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參數資料
| 型號： | AD9957BSVZ-REEL |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 通信及網絡 |
| 英文描述： | 1 GSPS Quadrature Digital Upconverter with 18-Bit IQ Data Path and 14-Bit DAC |
| 中文描述： | SPECIALTY TELECOM CIRCUIT, PQFP100 |
| 封裝： | ROHS COMPLIANT, MS-026AED-HD, TQFP-100 |
| 文件頁數： | 33/60頁 |
| 文件大小： | 840K |
| 代理商： | AD9957BSVZ-REEL |
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AD9957
CLOCK INPUT (REF_CLK)
REFCLK OVERVIEW
The AD9957 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. A block diagram of the REF_CLK
functionality is shown in Figure 47. The various input configu-
rations are controlled by means of the XTAL_SEL pin and
control bits in the CFR3 register. Figure 47 also shows how the
CFR3 control bits are associated with specific functional blocks.
Rev. 0 | Page 33 of 60
REF_CLK
REF_CLK
PLL
VCO
SELECT
DIVIDE
CHARGE
PUMP
OUT
IN
PLL_LOOP_FILTER
ENABLE
PLL_LOOP_FILTER
DRV
CFR3
<31:30>
2
REFCLK_OUT
XTAL_SEL
REFCLK
INPUT
SELECT
LOGIC
SYSCLK
ICP
CFR3
<21:19>
N
CFR3
<7:1>
VCO
CFR3
<26:24>
÷2
INPUT DIVIDER BYPASS
CFR3<15>
PLL_ENABLE
CFR3
<8>
INPUT DIVIDER
RESET
CFR3<14>
Figure 47. REF_CLK Block Diagram
94
95
2
90
91
0
1
0
1
2
7
3
0
1
0
The PLL enable bit is used to choose between the PLL path or
the direct input path. When the direct input path is selected, the
REF_CLK pins must be driven by an external signal source
(single-ended or differential). Input frequencies up to 2 GHz are
supported. For input frequencies greater than 1 GHz, the input
divider must be enabled for proper operation of the device.
When the PLL is enabled, a buffered clock signal is available at
the REFCLK_OUT pin. This clock signal is the same frequency
as the REF_CLK input. This is especially useful when a crystal
is connected, because it gives the user a replica of the crystal
clock for driving other external devices. The REFCLK_OUT
buffer is controlled by two bits as listed in Table 7.
Table 7. REFCLK_OUT Buffer Control
CFR3<31:30>
REFCLK_OUT Buffer
00
Disabled
01
Low output current
10
Medium output current
11
High output current
CRYSTAL DRIVEN REF_CLK
When using a crystal at the REF_CLK input, the resonant
frequency should be approximately 25 MHz. Figure 48 shows
the recommended circuit configuration.
0
REF_CLK
REF_CLK
39pF
39pF
XTAL
90
91
Figure 48. 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 49 for more details.
The REF_CLK input resistance is ~2.5 kΩ differential (~1.2 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 49
assume a signal source with a 50 Ω output impedance.
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