Q: How to amplify 15uV, 1.5MHz signal to read on Scopemeter

On Sat, 27 Oct 2007 11:38:40 -0400, John Popelish <jpopelish@rica.net>
wrote:

Phil Allison wrote:
"John Popelish"
JimG wrote:
(snip)
In fact, I've made a little progress with a crude circuit using a
LF411CN op amp and a negative feedback loop (yielding a gain of around
320).
(snip)

Not at 1.5 MHz, you didn't. That would take an opamp with a gain
bandwidth of at least 320*1.5 MHz = 480 MHz, at least.

The LF411 has a gain bandwidth of about 3 MHz, capable of a gain of 2, at
most, with 1.5 MHz signals.
http://cache.national.com/ds/LF/LF411.pdf

Now, if you have actually been talking about a 1.5 kHz signal, then that
is a horse of a different color.



** LOL

I kept wondering if that 15 uV figure ought to be 15 mV.

But of course, why the heck would a thermocouple amp need to chop at 1.5 MHz
??

Perhaps it is a micron sized thermocouple measuring the
temperature of a speck of radium as it radioactively decays.

Yeah, that is probably it. ;-)
Click to expand...
Nope, nothing nearly that exotic. It's a thermcouple simulator and DC
volt/millivolt source. When calibrated, it's output is 50 ppm +/- 3
uV from 11V to ~1 uV.

You probably understand how that fits with its chopper configuration,
but I don't. I just know I was told to be prepared to pick up a
signal around 15uV, which is what I am seeing now.

Unfortunately, the circuit diagram in the service manual has a
printing error, which led me down the 1.5MHz path for awhile.

Jim
 
JimG wrote:
On Sat, 27 Oct 2007 01:57:22 -0400, John Popelish <jpopelish@rica.net
wrote:

Looks like my crude preamp circuit is going to work. I've been able
to increase the gain to 1500, which at 1.2kHz is holding up OK on the
LF411CN op amp.
Click to expand...
If you use two opamp stages with equal gains of about the
square root of the gain of your single stage, you will get a
much better frequency response that you are getting, now.

The source signal from the chopper amp is indeed close to 15uV p-p,
which gets amplified to almost 25mV. This is a couple of divisions on
my Scopemeter and is easily visible.

But there is still a fair amount of noise on top of the basic 1.2kHz
signal. Any further suggestions for cheap ways to reduce this noise
are welcome.
Click to expand...
A different scope would help a lot. An old analog scope
with a phosphor screen would be better at visually averaging
many traces, so that the brightest part of the image would
be a lot smoother than you get on a scope meter. But better
digital scopes include a signal averaging choice that
mathematically averages multiple passes, and displays the
average. That would really clean it up. Perhaps you can
borrow one.

It's not clear to me if something like the Minicircuits GALI-39 that
ninja suggested is still appropriate for a 1.2kHz signal with 0.05 ohm
output impedance.
Click to expand...
No. Dropping the frequency requirement by 1000 means that
opamps are right in their sweet spot. But you could benefit
from one that has very low voltage noise for the first gain
stage, like an LT1028 or LT1037 or OP27, which all have
lower input noise than the LF411.

LF411 with about 25 nV/Hz^.5 at 10kHz:
http://cache.national.com/ds/LF/LF411.pdf

LT1028 with about 8 nV/Hz^.5 at 10kHz and over 10 times the
gain bandwidth product:
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1026,P1235,D3480

LT1037 with about 2.5 nV/Hz^.5 at 10kHz and about 20 times
the gain bandwidth product:
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1026,P1212,D2204

OP27 with about 10 nV/Hz^.5 at 10kHz but less than 1/5th the
gain bandwidth product:
http://www.analog.com/UploadedFiles/Data_Sheets/OP07.pdf
 
On Sat, 27 Oct 2007 11:43:06 -0400, John Popelish <jpopelish@rica.net>
wrote:
(snip)
Though, I would
still reserve 90% of the opamp gain bandwidth for the
harmonics of the signal, so I could see the wave shape,
clearly.
(snip)
Click to expand...

