overbrace with flowchart with mathematics

I am trying to make a flowchart for a controls system. I'm honestly unsure of the best way to do it. Some of the things that I would like to include is mathematics and an overbrace so that I can group functional terms to make it easier to understand. I've distilled it down to the following picture:

This is a minimum example of what I am looking to do.

Any advice on how to start would be greatly appreciated.

• This is a sketch on paper, not a MWE – DG' Feb 20 at 22:25

There is a decoration called brace. You may also be interested in its calligraphy variant but here is the basic version for the time being.

\documentclass[tikz, margin=3mm]{standalone}
\usetikzlibrary{decorations.pathreplacing,positioning}
\begin{document}
\begin{tikzpicture}[>=stealth]
\node[draw] (dWdt) {$\displaystyle\frac{\partial W}{\partial t}$};
\node[right=2cm of dWdt,draw] (dpside) {$\displaystyle\frac{\partial \psi}{\partial e'}$};
\path (dpside.south east) ++ (1,-1)
node[circle,draw,path picture={
\draw (path picture bounding box.south west) -- (path picture bounding
box.north east)
(path picture bounding box.south east) -- (path picture bounding box.north
west);}] (x){};
\draw[->] (dWdt) -- (dpside);
\draw (dpside.east) -| coordinate (aux) (x);
\draw[->] (aux) -- ++ (1,0) node[right]{out $f(x,t)$};
\draw[<-] (dWdt.west) -- ++ (-0.5,0) |- (x);
\draw[<-] (x.south) -- ++ (0,-1) node[below]{bias};
\draw[decoration={brace,raise=1pt,amplitude=4pt},decorate,thick]
(dWdt.north west)  -- node[above=1ex]{$W$} (dpside.north east) ;
\end{tikzpicture}
\end{document}


• Great work. However, the legend out f(x,t) goes near to second rectangle from left to right, in the right upper corner. – Cragfelt Feb 21 at 2:05
• @Cragfelt Thanks a lot! You are right, of course. Corrected. – Schrödinger's cat Feb 21 at 5:37
• @Schrödinger'scat Thank you. I appreciate it. It's actually for the documentation of an analog implementation of the wave function collapse via Schrödinger's equation. Wonderful coincidence. – b degnan Feb 21 at 14:16