\documentclass[border=1mm]{standalone}

\usepackage{cmap}
\usepackage[cp1250]{inputenc}	
\usepackage[T1]{fontenc}
\usepackage{graphicx}
\usepackage{lmodern}
\usepackage{tikz}
\usepackage{float}
\usepackage{siunitx}
\usepackage{amsmath}
\usepackage{upgreek}
\usepackage{pgfplots}
\pgfplotsset{compat=1.17}
\usepgfplotslibrary{units}
% \makeatletter
%\pgfplotsset{
%  unit code/.code 2 args=
%    \begingroup
%    \protected@edef\x{\endgroup\si{#2}}\x
%} 
%\makeatother

\pgfkeys{%
  /tikz/on layer/.code={
    \pgfonlayer{#1}\begingroup
    \aftergroup\endpgfonlayer
    \aftergroup\endgroup
  }
}

\begin{document}
	\begin{tikzpicture}
	
		\begin{axis}[
			name=main plot,
			height=4cm,	width=11cm,	grid=major,
    		y unit=\SI{}{\micro\ampere},
			ylabel={$i_\mathrm{inj}$},
			%ymin=-160,ymax=-40,
			xmin=0.1945e-6, xmax=0.196e-6,
			xtick distance = 250e-12,
			%ytick distance = 20,
			set layers,
			%x tick label style={/pgf/number format/.cd, set thousands separator={}},
			xticklabel shift = 2 pt,
			legend pos=north east,
			xticklabels =\empty,
			scaled x ticks = false,
			xlabel =
		]					
			
			\addplot +[solid, thick,color=blue,mark=none] table [x index={0}, y index={1}, col sep=comma] {Iinj_lowf.csv};
		\end{axis}
		
		\begin{axis}[
      		at={(main plot.below south west)},
      		height=6cm,	width=11cm,	grid=major,
      		anchor=north west,    % <-- added
      		yshift=1ex,          % <-- adjusted
      		xmin=0.1945e-6, xmax=0.196e-6,
      		ymin=-1.5,ymax=1.5,
      		ytick distance = 0.5,
      		xtick distance = 250e-12,
      		ylabel={$v_\mathrm{prev}$,$v_\mathrm{inj}$},
      		y unit=V,
      		xlabel = Time,
      		xticklabels =\empty,
			scaled x ticks = false,
			legend style={at={(0.5,0.4)},anchor=north}
      		]
      		
      		\addplot +[solid,thick, color=blue,mark=none] table [x index={0}, y index={1}, col sep=comma] {Vprev_lowf.csv};
      		\addlegendentry{$v_\mathrm{prev}$}
      		\addplot +[solid,thick, color=red,mark=none] table [x index={0}, y index={1}, col sep=comma] {Vout_lowf.csv};    
      		\addlegendentry{$v_\mathrm{inj}$}		
      		
      		
			\draw [<->] (axis cs:0.195e-6,0.23) -- (axis cs:0.1955e-6,0.23) ;
			\node [] at (axis cs:0.195235e-6,0.35) {500 ps};      		
      		
    	\end{axis}
		
	\end{tikzpicture}
\end{document}