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dashley |
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%$Header$ |
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\chapter[\crcszeroshorttitle{}]{\crcszerolongtitle{}} |
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\label{crcs0} |
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\section{Introduction} |
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\index{ratiometric conversion and calculation} |
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This chapter describes the construction and analysis of ratiometric conversion and |
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measurement systems. By \emph{ratiometric}, we mean that the system requires input |
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from multiple A/D channels to infer the data of interest, typically a potentiometer |
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position. Ratiometric conversion and calculation systems are most often used in |
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small microcontroller work because they can reduce cost by eliminating regulated |
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voltage supplies. Successive sections in the chapter describe the analysis of progressively |
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more complex ratiometric conversion and calculation systems. |
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\section{Ratiometric Conversion In Hardware Versus Ratiometric Calculation In Software} |
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Need to include a differentiation between conversion in hardware and |
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calculation in software. |
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%Section tag: srsy1 |
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% |
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\section{Potentiometer With $V_{+}$ Reference And Hardware Ratiometric Conversion} |
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The simplest ratiometric potentiometer system |
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that would be constructed in practice |
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is shown in Fig. \ref{crcs0:srsy1:smplsys0}. |
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In this system, microcontroller software must sense |
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the potentiometer position $R_{P1}/R_P$\footnote{We hope that |
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all of our readers have a background that allows them to |
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analyze resistor networks. For readers without this background, |
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we recommend reading and working through the exercises in an |
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undergraduate circuit analysis text.} even as |
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$V_{+}$ varies within the interval |
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$V_{+} \in [V_{+MIN}, V_{+MAX}]$. Such systems, with |
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additional filtering and current-limiting components, |
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are commonly used in automobiles to allow a microcontroller |
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software load to sense seat or |
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mirror position. |
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\index{seat position} |
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\index{mirror position} |
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\index{battery voltage} |
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Using automobile battery voltage as $V_{+}$ |
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has the advantage that a regulated voltage is not |
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required, thus saving the component cost and circuit board |
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area of a voltage regulator. |
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\begin{figure}[!tb] |
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\centering |
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\includegraphics[width=4.6in]{c_rcs0/s_rsy1/smplsys0.eps} |
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\caption{Simple Ratiometric Measurement System With Hardware Ratiometric Conversion} |
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\label{crcs0:srsy1:smplsys0} |
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\end{figure} |
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In the circuit of Fig. \ref{crcs0:srsy1:smplsys0}, the microcontroller |
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\index{A/D converter}A/D converter will convert $V_P$ using $V_R$ as a voltage |
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reference according to the relationship in (\ref{crcs0:srsy1:eq000}), where $N_{MAX}$ |
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is the maximum count of the A/D converter. The \index{floor function}$floor(\cdot{})$ |
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function in (\ref{crcs0:srsy1:eq000}) is used to model the effect of |
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\index{quantization}quantization---the |
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A/D count $N$ is required to be $\in \vworkintsetnonneg$. |
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\begin{equation} |
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\label{crcs0:srsy1:eq000} |
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N = \left\lfloor { \frac{N_{MAX} V_P}{V_R} } \right\rfloor |
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\end{equation} |
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%Section tag: srsy0 |
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% |
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\section{Fixed $r_{1}$, Fixed $r_{2}$ System} |
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The simplest ratiometric system that would be constructed in practice |
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is shown in Fig. \ref{crcs0:srsy0:fr1fr2a}. |
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In Fig. \ref{crcs0:srsy0:fr1fr2a}, |
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assume that the potentiometer is positioned so that |
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$R_{P1}$ is the resistance from the potentiometer wiper |
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to ground, and $R_{P2}$ is the resistance from the potentiometer |
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wiper to $V_{+}$. By definition, $R_{P} = R_{P1} + R_{P2}$. $z_R$ and |
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$z_P$ are the transfer coefficients which relate voltage to A/D counts. |
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These transfer coefficients are an analysis convenience, and correspond to |
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A/D converter characteristics. |
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\begin{figure}[!tb] |
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\centering |
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\includegraphics[height=2.5in]{c_rcs0/s_rsy0/smplsys0.eps} |
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\caption{Simple Ratiometric Measurement System With Software Ratiometric Calculation} |
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\label{crcs0:srsy0:fr1fr2a} |
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\end{figure} |
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The circuit is designed to allow |
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estimation of $R_{P1}$ (effectively, the potentiometer position) |
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under conditions of varying $V_{+}$. The economy of such a circuit |
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comes from the characteristic that $V_{+}$ need not be regulated, |
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thus allowing less expensive lower-capacity voltage regulators or |
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fewer voltage regulators to be used in an embedded system. |
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In an vehicle, for example, $V_{+}$ may be the battery voltage of |
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the vehicle, which will vary substantially based on which |
