PCM3500ESLASH2K datasheet(22/26 Pages) TI

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PCM3500

OUTPUT FILTER CIRCUITS FOR THE DAC

The PCM3500’s DAC uses delta-sigma conversion tech-

niques. It uses oversampling and noise shaping to improve

in-band (f = fS/2) signal-to-noise performance at the expense

of increased out-of-band noise. The DAC output must be

low-pass filtered to attenuate the out-of-band noise to a

reasonable level.

The PCM3500 includes a low-pass filter in the on-chip

output amplifier circuit. The frequency response for this

filter is shown in Figure 20. Although this filter helps to

lower the out-of-band noise, it is not adequate for many

applications. This is especially true for applications where

the sampling frequency is below 16kHz, since the out-of-

band noise above fS/2 is in the audio spectrum. An external

filter circuit, either passive or active, is required to provide

additional attenuation of the out-of-band noise. The low-

pass filter order will be dependent upon the out-of-band

noise requirements for a particular system. Generally, a 2nd-

order or better low-pass circuit will be required, with the

cut-off frequency set to fS/2 or less.

Burr-Brown Application Bulletin AB-034 provides infor-

mation for designing both Multiple Feedback and Sallen-

Key active filter circuits using software available from Burr-

Brown’s web site. Another excellent reference for both

passive and active filter design is the “Electronic Filter

Design Handbook, Third Edition” by Williams and Taylor,

published by McGraw-Hill.

ON-CHIP ANALOG FRONT END FOR THE ADC

The PCM3500 A/D converter includes a fully differential

input delta-sigma modulator. In order to simplify connection

for single-ended applications, an analog front end (AFE)

circuit has been included on the PCM3500 just prior to the

modulator. The AFE circuit is shown in Figure 21.

FIGURE 20. DAC Output Amplifier Filter Response.

FIGURE 21. On-Chip AFE Circuit for the ADC.

50k

Ω

COM

REF1

REF2

Delta-Sigma

Modulator

(+)

(–)

Reference

1.0

µF

OUTPUT FILTER

STOPBAND FREQUENCY RESPONSE

Frequency (Hz)

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

100

10k

100k

10M

OUTPUT FILTER

PASSBAND FREQUENCY RESPONSE

Frequency (Hz)

–1

–2

–3

–4

–5

–6

–7

–8

–9

–10

100

10k

100k

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