SigmaTel, Inc.
Data Sheet
STAC9721
Appendix B
+5.0V/+3.3V POWER SUPPLY OPERATION NOTES
The STAC9721 is capable of operating from a single 5V supply connected to both DVdd and AVdd.
Even though the STAC9721 has digital switching levels of 0.2Vdd to 0.5Vdd (See AC Link Electrical
Characteristics in this data book), we recommend that all digital interface signals to the AC-Link be 5V.
If digital interface signals below 5V are used, then appropriate level shifting circuitry must be provided to
ensure adequate digital noise immunity.
The STAC9721 can also operate from a 3.3V digital supply connected to DVdd while maintaining a 5V
analog supply on AVdd. On-chip level shifters ensure accurate logic transfers between the analog and
digital portions of the STAC9721. If digital interface signals above 3.3V are used (i.e. a +5V AC-Link
interface), then appropriate level shifting circuitry must be provided to ensure adequate digital noise
immunity and to prevent on-chip ESD protection diodes from turning on. (See Appendixes A concerning
SPLIT INDEPENDENT POWER SUPPLY OPERATION).
The STAC9723 must be run from a 3.3V supply connected to both DVdd and AVdd. If digital interface
signals above 3.3V are used (i.e. a +5V AC-Link interface), then appropriate level shifting circuitry must
be provided to ensure adequate digital noise immunity and to prevent on-ship ESD protection diodes
from turning on.
*Always operate the STAC97xx digital supply from the same supply voltage as the digital
controller supply.
*All the analog inputs must be ac-coupled with a capacitor of 0.1 µF or greater. It is recommended
that a resistor of about 47KΩ be connected from the signal side of the capacitor to analog GND as
shown below.
> 1.0 uF
SIGNAL
Analog Input
47K
*All the analog outputs must be ac-coupled. If an external amplifier is used, make sure that the
input impedance of the amplifier is at least 10KΩ and use an ac-coupling capacitor of 1.0 µF.
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