Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!rutgers!ub!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: What is the noise voltage of a 1.5V cell
Summary: Limited by thermal noise...
Message-ID: <4285@kitty.UUCP>
Date: 3 Jan 91 05:27:13 GMT
References: <1991Jan3.001303.483@ae.chalmers.se>
Organization: Recognition Research Corp., Clarence, NY
Lines: 34

In article <1991Jan3.001303.483@ae.chalmers.se>, hogstedt@ae.chalmers.se (PER HOGSTEDT) writes:
> I need to produce a voltage that is going to be used to bias
> a high sensitivity magnetometer (intrinsic noise < 1 pT/sqrt(Hz)) exposed
> to the earth's magnetic field. The long term stability demands on this
> voltage source are moderate, but the voltage noise has to be extremly low.
> ... 
> As a kind of last resort, I would like to know what the noise is of a 
> regular 1.5V (e.g. alkaline) cell, assuming the cell is kept at
> room temperature and loaded only by some microAmps?
> The bandwidth of primary interest is DC to maximum 50 Hz.

	The essential noise component of a simple electrochemical primary
cell is thermal noise (Johnson Noise, for Johnson fans).  According to
the good Dr. Nyquist, only three factors need concern you for this
application: (1) absolute temperature, (2) noise bandwidth, and (3) internal
resistance of the cell over the noise bandwidth.  Since your bandwidth
is only 50 Hz, we can ignore any frequency effects upon cell resistance.
Since you want to run at room temperature, we can figure 290 deg K for
absolute temperature.  Since you are asking about a 1.5 volt alkaline
cell, I would guess internal resistance to be around .25 ohm.

	My off-the-wall guess is that thermal noise will be less than
1 nanovolt.  You should be able to readily obtain the thermal noise
equation, plug in your values along with Boltzmann's constant, and get
a more exact figure.

	If you were drawing some serious power from the alkaline cell,
I would consider some other factors in evaluating noise (like temperature
shifts, gas generation, etc.), but offhand I can't see anything other than
thermal noise for a uA load.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
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