CS5394KS datasheet(12/22 Pages) CIRRUS

12 / 22 page

CS5394

DS258PP4

Synchronization of Multiple Devices

In systems where multiple ADCs are required, care

must be taken to achieve simultaneous sampling. It

is recommended that the rising edge of the CAL

signal be timed with a falling edge of MCLK to en-

sure that all devices will initiate a calibration and

synchronization sequence on the same rising edge

of MCLK. The absence of re-timing of the CAL

signal can result in a sampling difference of one

MCLK period.

Grounding and Power Supply Decoupling

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, may be run from the system

+5 V logic supply. Alternatively, VD may be pow-

ered from the analog supply via a ferrite bead. In

this case, no additional devices should be powered

from VD. Decoupling capacitors 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 0.1 µF, must be positioned to mini-

mize the electrical path from VREF and pin 3,

AGND. The CDB5394 evaluation board demon-

strates 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.

PERFORMANCE

Digital Filter

Figures 5-8 show the performance of the digital fil-

ter included in the ADC. All plots are normalized

to Fs. Assuming a sample rate of 48 kHz, the 0.5

frequency point on the plot refers to 24 kHz. The

filter frequency response scales precisely with Fs.

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