PCM1740ESLASH2K datasheet(18/26 Pages) TI2

18 / 26 page

PCM1740

SBAS128B

www.ti.com

The VTUN control voltage is generated by the MPEG-2

controller, which compares the MCKO output clock from

the PCM1740 with the clock count received from the trans-

mitter. VTUN is adjusted to retain clock synchronization

between the transmitted and received signals. The SCKO

output is used as the audio master clock for the audio

decoder and additional data converters.

COM Output

The unbuffered DC common-mode voltage output, VCOM

(pin 15), is brought out mainly for de-coupling purposes.

VCOM is nominally biased to VCC/2. The VCOM output may

be used to bias external circuits, but it must be connected to

a high-impedance node or buffered using a voltage follower.

Figure 18 shows examples of the proper use of the VCOM

output for external biasing applications.

DAC OUTPUT FILTERING

Delta-Sigma DACs utilize noise shaping techniques to im-

prove in-band signal-to-noise (SNR) performance at the

expense of generating increased out of band noise above the

Nyquist frequency, or fS/2. The out-of-band noise must be

low-pass filtered in order to provide optimal converter

performance. This is accomplished by a combination of on-

chip and external low-pass filtering.

The PCM1740 includes an on-chip low-pass filter as part of

the output amplifier stage. The frequency response for the

filter is shown in the Typical Characteristics section

of this data sheet. The –3dB cutoff frequency is fixed at

100kHz.

Figure 19 shows the recommended external low-pass active

filter circuits for dual and single-supply applications. These

circuits are second-order Butterworth filters using the Mul-

tiple Feedback (MFB) circuit arrangement. Both filters have

a cutoff frequency of 30kHz. Figure 19(a) is a dual-supply

filter with a gain of 1.85 (for a standard 2 VRMS line output

level). Figure 19(b) is a single-supply filter with a gain of 1.

Values for the filter components may be calculated using the

FilterPro program, available from the TI web site

(www.ti.com) and local sales offices. For more information

regarding MFB active filter design and the FilterPro pro-

gram, refer to TI Application Report SBFA001.

Since the overall system performance is defined primarily

by the quality of the DACs and their associated analog

output circuitry, op amps designed specifically for audio

applications are recommended for the active filters. TI’s

OPA2134, OPA2353, and OPA2343 dual op amps are ideal

for use with the PCM1740.

FIGURE 18. Using VCOM To Bias External Circuitry.

OUT

Non-Polarized

µF

PCM1740

1-10

µF

COM

PCM1740

OPA337

OPA343

1-10

µF

Use voltage follower

to buffer V

COM

To Bias

Nodes

COM

(a) Biasing an External Active Filter Stage

(b) Using a Buffer to Provide Bias for Multiple or

Low Input Impedance Nodes

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