<DIV class=msgLinks>
<TABLE width="100%">
<TBODY>
<TR vAlign=baseline>
<TD align=left><SPAN class=authorLink>from [<A href="http://www.kkn.net/archives/cgi-bin/namazu.cgi?query=%2Bfrom%3Adtayloe%40home.com&idxname=QRP-L&sort=date%3Alate"><EM>Dan Tayloe</EM></A>]</SPAN></TD>
<TD align=right><SPAN class=bookmarkLink>[<A href="http://www.kkn.net/archives/cgi-bin/mesg.cgi?a=QRP-L&i=39A9AD22.1AA0D9D6%40home.com">Permanent Link</A>]</SPAN><SPAN class=orgLink>[<A href="http://www.kkn.net/archives/cgi-bin/extract-mesg.cgi?a=QRP-L&m=2000-08&i=39A9AD22.1AA0D9D6%40home.com">Original</A>]</SPAN></TD></TR></TBODY></TABLE></DIV>
<DIV class=msgHead><!--X-Subject-Header-End--><!--X-Head-of-Message-->
<TABLE>
<TBODY>
<TR vAlign=baseline>
<TH align=right><STRONG>To</STRONG>: </TH>
<TD align=left><A href="mailto:henryf%40quartz.gly.fsu.edu">henryf@quartz.gly.fsu.edu</A>, qrp-l <<A href="mailto:qrp-l%40Lehigh.EDU">qrp-l@Lehigh.EDU</A>></TD></TR>
<TR vAlign=baseline>
<TH align=right><STRONG>Subject</STRONG>: </TH>
<TD align=left>Re: Low Noise Op Amps</TD></TR>
<TR vAlign=baseline>
<TH align=right><STRONG>From</STRONG>: </TH>
<TD align=left>Dan Tayloe <<A href="mailto:dtayloe%40home.com">dtayloe@home.com</A>></TD></TR>
<TR vAlign=baseline>
<TH align=right><STRONG>Date</STRONG>: </TH>
<TD align=left>Sun, 27 Aug 2000 17:06:58 -0700</TD></TR></TBODY></TABLE><!--X-Head-of-Message-End--><!--X-Head-Body-Sep-Begin--></DIV>
<DIV class=msgBody>
<TABLE cellSpacing=1 width="100%">
<TBODY>
<TR>
<TD><!--X-Head-Body-Sep-End--><!--X-Body-of-Message--><PRE>><I>I don't have much experience specing out op-amps. What does nV/rtHz</I>
><I>mean? nanovolts per ?hertz? Guess the lower the number the better?</I>
Henry:
It means what it says. If this is being used in the front end of a DC
receiver, 4 nV/SqrtHz really only means anything given a bandwidth. If
my receiver is going to have an audio filter 500 Hz wide, then the noise
of this op-amp referenced to the input side of the op-amp is:
4 * Sqrt(500) = 4 * 22.4 = 89.4 nV of noise
Now you need to include the series input resistors on the op-amp also.
If these were 1K and there were one one the + side, and one on the -
side,
you have 2K total (at least that is what the app note said). The noise
contribution of those resistors are 0.13*Sqrt(R)*Sqrt(Bandwidth) with a
units
of nV.
0.13 * Sqrt(2000) * Sqrt(500) = 130 nV of noise.
Notice that the 1K resistors have more noise than the op-amp! Since the
noise
is uncorrelated, the total input noise is the RMS sum or:
Sqrt(89.4*89.4 + 130*130) = 157 nV of RMS noise.
Now if the receiver has 100 db of gain (100,000 x) the "white noise" of
the
reciver at full gain will be 100,000 * 157e-9 = 15.7 mV RMS of noise
which will
indeed be audable.
On the other hand nV/SqrtHz needs to be closely eval‎uated. The data
sheet for
the op-amp will show a plot for noise vs frequency. Sometimes, a
non-audio
op-amp will show the rated noise level only at a higher frequency such
as 1 KHz.
The noise level is typically worse at lower frequencies, improving at
higher
frequencies. For cw type audio, I would like to see low noise
performance starting
at 200 Hz or so.
