Hi, xzp,
I have just posted at
the exact answer to your question.
You simply have to distinguish among the input impedance, Zin, source impedance, Zs, and noise resistance, Rn.
Zin does not affect SNR (some books say a different thing, including “The Art of electronics,” do not believe!).
Zs*In gives you the input noise component due to the OpAmp noise current.
Rn=Vn/In.
Zs=Rn gives you the lowest noise factor, or figure (noise contribution of the OpAmp), NF, but not the highest SNR.
Zeroing the reactive part in Zs, such that Zs=Rs, is called noise tuning.
Equating Rn to Rs with a transformer or by connecting a number, n, of OpAmps in parallel [such that Vnp=Vn/sqrt(n*Vni), Inp=In*sqrt(n Ini)] is called noise matching.
Noise tuning and noise matching give you the highest SNR.