CS5321 datasheet(20/38 Pages) CIRRUS

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CS5320/21/22

DS454PP1

2.4 Voltage Reference

The CS5320/21 is designed to operate with a volt-

age reference in the range of 4.0 to 4.5 volts. The

voltage reference is applied to the VREF+ pin with

the VREF- pin connected to the GND. A 4.5 V ref-

erence will result in the best S/N performance but

most 4.5 V references require a power supply volt-

age greater than 5.0 V for operation. A 4.0 V refer-

ence can be used for those applications which must

operate from only 5.0 V supplies, but will yield a

S/N slightly lower (1-2 dB) than when using a 4.5

V reference. The voltage reference should be de-

signed to yield less than 2

µV rms of noise in band

at the VREF+ pin of the CS5320/21. The CS5322

filter selection will determine the bandwidth over

which the voltage reference noise will affect the

CS5320/21/22 dynamic range.

For a 4.5 V reference, the LT1019-4.5 voltage ref-

erence yields low enough noise if the output is fil-

tered with a low pass RC filter as shown in Figure

21 Option A. The filter in Figure 21 Option A is ac-

ceptable for most spectral measurement applica-

tions, but a buffered version with lower source

impedance (Figure 21 Option B) may be preferred

for dc-measurement applications. Due to its dy-

namic (switched-capacitor) input the input imped-

ance of the +VREF pin of the CS5320/21 will

change any time MCLK or HBR is changed. There-

fore the current required from the voltage reference

will change any time MCLK or HBR is changed.

This can affect gain accuracy due to the high source

impedance of the filter resistor in Figure 20 and

Figure 21 Option A. If gain error is to be mini-

mized, especially when MCLK or HBR is changed,

the voltage reference should have lower output im-

pedance. The buffer of Figure 21 Option B offers

lower output impedance and will exhibit better sys-

tem gain stability.

2.5 Clock Source

For proper operation, the CS5320/21 must be pro-

vided with a CMOS-compatible clock on the

MCLK pin. The MCLK for the CS5320/21 is usu-

ally provided by the CS5322 filter. MCLK is usu-

ally 1.024 MHz to set the seven selectable output

word rates from the CS5322. The MCLK frequency

can be as low as 250 kHz and as high as 1.2 MHz.

The choice of clock frequency can affect perfor-

mance; see the Performance section of the data

0.1

µF

+9 to

15V

Ω

LT1019-4.5

200

Ω

49.9

Ω

100

Ω

10k

Ω

100

µF

100

µF

+9 to 15V

0.1

µF

Tant

0.1

µF

To VREF+

LT1007

Option A

Option B

Figure 21. 4.5 Voltage Reference with two filter options

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