Configurations

AB1

Test circuit for determination of 'A'

Test circuit for determination of 'B'

Transmission-1 matrix parameters

$$ T_{1}=\left(\begin{array}{cc} \frac{1}{n_{1}} & \frac{R}{n_{2}}\\ 0 & 0 \end{array}\right) $$

Output impedance

$$ Z_{o}=\frac{R\,n_{1}}{n_{2}} $$

AB2

Test circuit for determination of 'A'

Test circuit for determination of 'B'

Transmission-1 matrix parameters

$$ T_{1}=\left(\begin{array}{cc} \frac{C_{a}}{C_{a}+C_{b}} & \frac{C_{a}\,R_{o}}{C_{a}+C_{b}}\\ 0 & 0 \end{array}\right) $$

Output impedance

$$ Z_{o}=R_{o} $$

CD1

Test circuit for determination of 'C'

Test circuit for determination of 'D'

Transmission-1 matrix parameters

$$ T_{1}=\left(\begin{array}{cc} 0 & 0\\ s\,C_{a} & \frac{R\,s\,\left(C_{b}+\mathrm{n1}\,C_{a}\right)}{\mathrm{n1}} \end{array}\right) $$

Output impedance

$$ Z_{o}=\frac{R\,\left(C_{b}+\mathrm{n1}\,C_{a}\right)}{\mathrm{n1}\,C_{a}} $$

CD2

Test circuit for determination of 'C'

Test circuit for determination of 'D'

Transmission-1 matrix parameters

$$ T_{1}=\left(\begin{array}{cc} 0 & 0\\ s\,C_{a} & s\,C_{a}\,R_{o} \end{array}\right) $$

Output impedance

$$ Z_{o}=R_{o} $$

Comparison table

Discussion

Based on this comparison table, the choice is between AB1 and CD2. If the circuit complexity and the nonlinearity due to ESD protection are more important than the power efficiency and the flatness of the frequency response, CD2 is the best option.

The brute-force creation of the output impedance as shown in AB2 and CD2, requires extra voltage drive capability of the controller. For the given maximum signal level and power supply voltage, this method of realizing the output resistance does not impose show stoppers.