Not all microcontrollers output the same logic state after power-up or reset. 8051-type microcontrollers, for example, output logic high levels while other models power up with logic low levels after reset. In the configuration of Figure 7-2(a), the shutdown signal is applied on the cathode side of the photodiode within the optocoupler. A high logic level causes the OPA564-Q1 to be enabled, and a low logic level shuts the OPA564-Q1 down. In the configuration of Figure 7-2(b), with the logic signal applied on the anode side, a high level causes the OPA564-Q1 to shut down, and a low level enables the op amp.

Figure 7-6 Output Shutdown Output Impedance

Figure 7-7 OPA564-Q1: Output Shutdown Equivalent Circuit (With External Feedback)