low noise amplifier - can i build it?

[Roko]

hi everyone,

i'd like a bit of advice here: i need a preamplifier for measuring very
small vibrations on a piece of scientific kit. the sensor (a geophone -
i.e. a coil and magnet) will output voltages of about 0.1 microvolts
(in the range 0.1Hz to 1kHz), so i need to get hold of a really low
noise preamplifier.

firstly, does anyone know of a company might sell me what i want at a
reasonable price, i.e. less than $500 (Ł300)?

also, do you think i can build it myself to these specifications? i
have some experience with op-amp design and i know the basic theory,
but i'm not sure if i can actually get a good enough noise performance.


thanks in advance

roko mijic

 

[Rene Tschaggelar]

Roko wrote:

hi everyone,

i'd like a bit of advice here: i need a preamplifier for measuring very
small vibrations on a piece of scientific kit. the sensor (a geophone -
i.e. a coil and magnet) will output voltages of about 0.1 microvolts
(in the range 0.1Hz to 1kHz), so i need to get hold of a really low
noise preamplifier.

firstly, does anyone know of a company might sell me what i want at a
reasonable price, i.e. less than $500 (Ł300)?

also, do you think i can build it myself to these specifications? i
have some experience with op-amp design and i know the basic theory,
but i'm not sure if i can actually get a good enough noise performance.

Just to make sure the system cannot be excited
and a lock-in amplifier cannot be used.
Lownoise amplifiers are bought, not really built.
Since the source impedance appears to be low, have
a look at the LT1128, it is specified at 0.9nV/rtHz

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

 

[Ian Stirling]

Roko <rm382@cam.ac.uk> wrote:

hi everyone,

i'd like a bit of advice here: i need a preamplifier for measuring very
small vibrations on a piece of scientific kit. the sensor (a geophone -
i.e. a coil and magnet) will output voltages of about 0.1 microvolts
(in the range 0.1Hz to 1kHz), so i need to get hold of a really low
noise preamplifier.

A transformer in the front end might make things a little easier.
What's the impedance of the coil?

 

[Winfield Hill]

Rene Tschaggelar wrote...

Roko wrote:
i need a preamplifier for measuring very small vibrations on a
piece of scientific kit. the sensor (a geophone - i.e. a coil
and magnet) will output voltages of about 0.1 microvolts (in the
range 0.1Hz to 1kHz), so i need to get hold of a really low noise
preamplifier.

Since the source impedance appears to be low, have a look at
the LT1128, it is specified at 0.9nV/rtHz

http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1154,C1009,C1026,P1235

The LT1128 is a low-frequency version of the LT1028. I had not
noticed the huge 275pF high-voltage on-chip capacitor, one of
the largest I've seen. Perhaps Jim has seen (or used) bigger?

Anyway, Roko, back to your geophone amplifier... If it's low
frequencies you're interested in, the LT1128's spectral-noise
density starts going up below 250Hz, an effect that's called 1/f
noise. Thankfully its impact is not as dramatic as the graph,
page 5, implies, because when you integrate the noise density to
get the RMS noise level (and hence the peak-to-peak noise), the
sqrt-BW part of the calculation reduces low frequency influence.

A spectral noise of 0.9 to 2nV should be compared to the Johnson
noise of associated resistors in the circuit, e_n = sqrt (4kTR)
= 1.28nV for 100 ohms, and scales with sqrt R. For example your
geophone coil's DC resistance may be much higher than 100 ohms.
You also need to keep the value of critical gain resistors low.

There's 40 pages of detailed discussion about low-noise issues
in our book, The Art of Electronics. Recommended. :&gt


--
Thanks,
- Win

 

[Jim Thompson]

On 18 Jul 2005 06:12:42 -0700, Winfield Hill
<Winfield_member@newsguy.com> wrote:

[snip]

http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1154,C1009,C1026,P1235

The LT1128 is a low-frequency version of the LT1028. I had not
noticed the huge 275pF high-voltage on-chip capacitor, one of
the largest I've seen. Perhaps Jim has seen (or used) bigger?

[snip]

275pF, if square, would be about 565u X 565u, or about 22mil X 22 mil.

I've not seen one that big before, but if you've got real estate to
burn I guess it's OK ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[John Popelish]

Ken Smith wrote:

IIRC, the nominal impedance of a geophone is 600 Ohms. This impedance
normally appears as a resistance and not the inductance you would normally
expect from a coil.

You really need the spec sheet or to measure the resistance. I have

seen some units with very much higher resistance:
http://www.geosys.co.jp/GEO/Sensor/hs-10.html

 

[Winfield Hill]

John Popelish wrote...

Ken Smith wrote:

IIRC, the nominal impedance of a geophone is 600 Ohms. This impedance
normally appears as a resistance and not the inductance you would
normally expect from a coil.

Right, and hence a noise source.

