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📁 Input/Data Acquisition System Design for Human Computer Interfacing
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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.2//EN"><!--Converted with LaTeX2HTML 96.1 (Feb 5, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds --><HTML><HEAD><TITLE>5.3 Analog to Digital Conversion </TITLE><META NAME="description" CONTENT="5.3 Analog to Digital Conversion "><META NAME="keywords" CONTENT="sensors"><META NAME="resource-type" CONTENT="document"><META NAME="distribution" CONTENT="global"><LINK REL=STYLESHEET HREF="sensors.css" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/sensors.css"></HEAD><BODY BGCOLOR="#FFFFFF" TEXT="#000000" LANG="EN"> <A NAME="tex2html317" HREF="node24.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node24.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="next_motif.gif" tppabs="http://ccrma.stanford.edu/Images//next_motif.gif"></A> <A NAME="tex2html315" HREF="node20.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node20.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="up_motif.gif" tppabs="http://ccrma.stanford.edu/Images//up_motif.gif"></A> <A NAME="tex2html311" HREF="node22.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node22.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif" tppabs="http://ccrma.stanford.edu/Images//previous_motif.gif"></A>   <BR><B> Next:</B> <A NAME="tex2html318" HREF="node24.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node24.html">6 System Integration</A><B>Up:</B> <A NAME="tex2html316" HREF="node20.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node20.html">5 Data Acquisition</A><B> Previous:</B> <A NAME="tex2html312" HREF="node22.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node22.html">5.2 Sample and Hold</A><BR> <P><H2><A NAME="SECTION00053000000000000000">5.3 Analog to Digital Conversion </A></H2><P> The purpose of the analog to digital is to quantize the input signal from the sample and 
hold circuit to 2B discrete levels - where B is the number of bits of the analog to digital 
converter (ADC). The input voltage can range from 0 to Vref   (or -Vref to +Vref  for a 
bipolar ADC). What this means is that the voltage reference of the ADC is used to set the 
range of conversion of the ADC.  
 For a monopolar ADC, a 0V input will cause the converter to output all  zeros. If the 
input to the ADC is equal to or larger than  <IMG WIDTH=24 HEIGHT=18 ALIGN=TOP ALT="tex2html_wrap_inline2018" SRC="img101.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img101.gif"  >  then the converter will output all 
ones.  For inputs between these two voltage levels, the ADC will output binary numbers 
corresponding to the signal level. For a bipolar ADC, the minimum input is  <IMG WIDTH=33 HEIGHT=18 ALIGN=TOP ALT="tex2html_wrap_inline2064" SRC="img118.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img118.gif"  >  
not 0V.<P><H3><A NAME="SECTION00053100000000000000">5.3.1 Problem: Noise</A></H3><P> Because the ADC outputs only  <IMG WIDTH=17 HEIGHT=16 ALIGN=BOTTOM ALT="tex2html_wrap_inline2066" SRC="img119.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img119.gif"  >  levels there is inherently noise in the quantized 
output signal.  The ratio of the signal to this quantization noise is called SQNR. The 
SQNR in dB is approximately equal to 6 times the number of bits of the ADC:
 <P> <IMG WIDTH=153 HEIGHT=21 ALIGN=BOTTOM ALT="displaymath2068" SRC="img120.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img120.gif"  > <P>
 So for a 16 bit ADC this means that the SQNR is approximately equal to 96dB. 
 There are, of course, other sources of noise that corrupts the output of the ADC. These 
include noise from the sensor, from the signal conditioning circuitry, and from the 
surrounding digital circuitry<P><H3><A NAME="SECTION00053200000000000000">5.3.2 Solution: Maximize the Input Signal</A></H3><P> The key to reducing the effects of the noise is to maximize the input signal level. What 
this means is that the HCI designer should increase the gain of the signal conditioning 
circuitry until the maximum sensor output is equal to the  <IMG WIDTH=24 HEIGHT=18 ALIGN=TOP ALT="tex2html_wrap_inline2018" SRC="img101.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img101.gif"  >  of the ADC.  It is also 
possible to reduce  <IMG WIDTH=24 HEIGHT=18 ALIGN=TOP ALT="tex2html_wrap_inline2018" SRC="img101.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img101.gif"  >   down to the maximum level of the sensor.  The problem 
with this is that the noise will corrupt the small signals.  A good rule of thumb is to keep 
 <IMG WIDTH=24 HEIGHT=18 ALIGN=TOP ALT="tex2html_wrap_inline2018" SRC="img101.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img101.gif"  >   at least as large as the maximum digital signal, usually 5V.<P><HR><A NAME="tex2html317" HREF="node24.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node24.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="next_motif.gif" tppabs="http://ccrma.stanford.edu/Images//next_motif.gif"></A> <A NAME="tex2html315" HREF="node20.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node20.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="up_motif.gif" tppabs="http://ccrma.stanford.edu/Images//up_motif.gif"></A> <A NAME="tex2html311" HREF="node22.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node22.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif" tppabs="http://ccrma.stanford.edu/Images//previous_motif.gif"></A>   <BR><B> Next:</B> <A NAME="tex2html318" HREF="node24.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node24.html">6 System Integration</A><B>Up:</B> <A NAME="tex2html316" HREF="node20.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node20.html">5 Data Acquisition</A><B> Previous:</B> <A NAME="tex2html312" HREF="node22.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node22.html">5.2 Sample and Hold</A><P><ADDRESS><I>Tim Stilson <BR>Thu Oct 17 16:32:33 PDT 1996</I></ADDRESS></BODY></HTML>
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