Open-loop output impedance (Z_o) is a specification that is often omitted from the data sheets but is very important when performing a stability analysis. Zo forms a RC circuit in combination with a capacitive load and plays a key role in where the pole is created in the A_OL curve.

When modeled correctly, the Z_o helps in getting a more accurate settling time behavior under various capacitive loads. Z_o is also needed to calculate the proper component values when a compensation scheme is considered to assure the phase margin stays above 45 degrees. To learn more about Z_o, please refer to Texas Instruments Precision Lab video series on Stability.

Figure 2-12 below shows the "Open Loop Output Impedance (Z_o) Test Circuit." Z_o is output impedance looking from the output of the op amp.

Simulated open-loop output impedance (Z_o) is usually in decibels (dB). But, oftentimes data sheets represent this parameter in ohms (Ω) as shown in Figure 2-14. Figure 2-13 shows the simulation Z_o for TLV6742 in ohms and Figure 2-14 shows the data sheet Z_o plot for TLV6742. For a more complete simulation test analysis, please refer to the following EDN article, authored by Ian Williams. This circuit may be simulated by downloading the AN1516 Test Circuits in either TINA-TI™ or PSpice® for TI.