9.1.1 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 two 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 0.5-0.7 dB improvement can be obtained.

The bandgap reference and the anti-pop slow turnon circuit were improved in the LM4550B. 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 turnon of the audio stages, eliminating “pops and clicks”. Thus, turnon 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 24 and Figure 25, 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 less than 300 Hz. By using a smaller capacitor, the package size may be reduced, leading to a lower system cost.

9.1.2 Backwards Compatibility

The LM4550B is improved compared with the LM4550. If it is required to build a board that will use either part, a 10 kΩ resistor must be added from the V_REF pin (pin 27) to AV_DD for the LM4550. It is not required for the LM4550B. Addition of this resistor will slightly increase the temperature coefficient of the internal bandgap reference and slightly decrease the THD performance, but overall performance will still be better than the LM4550.

The LM4550 requires that pins 1 and 9 (DV_DD) connect directly to a 27 nH. inductor before going to the 3.3 Volt digital supply and the bypass capacitors. The inductor is not required for the LM4550B and should not be used.

9.2.1 Design Requirements

• Single codec output
• 1-Vrms input

9.2.2 Detailed Design Procedure

For all analog inputs a 1.0-µF capacitor should be tied to the input for proper decoupling. If the pin is unused then a 1.0-µF 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. See Figure 25.

Digital and analog voltage supplies should have proper decoupling capacitors that cover low and high frequency spikes. In our application we chose to go with 1.0-µF and 0.1-µF capacitors.