\documentclass[12pt,letterpaper]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}
\usepackage{tikz}
\usetikzlibrary{decorations.pathreplacing}
\makeatletter
\def\underbrace#1{%
\@ifnextchar_{\tikz@@underbrace{#1}}{\tikz@@underbrace{#1}_{}}}
\def\tikz@@underbrace#1_#2{%
\tikz[baseline=(a.base)] {\node[inner sep=0] (a) {\(#1\)};
\draw[line cap=round,decorate,decoration={brace,amplitude=5pt}]
(a.south east) -- node[below,inner sep=7pt] {\(\scriptstyle #2\)} (a.south west);}}
\def\overbrace#1{%
\@ifnextchar^{\tikz@@overbrace{#1}}{\tikz@@overbrace{#1}^{}}}
\def\tikz@@overbrace#1^#2{%
\tikz[baseline=(a.base)] {\node[inner sep=0] (a) {\(#1\)};
\draw[line cap=round,decorate,decoration={brace,amplitude=5pt}]
(a.north west) -- node[pos=.5,above,inner sep=7pt] {\(\scriptstyle #2\)} (a.north east);}}
\makeatother
\begin{document}
Test with braces:
\begin{equation}
\underbrace{ABCD}_{\text{this is a test}} + DEFG = \overbrace{HIJK}^{\text{this is a test}}
\end{equation}
\end{document}