How to define a fixed spacing between characters independent of font-size?

As it can be seen in the following two equations, the spacing between the first two A’s depends on the font size and changes if the new AA symbol is used in a fraction.

- ${Ahspace{-.7ex}A}{AA}$
- $frac{Ahspace{-.7ex}A}{AA}$

I have tried em, mm, and pt with no success. How can one define a negative spacing that functions the same way everywhere, independent of the font size?

Or similarly, how can I define a new composit symbol (e.g. AA) that looks the same everywhere?

documentclass{article}
newcommanddblA{mathchoice
{Ahspace{-.7ex}A}
{Ahspace{-.7ex}A}
{Ahspace{-.58ex}A}
{Ahspace{-.55ex}A}%
}
begin{document}
$dblA AA$

$scriptstyledblA AA$

$scriptscriptstyledblA AA$
end{document}

documentclass{article}

DeclareRobustCommand{AAA}{Amkern-5mu A}

begin{document}

[
AAAfrac{AAA}{AA}textstylefrac{AAA}{AA}scriptstylefrac{AAA}{AA}
]

end{document}