a low noise level opamp
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a low noise level opamp

 
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huiliu
Guest
Posted: Sun Dec 11, 2005 9:35 am    Post subject: a low noise level opamp Reply with quote
Can any one tell me where to get a low noise level opamp? It basically
statifies 10 micro V @ 0Hz -- 5kHz if powered by battery.

Very thanks

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Reg Edwards
Guest
Posted: Sun Dec 11, 2005 9:35 am    Post subject: Re: a low noise level opamp Reply with quote
There is nothing lower noise than a base-input, small collector
current, npn transistor.

If an op-amp has such an input transistor then all well and good.

For an even better performance - freeze it.
----
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Phil Allison
Guest
Posted: Sun Dec 11, 2005 9:35 am    Post subject: Re: a low noise level opamp Reply with quote
"Reg Edwards"
Quote:

There is nothing lower noise than a base-input, small collector
current, npn transistor.


** Huh ???

Has Reg never heard of source impedance issues ?

Never heard of low noise FETs either ?

Never heard of paralleling BJTs for low impedance, low noise usage ?




......... Phil

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Phil Allison
Guest
Posted: Sun Dec 11, 2005 9:35 am    Post subject: Re: a low noise level opamp Reply with quote
"huiliu" <huiliu.liu@gmail.com> wrote in message
news:1134279174.841710.297320@z14g2000cwz.googlegroups.com...



** Another Google Groupie.....

Quote:
Can any one tell me where to get a low noise level opamp? It basically
statifies 10 micro V @ 0Hz -- 5kHz if powered by battery.



** If that 10uV is an EIN figure - even a 741 should do.

What is the source impedance ?





........... Phil
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Ken Smith
Guest
Posted: Sun Dec 11, 2005 5:25 pm    Post subject: Re: a low noise level opamp Reply with quote
In article <dngn0n$1ig$1@nwrdmz03.dmz.ncs.ea.ibs-infra.bt.com>,
Reg Edwards <g4fgq,regp@ZZZbtinternet,com> wrote:
Quote:
There is nothing lower noise than a base-input, small collector
current, npn transistor.

I guess you are calling the following "nothing":

A medium power NPN transistor running at a modest collector current,
A low noise JFET such as the IF3601,
A varactor diode,
A SQUID,
A MASER.

I suspect that we could also add a power MOSFET to that list at low
frequencies.



--
--
kensmith@rahul.net forging knowledge
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huiliu
Guest
Posted: Sun Dec 11, 2005 5:35 pm    Post subject: Re: a low noise level opamp Reply with quote
It seems ok but not appropriate for my application. and I wanna have a
output with noise level less than 20 micro V < 5kHz, the load will be
Gohms.

Thanks
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Winfield Hill
Guest
Posted: Sun Dec 11, 2005 5:35 pm    Post subject: Re: a low noise level opamp Reply with quote
huiliu wrote...
Quote:

It seems ok but not appropriate for my application. and I wanna
have a output with noise level less than 20 micro V < 5kHz, the
load will be Gohms.

10uV rms with a 5kHz bandwidth implies an output noise density of
vn = 10uV/sqrt(5kHz) = 141nV/sqrt-Hz, assuming white noise. This
is an extremely high noise level you are allowing. Most opamps
have voltage-noise density, e_n, well under 35nV, and many are
under 5nV, some are even down to under 1nV.

Two other noise sources you may encounter are Johnson noise and
opamp current noise, both involving your feedback resistors. If
you use resistors below 150k-ohms the Johnson noise, sqrt(4kTR),
will be under 50nV/sqrt-Hz. And with 150k resistors, if you keep
your opamp's current noise, i_n, below 50nV/150k = 0.33pA/sqrt-Hz,
then that source will also be below 50nV. Opamp current noise is
just shot noise from the input bias current. If the bias current
is below Ib = i_n^2 / 2q = 0.34uA, that source will be well under
control. As you know most opamp bias currents are far below that.
If you use a JFET opamp, that term will disappear entirely.

If you have four 50nV noise sources, you'll have 100nV of noise
density, which is 7uV over a 5kHz bandwidth (the fourth source is
a placeholder for your signal source or voltage reference). You
may find that aspect harder to achieve, but if the first three
terms each contribute under 50nV, you'll be allowed 111nV/sqrt-Hz
for your signal source, while still meeting your 141nV budget.


--
Thanks,
- Win
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huiliu
Guest
Posted: Sun Dec 11, 2005 5:35 pm    Post subject: Re: a low noise level opamp Reply with quote
Sorry, I use it as a output not a input.
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huiliu
Guest
Posted: Mon Dec 12, 2005 9:35 am    Post subject: Re: a low noise level opamp Reply with quote
the analysis is very very useful, and can you show me where I can get
some sources on noise analysis? I checked some of electronic books and
got little information.
thanks again
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huiliu
Guest
Posted: Mon Dec 12, 2005 9:35 am    Post subject: Re: a low noise level opamp Reply with quote
:) very thanks
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Winfield Hill
Guest
Posted: Mon Dec 12, 2005 5:35 pm    Post subject: Re: a low noise level opamp Reply with quote
huiliu wrote...
Quote:

the analysis is very very useful, and can you show me where I can
get some sources on noise analysis? I checked some of electronic
books and got little information.

The Art of Electronics has 38 good pages of noise discussion.


--
Thanks,
- Win
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Pooh Bear
Guest
Posted: Mon Dec 12, 2005 5:35 pm    Post subject: Re: a low noise level opamp Reply with quote
huiliu wrote:
Quote:
the analysis is very very useful, and can you show me where I can get
some sources on noise analysis? I checked some of electronic books and
got little information.

There are 2 equations that you are likely to need regularly for noise.

Where a device has a specified input noise voltage density ( typically
measured in nV / sqrt Hz ) the wideband input noise *in a badwidth B* is
simply given by

Noise ( rms ) = noise_density * sqrt ( B )

When making noise calculations you are also likely to need to consider
the effect of thermal ( Johnson ) noise from resistors.

Thermal noise for a ressitor of value R at an absolute temperature T (
degrees Kelvin ) over the Bandwidth B ( in Hertz ) is given by.

Noise ( rms ) = sqrt ( 4 * k * T * R * B )

Where k is Boltmann's constant = 1.37 * 10^-23

Also when *adding* multiple noise sources be aware that due to its
'randomness' noise doesn't add arithmetically. Instead it adds as the
root of the sum of the squares.

So for several noise sources N1, N2 N3 etc.....

Noise total = sqrt ( N1^2 + N2^2 + N3^2...... ) etc


Graham
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huiliu
Guest
Posted: Mon Dec 12, 2005 5:35 pm    Post subject: Re: a low noise level opamp Reply with quote
thanks

oh, your book:) my pleasure to enjoy it.
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huiliu
Guest
Posted: Mon Dec 12, 2005 5:35 pm    Post subject: Re: a low noise level opamp Reply with quote
thanks
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huiliu
Guest
Posted: Mon Dec 12, 2005 5:35 pm    Post subject: Re: a low noise level opamp Reply with quote
a general but usful one.

got it:)

thanks!
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