\begin{tikzpicture}[->, >=stealth, shorten >= 0pt, draw=black!50, node distance=3.2cm, font=\sffamily]
    \tikzstyle{node}=[circle,fill=black,minimum size=2pt,inner sep=0pt]
    \tikzstyle{block}=[draw=black,rectangle,fill=none,minimum size=1.5cm, inner sep=0pt]

	\node[node] (xc) {};
    \node[block, right=1cm of xc] (ADC) {C-to-D};
    \node[block, right of=ADC, text width = 2cm, align= center] (DSP) {LTI \\ System};
    \node[block, right of=DSP] (DAC) {D-to-C};
	\coordinate[right=1cm of DAC] (yc) {};
	
	\coordinate (mid1) at ($(ADC.east)!0.5!(DSP.west)$) {};
	\coordinate (mid2) at ($(DSP.east)!0.5!(DAC.west)$) {};
		
    \path (xc) edge (ADC);
    \path (ADC) edge (DSP);
    \path (DSP) edge (DAC);
    \path (DAC) edge (yc);
    
    \node[above = 0.5mm of mid1] {$x[n]$};
    \node[below = 0.5mm of mid1] {$X(e^{j\omega})$};
    \node[above = 0.5mm of mid2] {$y[n]$};
    \node[below = 0.5mm of mid2] {$Y(e^{j\omega})$};
    \node[above = 0mm of xc, text width = 1cm, align=center] {$x_c(t)$};
    \node[below = 0mm of xc, text width = 1cm, align=center] {$X_c(j\Omega)$};
    \node[above = 0mm of yc, text width = 1cm, align=center] {$y_r(t)$}; 
    \node[below = 0mm of yc, text width = 1cm, align=center] {$Y_r(j\Omega)$};
    \node at ($(DSP.south)-(0, 0.25cm)$) {$h[n] \leftrightarrow H(e^{j\omega})$};
    \node[below, align=center, text width=2cm] at ($(ADC.south)$) {Sampling};
    \node at ($(DAC.south)-(0, 0.25cm)$) {Reconstruction};
    \ifdefined\Heff
    	\draw[dashed] ($(ADC.south west)-(0.3, 0.7cm)$) rectangle ($(DAC.north east)+(0.4, 0.4cm)$);
    	\node at ($(DSP.south)-(0,1cm)$) {$H_{eq}(j\Omega)$};
    \else
    	\relax
    \fi
\end{tikzpicture}