CS5317 datasheet(9/32 Pages) CIRRUS

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Initial Sampling

The CS5317 samples the analog input, AIN, at

one-half the master clock frequency (~2.5 MHz

max). The input sampling frequency, fs

in, appears

at CLKOUT regardless of whether the master

clock is generated on-chip (CLKG1 and CLKG2

modes) or driven directly into the CS5317

(CLKOR mode). The digital filter then processes

the input signal at the input sample rate.

Like any sampled-data filter, though, the digital

filter’s passband spectrum repeats around integer

multiples of the sample rate, fs

in. That is, when

the CS5317 is operating at its full-rated speed any

noise within

± 5 kHz bands around 2.5 MHz, 5

MHz, 7.5 MHz, etc. will pass unfiltered and alias

into the baseband. Such noise can only be filtered

by analog filtering before the signal is sampled.

Since the signal is heavily oversampled (2.5

MHz : 5 kHz, or 500 : 1), a single-pole passive

RC filter can be used as shown in Figure 2.

.25 F .5 F

-40

-80

-2.74 dB









sin

(128πƒT)

128sin

(πƒT)







3



Magnitude where:

T = 1/

ƒsin

ƒsin = input sampling frequency = CLKOUT frequency for all modes

= CLKIN/2 in CLKOR mode

= CLKIN*128 in CLKG1 mode

= CLKIN*256 in CLKG2 mode

F =

ƒsin/128 for all modes

ƒ = input frequency

ƒsout = ƒsin/128 = output data rate for CLKOR & CLKG1 = F

ƒsout = ƒsin/256 = output data rate for CLKG2 = F/2

Examples:

For

ƒsin = 2.56 MHz at ƒ = 5 kHz: Magnitude is -2.74 dB

For

ƒsin = 2.56 MHz at ƒ = 10 kHz: Magnitude is -11.8 dB

Figure 3. CS5317 Low-Pass Filter Response

Mag

H(ejω ) (dB)

1.2 K

0.01

µF

AIN

Input

Signal

Note: Any nonlinearities contributed by this filter

will be encoded as distortion by the CS5317.

Therefore a low distortion, high frequency ca-

pacitor such as COG-ceramic is recommended.

Figure 2. Anti-alias Filter

CS5317

DS27F4

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