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AD9912
Rev. D | Page 25 of 40
POWER SUPPLY PARTITIONING
The AD9912 features multiple power supplies, and their power
consumption varies with its configuration. This section covers
which power supplies can be grouped together and how the
power consumption of each block varies with frequency.
The numbers quoted here are for comparison only. Refer to the
Specifications section for exact numbers. With each group, use
bypass capacitors of 1 μF in parallel with a 10 μF.
The recommendations here are for typical applications, and for
these applications, there are four groups of power supplies:
3.3 V digital, 3.3 V analog, 1.8 V digital, and 1.8 V analog.
Applications demanding the highest performance may require
additional power supply isolation.
Important: All power supply pins must receive power regardless
of whether that block is used.
3.3 V SUPPLIES
DVDD_I/O (Pin 1) and AVDD3 (Pin 14)
Although one of these pins is analog and the other is digital,
these two 3.3 V supplies can be grouped together. The power
consumption on Pin 1 varies dynamically with serial port
activity.
AVDD3 (Pin 37)
This is the CMOS driver supply. It can be either 1.8 V or 3.3 V,
and its power consumption is a function of the output frequency
and loading of OUT_CMOS (Pin 38).
If the CMOS driver is used at 3.3 V, this supply should be
isolated from other 3.3 V supplies with a ferrite bead to avoid
a spur at the output frequency. If the HSTL driver is not used,
AVDD3 (Pin 37) can be connected (using a ferrite bead) to
AVDD3 (Pin 46, Pin 47, and Pin 49). If the HSTL driver is used,
connect AVDD3 (Pin 37) to Pin 1 and Pin 14, using a ferrite bead.
If the CMOS driver is used at 1.8 V, AVDD3 (Pin 37) can be
connected to AVDD (Pin 36).
If the CMOS driver is not used, AVDD3 (Pin 37) can be tied
directly to the 1.8 V AVDD (Pin 36) and the CMOS driver
powered down using Register 0x0010.
AVDD3 (Pin 46, Pin 47, and Pin 49)
These are 3.3 V DAC power supplies that typically consume
about 25 mA. At a minimum, a ferrite bead should be used to
isolate these from other 3.3 V supplies, with a separate regulator
being ideal.
1.8 V SUPPLIES
DVDD (Pin 3, Pin 5, and Pin 7)
These pins should be grouped together and isolated from the
1.8 V AVDD supplies. For most applications, a ferrite bead
provides sufficient isolation, but a separate regulator may be
necessary for applications demanding the highest performance.
The current consumption of this group increases from about
160 mA at a system clock of 700 MHz to about 205 mA at a
system clock of 1 GHz. There is also a slight (~5%) increase as
f
OUT
increases from 50 MHz to 400 MHz.
AVDD (Pin 11, Pin 19, Pin 23, Pin 24, Pin 36, Pin 42,
Pin 44, and Pin 45)
These pins can be grouped together and should be isolated from
other 1.8 V supplies. A separate regulator is recommended. At
a minimum, a ferrite bead should be used for isolation.
AVDD (Pin 53)
This 1.8 V supply consumes about 40 mA. The supply can be
run off the same regulator as the 1.8 V AVDD group, with a
ferrite bead to isolate Pin 53 from the rest of the 1.8 V AVDD
group. However, for applications demanding the highest
performance, a separate regulator is recommended.
AVDD (Pin 25, Pin 26, Pin 29, and Pin 30)
These system clock PLL power pins should be grouped together
and isolated from other 1.8 V AVDD supplies.
At a minimum, it is recommended that Pin 25 and Pin 30 be
tied together and isolated from the aggregate AVDD 1.8 V
supply with a ferrite bead. Likewise, Pin 26 and Pin 29 can also
be tied together, with a ferrite bead isolating them from the same
aggregate 1.8 V supply. The loop filter for the system clock PLL
should directly connect to Pin 26 and Pin 29 (see Figure 46).
Applications demanding the highest performance may need to
have these four pins powered by their on their own LDO.
If the system clock PLL is bypassed, the loop filter pin (Pin 31)
should be pulled down to analog ground using a 1 kΩ resistor.
Pin 25, Pin 26, Pin 29, and Pin 30 should be included in the large
1.8 V AVDD power supply group. In this mode, isolation of these
pins is not critical, and these pins consume almost no power.