TikZ how to draw lines to/from nodes correctly?

Question

Possible Duplicate:
Tikz: purely vertical arrow from nodeA.south to nodeB.north

I need to draw some diagrams for my thesis. I normally use PGF/TikZ for this purpose. I have often the problem as in the example below (extraction from bigger picture). I want to have all 3 arrows parallel (horizontally). If I use the FPGA node directly, I get diagonal lines from center to center (upper line).

Do you have an idea how to achive this in a nice fashion?

PS: It is not possible/intended to put the PW_PWM above the FPGA node or similar. There will be other nodes.

\documentclass[]{article}
\usepackage{tikz}
\usetikzlibrary{fit,matrix,scopes}
\begin{document}
\begin{tikzpicture}
[
    block/.style={draw},>=stealth
]

    \matrix
    [
        row sep=1cm,column sep=1.5cm
    ]
    {
        \coordinate (f1); &
        \node [block] (PW_PWM) {Pegelwandler}; &
        \coordinate (PWM);
        \\
        \node (f2) {FPGA}; &
        \node [block] (PW_DAC) {Pegelwandler}; &
        \coordinate (DAC);
        \\
        \coordinate (f3); &
        \node [block] (PW_ADCs) {Pegelwandler}; &
        \coordinate (ADCs);
        \\
    };
    \node [fit=(f1) (f2) (f3),draw,fill=white] (FPGA) {FPGA};
    { [->,thick]
        \draw (FPGA) -- (PW_PWM);
        \draw (PW_PWM) -- (PWM) node [anchor=south] {PWM};
        \draw (FPGA) -- (PW_DAC);
        \draw (PW_DAC) -- (DAC) node [anchor=south] {DAC};
        \draw [<->] (FPGA) |- (PW_ADCs);
        \draw [<->] (PW_ADCs) -- (ADCs) node [anchor=south] {ADCs};
    }
\end{tikzpicture}
\end{document}