CS5521 datasheet(42/56 Pages) CIRRUS

42 / 56 page

CS5521/22/23/24/28

DS317F2

2.6 Digital Filter

The CS5521/22/23/24/28 have eight different lin-

ear phase digital filters which set the output word

rates (OWRs) shown in Table 3. These rates as-

sume that XIN is 32.768 kHz. Each of the filters

has a magnitude response similar to that shown in

Figure 18. The filters are optimized to settle to full

accuracy every conversion and yield better than

80 dB rejection for both 50 and 60 Hz with output

word rates at or below 15.0 Hz.

The converter’s digital filters scale with XIN. For

example with an output word rate of 15 Hz, the fil-

ter’s corner frequency is typically 12.7 Hz using a

32.768 kHz

clock.

XIN

increased

65.536 kHz the OWR doubles and the filter’s cor-

ner frequency moves to 25.4 Hz.

2.7 Clock Generator

The CS5521/22/23/24/28 include a gate which can

be connected with an external crystal to provide the

master clock for the chip. The chips are designed to

operate using a low-cost 32.768 kHz “tuning fork”

type crystal. One lead of the crystal should be con-

nected to XIN and the other to XOUT. Lead lengths

should be minimized to reduce stray capacitance.

Note that the oscillator circuit will also operate

with a 100 kHz “tuning fork” type crystal.

The converters will operate with an external

(CMOS compatible) clock with frequencies up to

130 kHz (CS5521/23) or 200 kHz (CS5522/24/28).

Figures 19 and 20

detail

the

CS5521/23

and

CS5522/24/28’s performance (respectively) at in-

creased clock rates.

The 32.768 kHz crystal is normally specified as a

time-keeping crystal with tight specifications for

both initial frequency and for drift over tempera-

ture. To maintain excellent frequency stability,

these crystals are specified only over limited oper-

ating temperature ranges (i.e. -10° C to +60° C).

However,

applications

with

the

CS5521/22/23/24/28 don’t generally require such

tight tolerances.

Figure 18. Filter Response (Normalized to Output

Word Rate = 1)

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

110

130

XIN (kHz)

Figure 19. Typical Linearity Error for CS5521/23

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

0.001

0.0011

0.0012

0.0013

100 120 140 160 180 200

XIN (kHz)

Figure 20. Typical Linearity Error for CS5522/24/28

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