9.2.1 Design Requirements

For this example the following application parameters exist:

• Single Codec Output
• 1-Vrms input

This design is provided for a high-quality audio path and provides all analog functionality for a PC audio system.

The design has a single codec output with 1-Vrms input.

9.2.2 Detailed Design Procedure

For all analog inputs a 1-uF capacitor should be tied to the input for proper decoupling. If the pin is unused then a 1-uF capacitor should be used and tied to ground.

For analog input pins, a proper lowpass filter will be needed to filter out any high frequencies depending on the application. Please see Figure 22.

Digital and Analog voltage supplies should have proper decoupling capacitors that cover low- and high-frequency spikes. In this application, the user chooses to go with 1-uF and 0.1-uF capacitors.

9.3.1 Improving System Performance

The audio codec is capable of dynamic range performance in excess of 90 db, but the user must pay careful attention to several factors to achieve this. A primary consideration is keeping analog and digital grounds separate, and connecting them together in only one place. Some designers show the connection as a 0-Ω resistor, which allows naming the nets separately. Although it is possible to use a 2-layer board, TI recommends that a minimum of four layers be used, with the two inside layers being analog ground and digital ground. If EMI is a system consideration, then as many as eight layers have been successfully used. The 12- and 25-MHz. clocks can have significant harmonic content depending on the rise and fall times. Bypass capacitors should be very close to the package. The analog VDD pins should be supplied from a separate regulator to reduce noise. By operating the digital portion on 3.3 V instead of 5 V, an additional 0.5- to 0.7-db improvement can be obtained.

The bandgap reference and the anti-pop slow turnon circuit were improved in the LM4549B. A pullup resistor is not required on V_REF, pin 27. For an existing design, the 10-kΩ resistor can be left on the PCB, but the temperature coefficient will improve with no resistor on this pin. In addition, the THD will improve by 0.2–0.5 dB. The external capacitor is charged by an internal current source, ramping the voltage slowly. This results in slow turn-on of the audio stages, eliminating “pops and clicks”. Thus, turn-on performance is also improved. The pullup resistor, in conjunction with the internal impedance and the external capacitor, form a frequency dependent divider from the analog supply. Noise on the analog supply will be coupled into the audio path, with approximately 30 dB. of attenuation. Although this is not a large amount if the noise on the supply is tens of millivolts, it will prevent SNR from exceeding 80 dB.

In Figure 21 and Figure 22, the input coupling capacitors are shown as 1-µF capacitors. This is only necessary for extending the response down to 20 Hz. for music applications. For telematics or voice applications, the lower 3-dB point can be much higher. Using a specified input resistance of 10 kΩ, (40 kΩ typical), a 0.1-µF capacitor may be used. The lower 3-dB point will still be below 300 Hz. By using a smaller capacitor, the package size may be reduced, leading to a lower system cost.