You really need the spec sheet or to measure the resistance.
I have seen some units with very much higher resistance:
http://www.geosys.co.jp/GEO/Sensor/hs-10.html

And I'm quiet sure I've purchased geophones with lower resistance.
But they do tend to get up there, in their attempts for sensitivity.


--
Thanks,
- Win

 

[colin]

"Roko" <rm382@cam.ac.uk> wrote in message
news:1121683297.406897.172530@g14g2000cwa.googlegroups.com...

hi everyone,

i'd like a bit of advice here: i need a preamplifier for measuring very
small vibrations on a piece of scientific kit. the sensor (a geophone -
i.e. a coil and magnet) will output voltages of about 0.1 microvolts
(in the range 0.1Hz to 1kHz), so i need to get hold of a really low
noise preamplifier.

firstly, does anyone know of a company might sell me what i want at a
reasonable price, i.e. less than $500 (Ł300)?

also, do you think i can build it myself to these specifications? i
have some experience with op-amp design and i know the basic theory,
but i'm not sure if i can actually get a good enough noise performance.

some good sugestions so far, Also you need to consider 1/f noise wich wil
potentialy kill your noise budget at about 1hz,
1/f noise starts below a certain frequency wich may be quite high typically
1khz +

its hard to get an op amp below 0.1 uv pp noise in the range .1hz-10hz
many low noise amps with 1nv/rt hz at 10khz have total peak noise of 10uv pp
in the .1 10 hz range

One way to reduce 1/f noise is with an auto zeroing amplifier (also caled
zero drift), this cancels out the voltage offest and also the 1/f nosie wich
is equivalent to a quickly drifting offset voltage, parametric amplifiers
maybe the ultimate but they dont seem to be used much anymore.

Also you need to consider thermal drift wich will easily be in your
frequency range, thermoelectric effcets need to be considered, so thermal
mass and windproofing needed, best to put the pre amp as close as posible to
the source.

Ive been looking at similar problem of 1hz-10hz sub uv signals although I
have very high src impedance I am using the ad8552 for high src impedance
this has low noise (1.6uv pp) 0-10hz and extremly low curent noise 2fa/rthz
at 10hz, and low offset voltage 1uv

however there are better amplifiers for lower src impedances, the ad745 has
lower voltage noise of .38uv pp, but at 1hz the current noise is 100fa/rthz

ad797 has 50nv pp @.1-10hz but unspecified curent noise in this range,
although it specifies src impdeance of 2k for min noise at unspecified freq.

the lt1028 as sugested is best for source impedances of <400ohm at 10hz and
probably much lower impedance for lower frequencies.

also ad8628 has both low voltage and curent noise in this range, and many of
these data sheets are quite informative about getting good noise
performance.

If you look for 'ultra low noise' 'precision' 'zero drift' amplifier etc ..
im still looking atm

Colin =^.^=

 

[Robert Latest]

On Mon, 18 Jul 2005 15:32:46 GMT,
colin <no.spam.for.me@ntlworld.com> wrote
in Msg. <yYPCe.1820$Ox6.920@newsfe1-win.ntli.net>

its hard to get an op amp below 0.1 uv pp noise in the range .1hz-10hz
many low noise amps with 1nv/rt hz at 10khz have total peak noise of 10uv pp
in the .1 10 hz range

One way to reduce 1/f noise is with an auto zeroing amplifier (also caled
zero drift), this cancels out the voltage offest and also the 1/f nosie wich
is equivalent to a quickly drifting offset voltage,

Linear's LT1051 is spec'd at 1.5uVpp 0-10Hz and .4uVpp 0-1Hz. From there
on it's a pretty steady 70nV/rtHz up to 3kHz.

robert

 

[Pooh Bear]

Roko wrote:

hi everyone,

i'd like a bit of advice here: i need a preamplifier for measuring very
small vibrations on a piece of scientific kit. the sensor (a geophone -
i.e. a coil and magnet) will output voltages of about 0.1 microvolts
(in the range 0.1Hz to 1kHz), so i need to get hold of a really low
noise preamplifier.

firstly, does anyone know of a company might sell me what i want at a
reasonable price, i.e. less than $500 (Ł300)?

also, do you think i can build it myself to these specifications? i
have some experience with op-amp design and i know the basic theory,
but i'm not sure if i can actually get a good enough noise performance.

Your requirements aren't much different to those of a high spec microphone
preamplifier used in pro-audio equipment. Indeed, the bandwidth you need is
less.

Check out the specs of the SSM2019.

http://www.analog.com/en/prod/0,2877,SSM2019,00.html

Graham

 

[Ken Smith]

In article <dbg9sa0795@drn.newsguy.com>,
Winfield Hill <Winfield_member@newsguy.com> wrote:
[...]

geophone coil's DC resistance may be much higher than 100 ohms.
You also need to keep the value of critical gain resistors low.

