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[FAQ] TLV320AIC3254: Should I connect a capacitor to a MICBIAS pin? What should I do about noise?

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Part Number: TLV320AIC3254

Should I connect a capacitor directly to the output of the MICBIAS pin of the TLV320AIC3254, TLV320AIC3104 and other similar devices? 

Regards,
Collin Wells
Precision ADC Applications

  • 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.

    Regards,
    Collin Wells
    Precision ADC Applications

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