Resistor-referenced parallel-resonator response ($H=(R/R_s)/[1+iR(\omega C-1/(\omega L))]$)

When $R_s$ is in series with a parallel RLC circuit, the ratio of parallel-network voltage to voltage across $R_s$ is its impedance divided by $R_s$. Its peak is $R/R_s$, and its phase passes from $+\pi /2$ through zero to $-\pi /2$ as angular frequency rises through $1/\sqrt{LC}$. This is not the transfer ratio relative to the total input voltage, which includes the additional series voltage drop and has peak $R/(R+R_s)$. Identifying voltage endpoints is necessary before using the voltage divider formula.