AD7400A,pdf datasheet (Isolated Sigma-Delta Modulator)

The AD1886A contains an internal microphone preamp with 20 dB gain; in most cases a direct microphone connecTIon as shown in

Figure 16 is adequate. If the microphone level is too low, an external preamp can be added as shown in Figure 17. In either case the

microphone bias can be derived from the codec’s internal reference (VREFOUT) using a 2.2 kΩ resistor. For the preamp circuit, the

VREFOUT signal can also provide the midpoint bias for the amplifier.