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The output of the FG amplifier is provided on a pin, so the gain of the FG amplifier can be set by the user. Filter circuits can also be implemented. Note that the FG signal is also fed back internally to the speed control circuits. The FG signal that the DRV8308 device uses can be generated from a PCB trace under a motor, or it can be input from a logic-level TACH input, or it can be synthesized from the Hall sensor transitions (selectable by register FGSEL). If generated from Hall transitions, the resulting output can be either an exclusive-or function of the three Hall sensors, or the same as the HALL_U input, as shown in Figure 6. Selection of FG operating mode is through the FGSEL register bits. The FGOUT pin is an open-drain output and requires an external pullup resistor to the logic supply.
