CS5397 datasheet(20/40 Pages) CIRRUS

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CS5396 CS5397

DS229PP2

modulators via the AINR+/- and AINL+/- pins.

Each analog input will accept a maximum of

2.0 Vpp. The + and - input signals are 180° out of

phase resulting in a differential input voltage of

4.0 Vpp. Figure 8 shows the input signal levels for

full scale.

The analog modulator samples the input at

6.144 MHz (MCLK=24.576 MHz) corresponding

to Fs equal to 48 kHz in the 128

× Oversampling

Mode and Fs equal to 96 kHz in the 64

× Oversam-

pling Mode. The digital filter will reject signals

within the stopband of the filter. However, there is

rejection

for

input

signals

which

are

× 6.144 MHz) ± the digital passband frequency,

where n=0,1,2,...A 39

Ω resistor in series with the

analog input and a 6.8 nF COG capacitor between

the inputs will attenuate any noise energy at

6.144 MHz, in addition to providing the optimum

source impedance for the modulators. The use of

capacitors which have a large voltage coefficient

(such as general purpose ceramics) must be avoid-

ed since these can degrade signal linearity. If active

circuitry precedes the ADC, it is recommended that

the above RC filter is placed between the active cir-

cuitry and the AINR and AINL pins. The above ex-

ample frequencies scale linearly with output

sample rate.

The on-chip voltage reference and the common

mode voltage are available at VREF and VCOM

for the purpose of decoupling only. However, due

to the sensitivity of this node, the circuit traces at-

tached to these pins must be minimal in length and

no load current may be taken from VREF. It is pos-

sible to use VCOM as a reference voltage to bias

the input buffer circuits, if the circuit trace is very

short and VCOM is buffered at the converter (refer

to the CDB53965/97). The recommended decou-

pling scheme for VREF, Figure 1, is a 470 µF elec-

trolytic capacitor and a 0.1 µF ceramic capacitor

connected from VREF to AGND. The recommend-

ed decoupling scheme for VCOM, Figure 1, is a

100 µF electrolytic capacitor and a 0.1 µF ceramic

capacitor connected from VCOM to AGND.

GROUNDING AND POWER SUPPLY

DECOUPLING - ALL MODES

As with any high resolution converter, the ADC re-

quires careful attention to power supply and

grounding arrangements if its potential perfor-

mance is to be realized. Figure 1 shows the recom-

mended power arrangements, with VA and VL

connected to a clean +5 V supply. VD, which pow-

ers the digital filter, should be run from the system

+5 V logic supply, provided that it is not excessive-

ly noisy (< ±50 mV pk-to-pk). Decoupling capaci-

tors should be as near to the ADC as possible, with

the low value ceramic capacitor being the nearest.

The printed circuit board layout should have sepa-

rate analog and digital regions and ground planes,

with the ADC straddling the boundary. All signals,

especially clocks, should be kept away from the

VREF pin in order to avoid unwanted coupling into

the modulators. The VREF decoupling capacitors,

particularly the 0.01

µF, must be positioned to min-

imize the electrical path from VREF and pin 3,

AGND. The CDB5396/97 evaluation board dem-

onstrates the optimum layout and power supply ar-

rangements, as well as allowing fast evaluation of

the ADC.

To minimize digital noise, connect the ADC digital

outputs only to CMOS inputs.

+3.5 V

+2.5 V

+1.5 V

+3.5 V

+2.5 V

+1.5 V

CS5396/97

AIN+

AIN-

Full Scale Input level= (AIN+) - (AIN-)= 4.0 Vpp

Figure 8. Full scale input voltage

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