If the op-amp is used on the speaker side of the volume control (after
the normal
audio filter shaping), simple audio filtering such as a cap in parrallel
with the
feedback resistance will help kill the gain at high frequencies so that
you don't
hear very broad band, high frequency hiss.
- Dan Tayloe, N7VE; Phoenix, Az; Az ScQRPions
</PRE><!--X-Body-of-Message-End--><!--X-MsgBody-End--><!--X-Follow-Ups--></TD></TR></TBODY></TABLE></DIV>
<DIV class=msgLinks>
<TABLE width="100%">
<TBODY>
<TR vAlign=baseline>
<TD align=middle><SPAN class=subjectLink>[<A href="http://www.kkn.net/archives/cgi-bin/namazu.cgi?query=%2Bsubject:%2F%5E%28%3F:%5E%5Cs%2A%28re%7Csv%7Cfwd%7Cfw%29%5B%5C%5B%5C%5D%5Cd%5D%2A%5B:%3E-%5D%2B%5Cs%2A%29%2ALow%5Cs%2BNoise%5Cs%2BOp%5Cs%2BAmps%5Cs%2A%24%2F&idxname=QRP-L&sort=date%3Alate">More with this subject...</A>]</SPAN></TD></TR></TBODY></TABLE></DIV>
<DIV class=tSlice>
<TABLE cellSpacing=1 cellPadding=4 width="100%">
<TBODY>
<TR vAlign=baseline>
<TD align=left><A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg02021.html"><STRONG><Prev in Thread</STRONG></A>]</TD>
<TH align=middle width="100%"><STRONG>Current Thread</STRONG></TH>
<TD align=right>[<A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg01982.html"><STRONG>Next in Thread></STRONG></A> </TD></TR>
<TR class=tSliceList>
<TD colSpan=3>
<UL>
<LI><STRONG>Re: Low Noise Op Amps</STRONG>, <EM>(continued)</EM>
<UL>
<UL>
<UL>
<LI><B><A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg02203.html" name=02203>Re: Low Noise Op Amps</A></B>, <I>Paul Christensen</I>
<LI><B><A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg02253.html" name=02253>Re: Low Noise Op Amps</A></B>, <I>John Moriarity</I> </LI></UL></UL>
<LI><B><A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg01854.html" name=01854>Re: Low Noise Op Amps</A></B>, <I>Ian C. Purdie VK2TIP</I>
<LI><B><A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg01919.html" name=01919>Re: Low Noise Op Amps</A></B>, <I>Henry Freedenberg</I>
<UL>
<LI><B><A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg01927.html" name=01927>Noise 101 Was Re: Low Noise Op Amps (long)</A></B>, <I>Robert P. Okas</I>
<LI><B><A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg01988.html" name=01988>Re: Low Noise Op Amps</A></B>, <I>Glen Leinweber</I>
<UL>
<LI><B><A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg01992.html" name=01992>OT: Antenna problem</A></B>, <I>M0CQG</I>
<LI><B><A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg01997.html" name=01997>ANT- Gap Challenger in the wind</A></B>, <I>steve</I>
<LI><EM>Message not available</EM>
<LI><B><A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg02038.html" name=02038>Re: OT: Antenna problem</A></B>, <I>M0CQG</I> </LI></UL></LI></UL>
<LI><B><A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg02021.html" name=02021>Re: Low Noise Op Amps</A></B>, <I>Henry Freedenberg</I>
<LI><SPAN class=sliceCur><STRONG>Re: Low Noise Op Amps</STRONG>, <EM>Dan Tayloe</EM> <B></SPAN><=</B>
<LI><B><A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg01982.html" name=01982>Re: Low Noise Op Amps</A></B>, <I>Lau, Zack, W1VT</I>
<LI><B><A href="http://www.kkn.net/archives/html/QRP-L/2000-08/msg02209.html" name=02209>RE: Low Noise Op Amps</A></B>, <I>Kanalz, Karl</I> </LI></UL></LI></UL></TD></TR></TBODY></TABLE></DIV>
<!-- -->
