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APPLICATION INFORMATION

Basic Circuit Configurations

Analog Output Filter Circuits

PCM4104-EP

SBAS419 – JUNE 2007

This section provides practical information for system and hardware engineers that are designing in the

PCM4104.

Figure 19 and Figure 20 show typical circuit configurations for the PCM4104 operated in Standalone and

Software modes. Power supply bypass and reference decoupling capacitors should be placed as close to the

corresponding PCM4104 pins as possible. A common ground is shown in both figures, with the analog and

digital ground pins connected to a common plane. Separate power supplies are utilized for the analog and digital

sections, with 5 V required for the PCM4104 analog supplies and 3.3 V required for the digital supply.

The 5 V analog supply may be derived from a higher valued, positive analog power supply using a linear voltage

regulator, such as the REG103 available from Texas Instruments. The 3.3 V digital supply can be derived from a

primary 5 V digital supply using a linear voltage regulator, such as the REG1117, also from TI. The

PCM4104EVM evaluation module provides an example of how the common ground with separate supply

approach can be successfully implemented. The PCM4104EVM User's Guide includes schematics and PCB

layout plots for reference. The evaluation module is available through Texas Instruments' distributors and sales

representatives, or may be ordered online through the TI eStore, which can be accessed through the TI home

page at http://www.ti.com.

The master clock generator supplies the system clock for the PCM4104, as well as the audio data source, such

as a digital signal processor. The LRCK and BCK audio clocks should be derived from the system clock, in order

to ensure synchronous operation.

An external output filter is recommended for each differential output pair. The external output filter further

reduces the out-of-band noise energy produced by the delta-sigma modulator, while providing band limiting

suitable for audio reproduction. A 2nd-order Butterworth low-pass filter circuit with a –3 dB corner frequency

from 50 kHz to 180 kHz is recommended.

The configuration of the output filter circuit is dependent upon whether a single-ended or differential output is

required. Single-ended outputs are commonly used in consumer playback systems, while differential or balanced

outputs are used in many professional audio applications, such as recording or broadcast studios and live sound

systems.

Figure 21 illustrates an active filter circuit that uses a single op amp to provide both 2nd-order low-pass filtering

and differential to single-ended signal conversion. This circuit is used on the PCM4104EVM evaluation circuit

and meets the published typical Electrical Characteristics for dynamic performance. The single-ended output is

convenient for connecting to both headphone and power amplifiers when used for listening tests.

The quality of the op amp used is this circuit is important, as many devices will degrade the dynamic range

and/or total harmonic distortion plus noise (THD+N) specifications for the PCM4104. An NE5534A is shown in

Figure 21 and provides both low noise and distortion. Bipolar input op amps with equivalent specifications

should produce similar measurement results. Devices that exhibit higher equivalent input noise voltage, such as

the Texas Instruments OPA134 or OPA604 families, will produce lower dynamic range measurements

(approximately 1 dB to 2 dB lower than the typical PCM4104 specification), while having little or no impact on

the THD+N specification when measuring a full-scale output level.

Figure 22 illustrates a fully-differential output filter circuit suitable for use with the PCM4104. The OPA1632 from

Texas Instruments provides the fully differential signal path in this circuit. The OPA1632 features very low noise

and distortion, making it suitable for high-end audio applications.

Texas Instruments provides a free software tool, FilterProt, used to assist in the design of active filter circuits.

The software supports design of multiple feedback (MFB), Sallen-Key, and fully differential filter circuits. FilterPro

is available from the TI web site. Additionally, TI document number SBAF001A, also available from the TI web

site, provides pertinent application information regarding the proper usage of the FilterPro program.

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