IIRC, the nominal impedance of a geophone is 600 Ohms. This impedance
normally appears as a resistance and not the inductance you would normally
expect from a coil.

--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <42dba5a6$0$6301$ed2619ec@ptn-nntp-reader03.plus.net>,
Ian Stirling <root@mauve.demon.co.uk> wrote:

Roko <rm382@cam.ac.uk> wrote:
hi everyone,

i'd like a bit of advice here: i need a preamplifier for measuring very
small vibrations on a piece of scientific kit. the sensor (a geophone -
i.e. a coil and magnet) will output voltages of about 0.1 microvolts
(in the range 0.1Hz to 1kHz), so i need to get hold of a really low
noise preamplifier.

A transformer in the front end might make things a little easier.
What's the impedance of the coil?

The name "Geoformer" comes to mind as a transformer specifically intended
for this application.

--
--
kensmith@rahul.net forging knowledge

 

[John Popelish]

colin wrote:
(snip)

Ive been looking at similar problem of 1hz-10hz sub uv signals although I
have very high src impedance I am using the ad8552 for high src impedance
this has low noise (1.6uv pp) 0-10hz and extremly low curent noise 2fa/rthz
at 10hz, and low offset voltage 1uv
(snip)

I have been reviewing opamps for noise in this frequency range, but
haven't yet included the AD8552. But you might look at the auto
zeroing LTC1047 and LTC1051 and OP2604 or OPA604.

 

[Adrian Tuddenham]

Roko <rm382@cam.ac.uk> wrote:

hi everyone,

i'd like a bit of advice here: i need a preamplifier for measuring very
small vibrations on a piece of scientific kit. the sensor (a geophone -
i.e. a coil and magnet) will output voltages of about 0.1 microvolts
(in the range 0.1Hz to 1kHz), so i need to get hold of a really low
noise preamplifier.

firstly, does anyone know of a company might sell me what i want at a
reasonable price, i.e. less than $500 (Ł300)?

also, do you think i can build it myself to these specifications? i
have some experience with op-amp design and i know the basic theory,
but i'm not sure if i can actually get a good enough noise performance.

Be very careful reading the low-noise specification. Many devices
become noisy at lower frequencies and the specifications are often
written to conceal this fact.

Check that your amplifier/amplifying device is low noise over the whole
of the frequency range you are using. If it doesn't say so in the spec
sheet, it probably isn't so.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

 

[Roko]

wow! thanks for the prompt (and numerous) replies - especially to Win -
i can't believe that - i was reading your book today trying to figure
out how to make this thing! wow!!!!! great book btw.

ok, a few points: the sensor will be at 0.3 degrees kelvin, so thermal
noise is less of a problem, and it has a resistance of 250 ohms. this
means we're getting a thermal spectral noise density of 0.06 nV /root
Hz.

Also, since i'll be plugging the output into a spectrum analyser, the
fact that the 0.1Hz - 10Hz band doesn't contain much bandwidth (and
therefore little noise power) doesn't help, because the component of my
signal at those frequencies won't contain much power either.

i'm not sure about what an "auto zeroing amplifier" is but it sounds
like it might just eradicate everything below a certain frequency,
which isn't what i want, my signal is at 1 - 2 Hz! is this the case?

The LLT 1128 looks good - it specifies 35 nV of noise from 0.1Hz - 10
Hz, so would i be correct in thinking that the spectral density will be
approximately 35/root10 = 10 nV/rootHz? this looks good.

the SSM2019 looks nice at $3 - i may as well just buy it, but it only
specifies the noise at 1kHz.

thanks guys, i'll look at the chips and maybe start ordering. am i
correct in thinking that these chips will need putting on a PCB and a
few resistors soldering on, or they contain all the components already?

cheers

Roko Mijic

 

[Pooh Bear]

Roko wrote:

wow! thanks for the prompt (and numerous) replies - especially to Win -
i can't believe that - i was reading your book today trying to figure
out how to make this thing! wow!!!!! great book btw.

ok, a few points: the sensor will be at 0.3 degrees kelvin, so thermal
noise is less of a problem, and it has a resistance of 250 ohms. this
means we're getting a thermal spectral noise density of 0.06 nV /root
Hz.

Also, since i'll be plugging the output into a spectrum analyser, the
fact that the 0.1Hz - 10Hz band doesn't contain much bandwidth (and
therefore little noise power) doesn't help, because the component of my
signal at those frequencies won't contain much power either.

i'm not sure about what an "auto zeroing amplifier" is but it sounds
like it might just eradicate everything below a certain frequency,
which isn't what i want, my signal is at 1 - 2 Hz! is this the case?