I don't yet understand what you're telling me here, but I'll do some
investigation and see if I can figure it out.

I currently have it set up for around 30 dB gain. If I am
interpreting the op amp spec's correctly, it can handle 70 dB at 1kHz.

Thanks.

Jim
 
JimG wrote:
On Sat, 27 Oct 2007 11:43:06 -0400, John Popelish <jpopelish@rica.net
wrote:
(snip)
Though, I would
still reserve 90% of the opamp gain bandwidth for the
harmonics of the signal, so I could see the wave shape,
clearly.
(snip)

I don't yet understand what you're telling me here, but I'll do some
investigation and see if I can figure it out.

I currently have it set up for around 30 dB gain. If I am
interpreting the op amp spec's correctly, it can handle 70 dB at 1kHz.
Click to expand...
At the fundamental frequency of 1.2 kHz, lets say you want a
gain of 100. That requires at least 100*1200=120kHz gain
bandwidth product from the opamp. but to produce that gain
for the 10th harmonic requires at least 100*1200*10=1.2 MHz
gain bandwidth produce. That is the factor of 10 I was
talking about. I would not ask the opamp to produce more
than 1/10th of the fundamental gain bandwidth (gain times
the fundamental frequency of the wave) so that it had
plenty of gain to accurately also amplify enough harmonics
of the wave frequency to accurately maintain the wave shape.
 
On Sat, 27 Oct 2007 12:28:54 -0400, John Popelish <jpopelish@rica.net>
wrote:

At the fundamental frequency of 1.2 kHz, lets say you want a
gain of 100. That requires at least 100*1200=120kHz gain
bandwidth product from the opamp. but to produce that gain
for the 10th harmonic requires at least 100*1200*10=1.2 MHz
gain bandwidth produce. That is the factor of 10 I was
talking about. I would not ask the opamp to produce more
than 1/10th of the fundamental gain bandwidth (gain times
the fundamental frequency of the wave) so that it had
plenty of gain to accurately also amplify enough harmonics
of the wave frequency to accurately maintain the wave shape.
Click to expand...
Understood - thanks.

I take that to mean that I should limit the gain to around 300:

300 * 1.2kHz * 10 = 3600kHz

Gain bandwidth of the op amp looks like it's 4MHz at 25C.

Jim
 
JimG wrote:
On Sat, 27 Oct 2007 12:28:54 -0400, John Popelish <jpopelish@rica.net
wrote:

At the fundamental frequency of 1.2 kHz, lets say you want a
gain of 100. That requires at least 100*1200=120kHz gain
bandwidth product from the opamp. but to produce that gain
for the 10th harmonic requires at least 100*1200*10=1.2 MHz
gain bandwidth produce. That is the factor of 10 I was
talking about. I would not ask the opamp to produce more
than 1/10th of the fundamental gain bandwidth (gain times
the fundamental frequency of the wave) so that it had
plenty of gain to accurately also amplify enough harmonics
of the wave frequency to accurately maintain the wave shape.

Understood - thanks.

I take that to mean that I should limit the gain to around 300:

300 * 1.2kHz * 10 = 3600kHz

Gain bandwidth of the op amp looks like it's 4MHz at 25C.
Click to expand...
That is what I am suggesting. If you want more gain than
that, cascade more amplifiers.
 
On Sat, 27 Oct 2007 13:28:50 -0400, John Popelish <jpopelish@rica.net>
wrote:

That is what I am suggesting. If you want more gain than
that, cascade more amplifiers.
Click to expand...
Will do. Many thanks.

Jim
 
"JimG"

But there is still a fair amount of noise on top of the basic 1.2kHz
signal. Any further suggestions for cheap ways to reduce this noise
are welcome.
Click to expand...

** Errrrrrr - a LP filter ??

Fuckwit IT geek.



