Features: Good common mode rejection - 80 db

This implementation uses the POM-2730L-HD-R microphone with TLV320ADC6210 ADC.

POM-2730L-HD-R Specifications:

Sensitivity -30db 0db=1V/Pascal with load resistor, output impedance 2.2 kΩ, SNR 74 db SPL of 94 db or 1 Pascal.

TLV320ADCX120 Settings:

• Vref = 2.75 V
• Z_in = 10 kΩ
• DC coupled
• Analog differential input
• P0_R58 (7:6) is set at mode 1
• DRE disabled

The following figures illustrate the circuit for a electret condensor microphone set up to give a differential output.

The output stage of an electret condenser microphone is a FET element. A pull-up to MicBias and pull-down to ground results in the circuit working as a phase splitter. With an incoming sound input, the INxP and INxM pins are moving in phase opposition. When the level of INxP reduces, the level of INxM increases. To achieve the maximum undistorted signal handling, the voltage difference between INxP and INxM must always be greater then zero at the peak of the signal. This keeps the FET element of the microphone working in linear range.

For maximum signal handling the static DC bias across the microphone terminal is kept at VMICBIAS/2.

Figure 4-6 DC- Coupled Differential Microphone

Figure 4-7 DC Bias for Microphone

In this system $\mathrm{R}=\frac{{\mathrm{R}}_{\mathrm{O}}}{2}\mathrm{ }=1100\mathrm{ }\left(\mathrm{\Omega }\right)$.

Figure 4-8 Waveform DC-Coupled Differential Microphone

Figure 4-9 Circuit DC-Coupled Differential Microphone

The DC level on the INxP and INxM pins without any audio signal is 2.25 V and 0.75 V respectively. Sounds received by the microphone result in out-of-phase signals on the pins as shown by the figure.

1-VRMS differential signal can be digitized by this circuit.

• An economy circuit shown below uses a condenser microphone placed at a distance from the ADC.
• The connecting cable can be inexpensive twisted cable or even an ordinary two-core wire. This is a differential application which relies on the good CMRR of the device to remove noise even with an inexpensive cable.
• Microphone needs no separate PCB or power supply.

Figure 4-10 Economy Circuit