The LLT 1128 looks good - it specifies 35 nV of noise from 0.1Hz - 10
Hz, so would i be correct in thinking that the spectral density will be
approximately 35/root10 = 10 nV/rootHz? this looks good.

the SSM2019 looks nice at $3 - i may as well just buy it, but it only
specifies the noise at 1kHz.

On the data sheet the noise is shown down to 1Hz IIRC. It's about 3nV/rtHz
at that point.

It wasn't actually designed for LF use but is still pretty decent.

thanks guys, i'll look at the chips and maybe start ordering. am i
correct in thinking that these chips will need putting on a PCB and a
few resistors soldering on, or they contain all the components already?

They'll need a circuit designed around them. As you say, a few Rs mainly
but don't forget power supply decoupling etc... Talking of which, do you
have a bipolar DC supply for this amplifier ?

Graham

 

[colin]

"Roko" <rm382@cam.ac.uk> wrote in message
news:1121725694.184469.325250@o13g2000cwo.googlegroups.com...

ok, a few points: the sensor will be at 0.3 degrees kelvin, so thermal
noise is less of a problem, and it has a resistance of 250 ohms. this
means we're getting a thermal spectral noise density of 0.06 nV /root
Hz.

250 ohms makes things easier, low noise voltage is usualy mutualy exclusive
with low noise curent.

i'm not sure about what an "auto zeroing amplifier" is but it sounds
like it might just eradicate everything below a certain frequency,
which isn't what i want, my signal is at 1 - 2 Hz! is this the case?

no the auto zero doesnt zero the signal, just cancels out the errors in the
op amp, otherwise the op amp behaves as ay normal op amp from dc to its
gbw. for low impedance you can get just as good results with ordinary type.

The LLT 1128 looks good - it specifies 35 nV of noise from 0.1Hz - 10
Hz, so would i be correct in thinking that the spectral density will be
approximately 35/root10 = 10 nV/rootHz? this looks good.

the spectral density of the noise voltage rises below its corner with
falling frequency @ 1/f
the curent noise also rises @ 1/f below its corner frequency but the corner
frequency is often higher.
the 1/f noise only goes down to some lower frequency so noise density isnt
very useful at this frequency range its best to look at the noise graphs on
the spec sheets very carefuly, or the quoted dc to 10hz total noise. some
noise spces have a very alarmingly steep rise at the bopom of their graph.
The reference to pop-corn noise in AoE is amusingly apropriate I gues.

the SSM2019 looks nice at $3 - i may as well just buy it, but it only
specifies the noise at 1kHz.

that looks good, it is probably specified at its lowest noise frequency, so
below 1khz it will probably rise @ 1/f? dont forget although the current
noise looks low too it will rise faster with falling frequency, even with
250 ohms it might become dominant at 0.1hz. if you want to be sure its best
to use an ic that has a specified noise performance at the frequecny you are
using. I think Win was interested in something like that for his programable
gain amp.

thanks guys, i'll look at the chips and maybe start ordering. am i
correct in thinking that these chips will need putting on a PCB and a
few resistors soldering on, or they contain all the components already?

just a few resistors to set the gain (1 if your lucky), coupling capacitors,
bias resistors, couple of voltage regulators and some good supply decoupling
capacitors, and a filter network to filter out unwanted noise, and possibly
a second stage of amplification too, or combination of filter/amplification.
there are some good examples in the many spec sheets/app notes. in fact i
wouldnt be surprised if there is an example for a geophoneamplifier
somewhere.

Colin =^.^=

 

[Rene Tschaggelar]

Pooh Bear wrote:

Roko wrote:

They'll need a circuit designed around them. As you say, a few Rs mainly
but don't forget power supply decoupling etc... Talking of which, do you
have a bipolar DC supply for this amplifier ?

Indeed, with a switcher, everything is lost.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

 

[Fred Bartoli]

"Rene Tschaggelar" <none@none.net> a écrit dans le message de
news:42dcaa87$0$1148$5402220f@news.sunrise.ch...

Pooh Bear wrote:

Roko wrote:

They'll need a circuit designed around them. As you say, a few Rs mainly
but don't forget power supply decoupling etc... Talking of which, do
you
have a bipolar DC supply for this amplifier ?


Indeed, with a switcher, everything is lost.

Not with a SA for analyzing the output signal.



--
Thanks,
Fred.

 

[Uwe Bonnes]

Roko <rm382@cam.ac.uk> wrote:

wow! thanks for the prompt (and numerous) replies - especially to Win -
i can't believe that - i was reading your book today trying to figure
out how to make this thing! wow!!!!! great book btw.

ok, a few points: the sensor will be at 0.3 degrees kelvin, so thermal
noise is less of a problem, and it has a resistance of 250 ohms. this
means we're getting a thermal spectral noise density of 0.06 nV /root
Hz.

There are also LN2 cooled amplifiers, at least in the area of nuclear
physics. Cooling the first stage can greatly enhance your SNR.

Bye
--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------