......... Phil
 
"JimG" <jggall01@yahoo.com> wrote in message
news:k8i5i35blk6kilu7m94fm4trjstljerrc0@4ax.com...
On Fri, 26 Oct 2007 15:25:37 -0700, "Ninja"
Wannacat@NoSpam.adelphia.net> wrote:


The output impedance of the calibrator is very low - less than 0.05
ohm. Does the low impedance make the preamp you suggested more, or
less, feasible?
Click to expand...
I realize that the thread has taken in a different direction now that the
bandwidth requirement has been reduced by a factor of 1000, but for the sake
of completeness . . .

The thermal noise floor (voltage wise) is proportional to the square root of
the impedance level. A signal from a low impedance source would be competing
with a lower noise floor, so maintaining a good signal to noise ratio is
easier.

Congratulations on your success. There's nothing like rolling up your
sleeves and digging into a problem to further your understanding.
 
Phil Allison wrote:

** Go stick your head in a gas oven.




....... Phil
Click to expand...

fuch off Moron. I would step you into before my Ass sees the burning...


begone.... (anti-Nazi)
 
Phil Allison wrote:

** Go stick your head in a gas oven.




....... Phil
Click to expand...
latinic fucktard

crawl back to your moronic earth-hole :))))))


You make me laugh :), you are fooni.



(when your comments show your electronic judiciousness,
:))))))))))))))))



Keep on Clown.



Best Regards,

Daniel Mandic
 
Phil Allison wrote:

"Daniel Mandic"

Turn Off



** Go stick your head in a gas oven.




....... Phil
Click to expand...

How can you be so Styrian? (County in Austria)


I have been fishing 20punds Carps wit' a 6mil synthetic core :)))))

show me the same ;.)))))))))))))!



C'mon, you are a totally looooooooooseeeeeeer. You could not even match
in the Basics with me.

Phil........ :))))))))))))


(yeah..... fill'em)



:)
 
"Daniel Mandic"



http://www.yates.com.au/Products/PestControl/RatsMiceHouseholdPests/RatsakDoubleStrengthRTURatBait.asp






....... Phil
 
"Daniel Mandic" <daniel_mandic@aon.at> wrote in message
news:4723ff19$0$21926$91cee783@newsreader02.highway.telekom.at...
Phil Allison wrote:

Fuckwit IT geek.


Turn Off
Click to expand...
Apologies for this Aus (?) character. Most of us are friendly people who can
discuss things. He is an exception. Most of us have blocked him and only see
his rubbish if quoted!
 
On Sun, 28 Oct 2007 16:08:15 +1100, "Suzy" <not@valid> wrote:

"Daniel Mandic" <daniel_mandic@aon.at> wrote in message
news:4723ff19$0$21926$91cee783@newsreader02.highway.telekom.at...
Phil Allison wrote:

Fuckwit IT geek.


Turn Off

Apologies for this Aus (?) character. Most of us are friendly people who can
discuss things. He is an exception. Most of us have blocked him and only see
his rubbish if quoted!
Click to expand...
---
Aus(trian) or Aus(tralian)?


--
JF
 
"John Fields"
"Suzy the Self Appointed Net Kop Mad Bitch "
"Daniel Mandick"

Phil Allison wrote:

Fuckwit IT geek.


Turn Off

Apologies for this Aus (?) character. Most of us are friendly people who
can
discuss things. He is an exception. Most of us have blocked him and only
see
his rubbish if quoted!

---
Aus(trian) or Aus(tralian)?
Click to expand...


** Fraulein " Suzy the Net Kop " is likely the Austrian one.

Uncle Adolph would be proud of the dumb a dog shit NAZI slut.





....... Phil
 
Phil Allison wrote:

"Daniel Mandic"
fuch off Moron. I would step you into before my Ass sees the
burning...
Click to expand...

http://www.yates.com.au/Products/PestControl/RatsMiceHouseholdPests/Ra
tsakDoubleStrengthRTURatBait.asp
Click to expand...

yar..... you can take it with you when going into... :))))
 
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