AD9549
Rev. D | Page 11 of 76
Pin No.
Input/
Output
Pin Type
Mnemonic
Description
20, 21
O
PFD_VRB,
PFD_VRT
22
O
Current set
resistor
PFD_RSET
27
I
Differential
input
SYSCLK
System Clock Input. The system clock input has internal dc biasing and should
always be ac-coupled, except when using a crystal. Single-ended 1.8 V CMOS
can also be used, but it may introduce a spur caused by an input duty cycle
that is not 50%. When using a crystal, tie the CLKMODESEL pin to AVSS, and
connect crystal directly to this pin and Pin 28.
28
I
Differential
input
SYSCLKB
Complementary System Clock. Complementary signal to the input provided
on Pin 27. Use a 0.01 μF capacitor to ground on this pin if the signal provided
on Pin 27 is single-ended.
31
O
LOOP_FILTER
System Clock Multiplier Loop Filter. When using the frequency multiplier to
drive the system clock, an external loop filter must be constructed and attached to
this pin. This pin should be pulled down to ground with a 1 k resistor when
the system clock PLL is bypassed. S
ee Figure 44 for a diagram of the system
clock PLL loop filter.
32
I
1.8 V CMOS
CLKMODESEL
Clock Mode Select. Set to GND when connecting a crystal to the system clock
input (Pin 27 and Pin 28). Pull up to 1.8 V when using either an oscillator or an
external clock source. This pin can be left floating when the system clock PLL is
33, 39, 43, 52
O
GND
AVSS
Analog Ground. Connect to ground.
34
O
1.8 V HSTL
OUTB
35
O
1.8 V HSTL
OUT
sections for details.
37
I
Power
AVDD3
Analog Supply for CMOS Output Driver. This pin is normally 3.3 V but can be
1.8 V. This pin should be powered even if the CMOS driver is not used. See the
38
O
3.3 V CMOS
OUT_CMOS
40
I
Differential
input
FDBK_INB
Complementary Feedback Input. In standard operating mode, this pin is
connected to the filtered DAC_OUTB output. This internally biased input is
typically ac-coupled, and when configured as such, can accept any differential
signal whose single-ended swing is at least 400 mV.
41
I
Differential
input
FDBK_IN
Feedback Input. In standard operating mode, this pin is connected to the
filtered DAC_OUT output.
48
O
Current set
resistor
DAC_RSET
DAC Output Current Setting Resistor. Connect a resistor (usually 10 k) from
50
O
Differential
output
DAC_OUT
DAC Output. This signal should be filtered and sent back on chip through
FDBK_IN input. This pin has an internal 50 pull-down resistor.
51
O
Differential
output
DAC_OUTB
Complementary DAC Output. This signal should be filtered and sent back on
chip through FDBK_INB input. This pin has an internal 50 pull-down resistor.
56
I/O
3.3 V CMOS
REFSELECT
Reference Select Input. In manual mode, the REFSELECT pin operates as a high
impedance input pin; and in automatic mode, it operates as a low impedance
output pin. Logic 0 (low) indicates/selects REFA. Logic 1 (high) indicates/selects
REFB. There is no internal pull-up/pull-down resistor on this pin.
57
I/O
3.3 V CMOS
HOLDOVER
Holdover (Active High). In manual holdover mode, this pin is used to force the
AD9549 into holdover mode. In automatic holdover mode, it indicates
holdover status. There is no internal pull-up/pull-down resistor on this pin.
58
I
3.3 V CMOS
PWRDOWN
Power-Down. When this active high pin is asserted, the device becomes
inactive and enters the full power-down state. This pin has an internal 50 k
pull-down resistor.
59
I
3.3 V CMOS
RESET
Chip Reset. When this active high pin is asserted, the chip goes into reset. Note
that on power-up, it is recommended that the user assert a high to low edge
after the power supplies reach a threshold and stabilize. This pin has an
internal 50 k pull-down resistor.