PCM1740ESLASH2K datasheet(7/26 Pages) TI2

7 / 26 page

PCM1740

SBAS128B

www.ti.com

3rd ORDER

∆Σ MODULATOR

Frequency (kHz)

–20

–40

–60

–80

–100

–120

–140

–160

STEREO DIGITAL-TO-ANALOG

CONVERTER

The stereo DACs of the PCM1740 use a multi-level delta-

sigma architecture. Based upon a 3rd-order noise shaper and

a 5-level amplitude quantizer, this section converts the 8x

oversampled, 18-bit input data from the interpolation filter

to a 5-level delta-sigma format. A block diagram of the

multi-level delta-sigma modulator is shown in Figure 1.

This architecture has the advantage of improved stability

and increased tolerance to clock jitter when compared to the

one-bit (2-level) delta-sigma DACs.

The combined oversampling rate of the delta-sigma modu-

lator and the 8x interpolation filter is 48fS for a 384fS system

clock, and 64fS for a 256fS system clock. The theoretical

quantization noise performance for the 5-level delta-sigma

modulator is shown in Figure 2.

The output of the delta-sigma modulator is low-pass filtered

and buffered by an on-chip output amplifier. For best

performance, an external low-pass filter is recommended.

Refer to the Applications Information section of this data

sheet for details regarding DAC output filter recommenda-

tions.

The PCM1740 includes two analog outputs, VOUTL (pin 14)

and VOUTR (pin 11), corresponding to the left and right audio

outputs. The full-scale output amplitude is 0.62

× V

CC, or

3.1VPP with a +5V supply and an AC-coupled load of 5kΩ or

greater. The analog outputs are centered about the DC com-

mon-mode voltage, which is typically VCC/2.

The DC common-mode voltage is made available at the

VCOM output (pin 15). This is an unbuffered output, prima-

Out

48f

S (384fS)

64f

S (256fS)

18-Bit

5-level Quantizer

–

Z–1

–

Z–1

FIGURE 2. Quantization Noise Spectrum.

rily used for decoupling purposes. See the Applications

Information section of this data sheet for more information

regarding the use of the VCOM output for biasing external

circuitry.

VOLTAGE CONTROLLED CRYSTAL OSCILLATOR

(VCXO)

The PCM1740 includes an on-chip voltage-controlled crys-

tal oscillator, or VCXO, which is used to generate the

27MHz master clock required by most digital broadcast and

MPEG-2 decoding applications.

FIGURE 1. 5-Level

∆Σ Modulator Block Diagram.

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