-+---+----!!-- ------ OP-amp input
coil ! !. !
--- ).(
---C2 ).(
! ).(
-----[RF bead]-+---+----!!-- . !
! C3B ! ------ GND near op-amp
! SHIELD
!
--- C1B
---
!
GND


The beads and the C1A and C1B serve to keep the local radio stations out
of the system. C2, tunes the receiver coil. C3A and C3B are fairly
optional. The tune the transformer. The transformer is a step up by
about 10:1.

Ken,

Thanks very much! It's very clear. Although I am not certain what C3A
& C3B do, but you said they're optional. An increased voltage is on C2
because it's in resonance with the coil. The beads block most of
higher frequencies. C1A & C1B routes most of the higher frequencies to
ground. And of course the shielded T1 ups the 37Hz signal by 10 to the
op-amp. I might even get 200:1 from the Sp-4. It's an audio
transformer so the designers should have had a goal of minimum noise in
mind. If it has too much noise then there's always the
nano-crystalline cores, which have extremely low avalanche noise
because they're domains are magnitudes smaller than most cores.

I'll probably buy a few LT1028 op-amps today-- Voltage gain of 7
million min., 0.85nV/Sqrt(Hz) @ 1KHz, CMRR=126dB.
http://www.linear.com/pc/downloadDocument.do;jsessionid=C6G0DR5alY0CF3IWwMa2dhMCUVepcerqCnr6z5UMWxgydwXXm6ea!1137097510?navId=H0,C1,C1154,C1009,C1026,P1234,D3480
Ideally the T1 secondary should have low R. It seems op-amp source R
makes a huge difference in noise. In that case, there might be a small
chance that even 1:1 transformer could be better since it will have
less R. On the other hand, it seems we lose voltage gain with lower
load resistance. Here's a thought, maybe a bead could be placed across
the secondary in parallel to lower the op-amps source resistance. I
wonder if that counts, lol?

Perhaps a Summing Amplifier could be used to combine both the pickup
coil and noise cancellation coil. Of course the wires of noise
cancellation coil would be switched to they subtract, not combine.
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar5.html
Simply by changing R1 I can balance both coils to achieve optimum
cancellation. I think voltage gain = [ (V1/R1) + (V2/R2) ] * R3
Here's the page on a Differential Amplifier:
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar7.html
I wonder why the voltage gain of the Differential Amplifier is more
complex than the Summing Amplifier. It says Vout = V2 * (R3+R1)*R4 / [
(R4+R2)*R1 ] - V1*(R3/R1)

Here's a list of op-amp types:
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar.html

Thanks for the superb help!
Paul

 

Robert Latest wrote:

On 2005-08-09, pmlonline@gmail.com <pmlonline@gmail.com> wrote:

noise. Hopefully some clever math technique, possibly taking advantage
of the known signal phase, can bring the detection time under a minute.

The fact that the signal phase is known suggests that you have
access to the generator of the signal, which would make your
problem a piece of cake for a commercial lock-in amplifier.

Now that is very interesting. I see it outputs DC, which is great. I
have access to the 37Hz signal only by EM, but I also know the phase.
It's the amplitude that I need to detect-- it could from zero on up.
Here's a great lock-in amplifier site:
http://www.cpm.uncc.edu/lock_in_1.htm

Thank Robert! That will make a huge difference. I'm surprised to not
find any lock-in amplifier chips! Sure, you can get as complex as one
wants, but it really isn't that complex. So I'll use Ken's circuit as
a preamp and then feed it to an analog mixer and then to a low pass
filter. Do you have any mixers in mind. The first one I came to was
MC12002. Not sure if it works at 37Hz since some mixers have built in
caps and generally used for VHF. It would be nice to have a built in
oscillator, but problem no at such low frequencies. I could use a
simple audio oscillator.

Thanks again,
Paul

 

[Don Lancaster]

Pooh Bear wrote:

pmlonline@gmail.com wrote:


Winfield Hill wrote:

pmlonline@gmail.com wrote...

Could someone so kindly tell me exactly what 1.3nV / Sqrt(Hz) @ 1KHz
means on a differential amp spec? What would the noise be between
37.00 Hz and 37.01 Hz? Would it be 1.3nV / Sqrt(37.00 - 37.01) or
1.3nV / Sqrt(37)

That's 1.3nV times sqrt 0.01Hz.

Thanks, it makes sense. So at 0.01 Hz bandwidth it comes to 130pV.
Where does the noise come from? Is it purely thermal or just a
byproduct of the amplification process?

Here's my problem. I am trying to get a rough guestimate of the values
to see if it is even plausible. I am estimating that a signal will be
picking up ~ 1pV of RF at 37Hz on a coil.


37Hz isn't an RF frequency. It's in the audio band !

Graham

If it is receiving electromagnetic waves through an antenna like device,
it IS a RF frequency.

