Datasheet
AD9913
Rev. A | Page 18 of 32
CLOCK INPUT (REF_CLK)
REF_CLK OVERVIEW
The AD9913 supports a number of options for producing the
internal SYSCLK signal (that is, the DAC sample clock) via the
REF_CLK input pins. The REF_CLK input can be driven
directly from a differential or single-ended source, or it can
accept a crystal connected across the two input pins. There is
also an internal phase-locked loop (PLL) multiplier that can be
independently enabled. The various input configurations are
controlled by means of the control bits in the CFR2 [7:5]
register.
Table 6. Clock Input Mode Configuration
CFR2 [7:5] Mode Configuration
000 Differential Input, PLL Enabled
001 Differential Input, PLL Disabled (Default)
x10
1
XTAL Input, PLL Enabled
x11
1
XTAL Input, PLL Disabled
100 CMOS Input, PLL Enabled
101 CMOS Input PLL Disabled
1
x = don’t care.
XTAL
REF_CLK
REF_CLK
CMOS
1
0
1
0
10
00
1
0
1
0
÷2
÷2
PLL
CFR2[5]
CFR2[6]
DIVIDE
CONTROL
CFR2[5:0]CFR2[14:9]
DIFFERENTIAL/
SINGLE
CFR2[15]
CFR2[3]
CFR2[7:6]
13
14
SYSTEM
CLOCK
07002-020
Figure 26. Internal Clock Path Functional Block Diagram
CRYSTAL-DRIVEN REF_CLK
When using a crystal at the REF_CLK input, the resonant
frequency should be approximately 25 MHz. Figure 27 shows
the recommended circuit configuration.
REFCLK
REFCLK
39pF39pF
XTAL
13
14
07002-021
Figure 27. Crystal Connection Diagram
DIRECT-DRIVEN REF_CLK
When driving the REF_CLK inputs directly from a signal
source, either single-ended or differential signals can be used.
With a differential signal source, the REF_CLK pins are driven
with complementary signals and ac-coupled with 0.1 μF
capacitors. With a single-ended signal source, either a single-
ended-to-differential conversion can be employed or the
REF_CLK input can be driven single-ended directly. In either
case, 0.1 μF capacitors are used to ac couple both REF_CLK
pins to avoid disturbing the internal dc bias voltage of ~1.35 V.
See Figure 28 for more details.
The REF_CLK input resistance is ~2.7 kΩ differential (~1.35 kΩ
single-ended). Most signal sources have relatively low output
impedances. The REF_CLK input resistance is relatively high,
therefore, its effect on the termination impedance is negligible
and can usually be chosen to be the same as the output imped-
ance of the signal source. The bottom two examples in Figure 28
assume a signal source with a 50 Ω output impedance.
TERMINATION
REF_CLK
DIFFERENTIAL SOURCE,
DIFFERENTIAL INPUT
SINGLE-ENDED SOURCE,
DIFFERENTIAL INPUT
S
INGLE-ENDED SOURCE,
SINGLE-ENDED INPUT
13
14
0.1µF
0.1µF
LVPECL,
OR
LVDS
DRIVER
REF_CLK
13
14
50Ω
0.1µF
0.1µF
BALUN
(1:1)
REF_CLK
REF_CLK
REF_CLK
REF_CLK
13
14
0.1µF
0.1µF
50Ω
07002-022
Figure 28. Direct Connection Diagram
CMOS-DRIVEN REF_CLK
This mode is enabled by writing CFR2 [7] to be true. In this
state, the AD9913 must be driven at Pin 13 with the reference
clock source. Additionally, it is recommended that Pin 14 in
CMOS mode be tied to ground through a 10 kΩ resistor.
13
14
REF_CLK
REF_CLK
10kΩ
CMOS
DRIVE
R
0
7002-023
Figure 29. CMOS-Driven Diagram
PHASE-LOCKED LOOP (PLL) MULTIPLIER
An internal phase-locked loop (PLL) provides users of the
AD9913 the option to use a reference clock frequency that is
lower than the system clock frequency. The PLL supports a wide
range of programmable frequency multiplication factors (1× to
64×). See Table 7 for details on configuring the PLL multipli-
cation factor. The PLL is also equipped with a PLL_LOCK bit.
CFR2 [15:8] and CFR2 [5:1] control the PLL operation. Upon
power-up, the PLL is off. To initialize the PLL, CFR2 [5] must
be cleared and CFR2 [1] must be set. The function of CFR2 [1]