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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>4.7 Additional Signal Conditioning Circuits</TITLE><META NAME="description" CONTENT="4.7 Additional Signal Conditioning Circuits"><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="tex2html276" HREF="node20.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node20.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="tex2html274" HREF="node12.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node12.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="tex2html270" HREF="node18.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node18.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="tex2html277" HREF="node20.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node20.html">5 Data Acquisition</A><B>Up:</B> <A NAME="tex2html275" HREF="node12.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node12.html">4 Signal Conditioning</A><B> Previous:</B> <A NAME="tex2html271" HREF="node18.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node18.html">4.6 Capacitance to Voltage</A><BR> <P><H2><A NAME="SECTION00047000000000000000">4.7 Additional Signal Conditioning Circuits</A></H2><P><H3><A NAME="SECTION00047100000000000000">4.7.1 Comparators</A></H3><P>Sometimes there is no need to send the entire range of voltages from a sensor to the
analog-to-digital converter (ADC). Instead, many times a sensor is used simply as a
switch. Figure <A HREF="node19.html#FSRswitch" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node19.html#FSRswitch">40</A> contains a circuit called a <EM>comparator</EM> which takes an analog sensor
voltage and compares it to a threshold voltage, <IMG WIDTH=18 HEIGHT=16 ALIGN=TOP ALT="tex2html_wrap_inline2040" SRC="img109.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img109.gif" > . If the sensor's voltage is greater
than the threshold, the output of the circuit is maximum (typically 5V). If the sensor's
output is less than the threshold, the output of the circuit is minimum (usually 0V). The
threshold voltage is set by adjusting the potentiometer labeled <IMG WIDTH=20 HEIGHT=16 ALIGN=TOP ALT="tex2html_wrap_inline2042" SRC="img110.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img110.gif" > . The output of
the sensor can also be reduced by using the resistor divider network as shown if desired.
Notice that the circuit has a positive feedback resistor <IMG WIDTH=16 HEIGHT=16 ALIGN=TOP ALT="tex2html_wrap_inline2044" SRC="img111.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img111.gif" > which assures that the
output of the comparator will swing quickly and completely from maximum output to
minimum output (also called ``rail to rail'').<P><B>Example - Using an FSR as a Switch</B><P>An example of when this might be useful is in the case of the force sensing resistor. As
depicted in Figure <A HREF="node8.html#FSRforce_log" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node8.html#FSRforce_log">6</A>, typical force sensing resistors exhibit a region in which the
resistance varies rapidly in response to a rather small variation in force. The force which
defines this 慿nee' in the force vs. resistance curce is known as the break force. This
behavior can be used to create a switch out of an FSR. This type of behaviour might be
useful in devices such as membrane style keypads.<P><P ALIGN=CENTER><A NAME="835"> </A><A NAME="FSRswitch"> </A> <IMG WIDTH=203 HEIGHT=154 ALIGN=BOTTOM ALT="figure834" SRC="img112.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img112.gif" > <BR>
</P>
<STRONG>Figure 40:</STRONG> An FSR in a Switch Configuration, Using a Comparator <BR><P><P>Figure <A HREF="node19.html#FSRswitch" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node19.html#FSRswitch">40</A> shows a circuit which implements a switch based upon a force sensing
resistor. The variable resistor, <IMG WIDTH=20 HEIGHT=16 ALIGN=TOP ALT="tex2html_wrap_inline2042" SRC="img110.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img110.gif" > is used to set the sensitivity of the switch.<P><H3><A NAME="SECTION00047200000000000000">4.7.2 Signal Energy to Voltage</A></H3><P><B>Motivation</B><P>Sometimes, even though a sensor's output is an extremely complex waveform, only the
energy of the waveform is needed. This is accomplished by rectifying the waveform
(taking the absolute value) and then smoothing (lowpass filtering) the result. This process
is shown in the circuit below.<P><P ALIGN=CENTER><A NAME="842"> </A><A NAME="rectifier1"> </A> <IMG WIDTH=507 HEIGHT=176 ALIGN=BOTTOM ALT="figure841" SRC="img113.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img113.gif" > <BR>
</P>
<STRONG>Figure 41:</STRONG> Energy to Voltage Conversion <BR><P><P>Notice that the output is a rectified version of the input. Figure <A HREF="node19.html#rectifier2" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node19.html#rectifier2">42</A> adds a couple
capacitors to the above circuit to smooth the output.<P><P ALIGN=CENTER><A NAME="849"> </A><A NAME="rectifier2"> </A> <IMG WIDTH=380 HEIGHT=262 ALIGN=BOTTOM ALT="figure848" SRC="img114.gif" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/img114.gif" > <BR>
</P>
<STRONG>Figure 42:</STRONG> A Complete Energy Measurement Circuit <BR><P><P><B>Example - EMG Measurement Using an Electrode</B><P>As was discussed previously, the output of the electrode is a voltage waveform that
measures underlying neural activity. Many times, when measuring the EMG, the complex
waveform that is created by many neural motor units firing is irrelevant. What is of interest
is the average energy caused by the muscle exertion. The circuit shown in Figure <A HREF="node19.html#rectifier2" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node19.html#rectifier2">42</A> is
used to obtain this energy. The capacitors are usually chosen to smooth off changes faster
than 5-10Hz.<P><HR><A NAME="tex2html276" HREF="node20.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node20.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="tex2html274" HREF="node12.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node12.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="tex2html270" HREF="node18.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node18.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="tex2html277" HREF="node20.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node20.html">5 Data Acquisition</A><B>Up:</B> <A NAME="tex2html275" HREF="node12.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node12.html">4 Signal Conditioning</A><B> Previous:</B> <A NAME="tex2html271" HREF="node18.html" tppabs="http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node18.html">4.6 Capacitance to Voltage</A><P><ADDRESS><I>Tim Stilson <BR>Thu Oct 17 16:32:33 PDT 1996</I></ADDRESS></BODY></HTML>⌨️ 快捷键说明
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