Bizarre as it seems.


--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: don@tinaja.com

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Don Lancaster]

Robert Latest wrote:

On 2005-08-09, pmlonline@gmail.com <pmlonline@gmail.com> wrote:


noise. Hopefully some clever math technique, possibly taking advantage
of the known signal phase, can bring the detection time under a minute.


The fact that the signal phase is known suggests that you have
access to the generator of the signal, which would make your
problem a piece of cake for a commercial lock-in amplifier.

robert

Or, to use a much more modern term, synchronous demodulation.

CMOS 4016 ferinstance.


--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: don@tinaja.com

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Don Lancaster]

pmlonline@gmail.com wrote:

Ken Smith wrote:

ASCII-ART

GND
!
--- C1A
--- T1 = Triad SP-4?
! C3A
from--[RF bead]-+---+----!!-- ------ OP-amp input
coil ! !. !
--- ).(
---C2 ).(
! ).(
-----[RF bead]-+---+----!!-- . !
! C3B ! ------ GND near op-amp
! SHIELD
!
--- C1B
---
!
GND


The beads and the C1A and C1B serve to keep the local radio stations out
of the system. C2, tunes the receiver coil. C3A and C3B are fairly
optional. The tune the transformer. The transformer is a step up by
about 10:1.



Ken,

Thanks very much! It's very clear. Although I am not certain what C3A
& C3B do, but you said they're optional. An increased voltage is on C2
because it's in resonance with the coil. The beads block most of
higher frequencies. C1A & C1B routes most of the higher frequencies to
ground. And of course the shielded T1 ups the 37Hz signal by 10 to the
op-amp. I might even get 200:1 from the Sp-4. It's an audio
transformer so the designers should have had a goal of minimum noise in
mind. If it has too much noise then there's always the
nano-crystalline cores, which have extremely low avalanche noise
because they're domains are magnitudes smaller than most cores.

I'll probably buy a few LT1028 op-amps today-- Voltage gain of 7
million min., 0.85nV/Sqrt(Hz) @ 1KHz, CMRR=126dB.
http://www.linear.com/pc/downloadDocument.do;jsessionid=C6G0DR5alY0CF3IWwMa2dhMCUVepcerqCnr6z5UMWxgydwXXm6ea!1137097510?navId=H0,C1,C1154,C1009,C1026,P1234,D3480
Ideally the T1 secondary should have low R. It seems op-amp source R
makes a huge difference in noise. In that case, there might be a small
chance that even 1:1 transformer could be better since it will have
less R. On the other hand, it seems we lose voltage gain with lower
load resistance. Here's a thought, maybe a bead could be placed across
the secondary in parallel to lower the op-amps source resistance. I
wonder if that counts, lol?

Perhaps a Summing Amplifier could be used to combine both the pickup
coil and noise cancellation coil. Of course the wires of noise
cancellation coil would be switched to they subtract, not combine.
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar5.html
Simply by changing R1 I can balance both coils to achieve optimum
cancellation. I think voltage gain = [ (V1/R1) + (V2/R2) ] * R3
Here's the page on a Differential Amplifier:
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar7.html
I wonder why the voltage gain of the Differential Amplifier is more
complex than the Summing Amplifier. It says Vout = V2 * (R3+R1)*R4 / [
(R4+R2)*R1 ] - V1*(R3/R1)

Here's a list of op-amp types:
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar.html

Thanks for the superb help!
Paul

Needless to say, any decent Q at 37Hz resonance would limit the
allowable rate of change of the input signal to be well under one Hertz.


--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: don@tinaja.com

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Don Lancaster]

pmlonline@gmail.com wrote:

Thank Robert! That will make a huge difference. I'm surprised to not
find any lock-in amplifier chips!

Thanks again,
Paul

Huh?

One quarter of a ten cent CMOS 4016 hung on any opamp to make +1 -1 gain.

There's even an ugly and unsubstantiated rumor that someone once wrote a
book on this.


--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: don@tinaja.com

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Kevin]

pmlonline@gmail.com wrote:
....>

I'll probably buy a few LT1028 op-amps today-- Voltage gain of 7
million min., 0.85nV/Sqrt(Hz) @ 1KHz, CMRR=126dB.

....
Paul

Paul,

Be aware that the quoted gain of an op-amp is the DC open loop gain and
normally the feedback is selected to bring the closed loop gain to very
much less - typically lower than 1000 per stage.

The gain will typically start dropping off at a very low frequency in
the 1Hz region.

Not to say that this device is not a good choice for your application,
it does have extremely good low frequency noise characteristics.

Also you were asking questions about why Differential Amplifiers had
much less gain than opamps: This is because usually such an amplifier
is actually an opamp with feedback all in the same package to bring the
gain to a well controlled figure (usually in the range of 1 to 100) eg
TIs INA114 series.

kevin