参数资料
型号: LTC2239IUH#TRPBF
厂商: Linear Technology
文件页数: 8/24页
文件大小: 0K
描述: IC ADC 10BIT 80MSPS 3V 32-QFN
标准包装: 2,500
位数: 10
采样率(每秒): 80M
数据接口: 并联
转换器数目: 1
功率耗散(最大): 246mW
电压电源: 单电源
工作温度: -40°C ~ 85°C
安装类型: 表面贴装
封装/外壳: 32-WFQFN 裸露焊盘
供应商设备封装: 32-QFN 裸露焊盘(5x5)
包装: 带卷 (TR)
输入数目和类型: 1 个单端,双极; 1 个差分,双极
16
LTC2239
2239fa
Figures 12 and 13 show alternatives for converting a
differential clock to the single-ended CLK input. The use of
a transformer provides no incremental contribution to
phase noise. The LVDS or PECL to CMOS translators
provide little degradation below 70MHz, but at 140MHz
will degrade the SNR compared to the transformer solu-
tion. The nature of the received signals also has a large
bearing on how much SNR degradation will be experi-
enced. For high crest factor signals such as WCDMA or
OFDM, where the nominal power level must be at least 6dB
to 8dB below full scale, the use of these translators will
have a lesser impact.
The transformer shown in the example may be terminated
with the appropriate termination for the signaling in use.
The use of a transformer with a 1:4 impedance ratio may
be desirable in cases where lower voltage differential
signals are considered. The center tap may be bypassed to
ground through a capacitor close to the ADC if the differ-
ential signals originate on a different plane. The use of a
capacitor at the input may result in peaking, and depend-
ing on transmission line length may require a 10
to 20
ohm series resistor to act as both a low pass filter for high
frequency noise that may be induced into the clock line by
neighboring digital signals, as well as a damping mecha-
nism for reflections.
Maximum and Minimum Conversion Rates
The maximum conversion rate for the LTC2239 is 80Msps.
For the ADC to operate properly, the CLK signal should
have a 50% (
±5%) duty cycle. Each half cycle must have
at least 5.9ns for the ADC internal circuitry to have enough
settling time for proper operation.
An optional clock duty cycle stabilizer circuit can be used
if the input clock has a non 50% duty cycle. This circuit
uses the rising edge of the CLK pin to sample the analog
input. The falling edge of CLK is ignored and the internal
falling edge is generated by a phase-locked loop. The input
clock duty cycle can vary from 40% to 60% and the clock
duty cycle stabilizer will maintain a constant 50% internal
duty cycle. If the clock is turned off for a long period of
time, the duty cycle stabilizer circuit will require a hundred
clock cycles for the PLL to lock onto the input clock. To use
the clock duty cycle stabilizer, the MODE pin should be
connected to 1/3VDD or 2/3VDD using external resistors.
The lower limit of the LTC2239 sample rate is determined
by droop of the sample-and-hold circuits. The pipelined
architecture of this ADC relies on storing analog signals on
small valued capacitors. Junction leakage will discharge
the capacitors. The specified minimum operating fre-
quency for the LTC2239 is 1Msps.
APPLICATIO S I FOR ATIO
WU
UU
CLK
100
0.1
F
4.7
F
FERRITE
BEAD
CLEAN
SUPPLY
IF LVDS USE FIN1002 OR FIN1018.
FOR PECL, USE AZ1000ELT21 OR SIMILAR
2239 F12
LTC2239
CLK
5pF-30pF
ETC1-1T
0.1
F
VCM
FERRITE
BEAD
DIFFERENTIAL
CLOCK
INPUT
2239 F13
LTC2239
Figure 12. CLK Drive Using an LVDS or PECL to CMOS Converter
Figure 13. LVDS or PECL CLK Drive Using a Transformer
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