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On AIC devices, such as the TLV320AIC3104 and TLV320AIC3254, it is a common misconception that the MICBIAS is an LDO. In reality, it is actually an amplifier or a switch to VDD, depending on the configured mode. This is shown in Figure 1, below.
Figure 1: MICBIAS Options
When MICBIAS is configured as an amplifier, its output voltage is derived from a band-gap reference. This output provides several low-noise programmable voltage levels, capable of driving resistive loads up to ~4mA. However, contrary to an LDO, this internally compensated amplifier should not have a capacitor directly connected to its output. For cases where board noise is an issue, the bias resistor can be split to employ an RCR close to the microphone. Also, a differential configuration helps, as common noise coupled into MICBIAS trace and +/- signal traces are rejected. Also, it is possible to place small capacitors close to the input pins to notch RF noise, such as GSM.
When MICBIAS is configured as a switch, it is ok to have a capacitor directly connected to the pin. In general, it is a good idea to have footprints for capacitors in as many places as possible, space permitting. This allows experimenting with many variables in case that unexpected noise gets into the design.
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