Investigating the start-up behavior of the system is also interesting. Especially determining if the minimum current of less than 4mA can be maintained during the whole start-up phase and how long the start-up phase takes. Figure 4-40 shows how the start-up is monitored in a first step.

Taking a measurement using the configuration in Figure 4-40 yields the scope picture illustrated in Figure 4-41.

Figure 4-42 shows why the current looks as measured. The current at the very beginning is defined only by the bypass resistor R2, which is the only part allowing current flow before the internal voltage is up and the amplifier is powered and regulating the current.

After 250ms the voltage is high enough, so that the LDO turns on and provides 1.8V internally. After 750ms more, the internal power switch turns on. This switch monitors the intermediate 3.3V rail to be stable and adds some delay. Turning on enables the MCU and the IO rail of the AFE. This allows the AFE to leave the power on reset and the current regulation begins, though the measured input current steps to 3mA.

With a supply of 24V the system starts up much faster, as this initial current is higher. Figure 4-43 shows the same measurement as before but with 24V loop voltage. The current at the beginning of power up sequence is approximately 2mA. The MCU completes the full start-up process in about 150ms.

Figure 4-44 to Figure 4-47 show the same test for different configurations.