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electrical loads are turned on, whether the starter motor is |
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engaged, etc. |
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The critical analysis question is, |
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how accurately can $R_{P1}/R_P$ be estimated under conditions |
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of varying $V_{+} \in [V_{+MIN}, V_{+MAX}]$? Or, equivalently, |
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given measured values of $y_R, y_P \in \vworkintsetnonneg$ |
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and given $V_{+} \in [V_{+MIN}, V_{+MAX}]$, |
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what inequality describes the possible values of $R_{P1}/R_P$ |
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(i.e. how much can be inferred or implied from the observation)? |
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From analysis of the circuit of Fig. \ref{crcs0:srsy0:fr1fr2a}, |
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it can be shown that (\ref{crcs0:srsy0:eq000}) applies. |
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However, because an A/D |
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count is necessarily $\in \vworkintsetnonneg$, (\ref{crcs0:srsy0:eq000b}) must be |
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used for analysis. |
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\begin{equation} |
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\label{crcs0:srsy0:eq000} |
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y_R = \frac{R_1 z_R V_{+}}{R_1 + R_2} |
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\end{equation} |
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\begin{equation} |
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\label{crcs0:srsy0:eq000b} |
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y_R = \left\lfloor\frac{R_1 z_R V_{+}}{R_1 + R_2}\right\rfloor |
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\end{equation} |
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Similarly, (\ref{crcs0:srsy0:eq000c}) describes $y_P$ for analysis. |
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\begin{equation} |
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\label{crcs0:srsy0:eq000c} |
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y_P = \left\lfloor\frac{R_{P1} z_R V_{+}}{R_P}\right\rfloor |
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\end{equation} |
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\section{Unplaced Equations} |
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This section is a holding place for equations until can get my |
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thoughts together. |
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\begin{equation} |
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y_P = \frac{R_{P1}}{R_P} V_{+} |
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\end{equation} |
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\begin{equation} |
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V_{+} = y_P \left( {\frac{R_P}{R_{P1}}} \right) |
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\end{equation} |
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\begin{equation} |
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y_R = \frac{R_1}{R_1 + R_2} V_{+} |
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\end{equation} |
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\begin{equation} |
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V_{+} = \frac{y_R ( R_1 + R_2)}{R_1} |
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\end{equation} |
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\begin{equation} |
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y_P \left( {\frac{R_P}{R_{P1}}} \right) = y_R \left( {\frac{R1 + R2}{R1}} \right) |
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\end{equation} |
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\begin{equation} |
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\frac{R_P}{R_{P1}} = \frac{y_R}{y_P} \left( {\frac{R_1 + R_2}{R_1}} \right) |
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\end{equation} |
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\begin{equation} |
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\frac{R_{P1}}{R_P} = \frac{y_P}{y_R} \left( {\frac{R_1}{R_1 + R_2}} \right) |
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\end{equation} |
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\begin{equation} |
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\frac{R_P V}{R_P V + 1} < \frac{\lfloor R_P V \rfloor}{\lfloor R_R V \rfloor} < \frac{R_P V + 1}{R_R V} |
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\end{equation} |
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\vfill |
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\begin{figure}[b] |
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\noindent\rule[-0.25in]{\textwidth}{1pt} |
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\begin{tiny} |
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\begin{verbatim} |
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$RCSfile: c_rcs0.tex,v $ |
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$Source: /home/dashley/cvsrep/e3ft_gpl01/e3ft_gpl01/dtaipubs/esrgubka/c_rcs0/c_rcs0.tex,v $ |
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$Revision: 1.2 $ |
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$Author: dtashley $ |
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$Date: 2001/07/01 19:46:09 $ |
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\end{verbatim} |
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\end{tiny} |
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\noindent\rule[0.25in]{\textwidth}{1pt} |
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\end{figure} |
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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% $Log: c_rcs0.tex,v $ |
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% Revision 1.2 2001/07/01 19:46:09 dtashley |
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% Move out of binary mode for use with CVS. |
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% |
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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% $History: c_rcs0.tex $ |
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% |
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% ***************** Version 5 ***************** |
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% User: Dashley1 Date: 12/22/00 Time: 12:56a |
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% Updated in $/uC Software Multi-Volume Book (A)/Chapter, RCS0, Ratiometric Conversion And Measurement Systems |
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% Tcl automated method of build refined. |
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% |
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% ***************** Version 4 ***************** |
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% User: Dashley1 Date: 6/28/00 Time: 12:09p |
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% Updated in $/uC Software Multi-Volume Book (A)/Chapter, RCS0, Ratiometric Conversion And Measurement Systems |
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% Substantial edits, forming thoughts. |
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% |
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% ***************** Version 3 ***************** |
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% User: David T. Ashley Date: 6/27/00 Time: 11:51p |
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% Updated in $/uC Software Multi-Volume Book (A)/Chapter, RCS0, Ratiometric Conversion And Measurement Systems |
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% Initial check-in. |
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% |
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% ***************** Version 2 ***************** |
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% User: Dashley1 Date: 6/27/00 Time: 7:36p |
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% Updated in $/uC Software Multi-Volume Book (A)/Chapter, RCS0, Ratiometric Conversion And Measurement Systems |
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% Edits for rationmetric conversion systems. |
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%End of file C_RCS0.TEX |