(YHDC TV19) Circuit Diagram
Since you are trying to measure an AC signal with an MCU, the best approach can be offseting the signal. VDD/2 offset can be acceptable.

According to the [the datasheet][1], both the primary and the secondary currents should not exceed 5mA.







simulate this circuit &ndash; Schematic created using CircuitLab

The transformer has a unity turns ratio. So the voltage across the secondary will be equal to the voltage across the primary.

First, measure the primary inductance (\$L_p\$). The reactance of the primary inductance at the working frequency, \$\mathrm f\$, is \$\mathrm{X_L = 2\pi f L_p}\$. The net impedance seen by the source is \$\mathrm{Z_i = RS1 + j\omega X_L}\$. So,

$$
\mathrm{|Z| = \sqrt{X_L^2+RS1^2}}
$$
So the current drawn from the source is,
$$
\mathrm{i_{pri} = \frac{V_{src}}{|Z|}}
$$
and thus,
$$
\mathrm{i_{pri-max} = \frac{V_{src-max}}{|Z|} \leq 5mA}
$$
Pick RS1 according to that.

Finally, the voltage across the primary (and thus the secondary) will be
$$
\mathrm{V_{pri}=V_{sec}=i_{pri} \cdot X_L}
$$
and the secondary current will be
$$
\mathrm{i_{sec} = \frac{V_{sec}}{RS2} \leq 5mA}
$$
Pick RS2 value according to that. And please note that it shold be lower enough than the parallel combination of R1 and R2 so that it can be neglected.