參數(shù)資料
型號: LTC1745CFW#PBF
廠商: Linear Technology
文件頁數(shù): 8/20頁
文件大?。?/td> 0K
描述: IC ADC 12BIT 25MSPS LN 48TSSOP
標準包裝: 39
位數(shù): 12
采樣率(每秒): 25M
數(shù)據(jù)接口: 并聯(lián)
轉換器數(shù)目: 1
功率耗散(最大): 455mW
電壓電源: 單電源
工作溫度: 0°C ~ 70°C
安裝類型: 表面貼裝
封裝/外殼: 48-TFSOP(0.240",6.10mm 寬)
供應商設備封裝: 48-TSSOP
包裝: 管件
輸入數(shù)目和類型: 2 個單端,雙極;1 個差分,雙極
16
LTC1745
1745f
APPLICATIO S I FOR ATIO
WU
UU
Any noise present on the encode signal will result in
additional aperture jitter that will be RMS summed with the
inherent ADC aperture jitter.
In applications where jitter is critical (high input frequen-
cies) take the following into consideration:
1. Differential drive should be used.
2. Use as large an amplitude as possible; if transformer
coupled use a higher turns ratio to increase the
amplitude.
3. If the ADC is clocked with a sinusoidal signal, filter the
encode signal to reduce wideband noise.
4. Balance the capacitance and series resistance at both
encode inputs so that any coupled noise will appear at
both inputs as common mode noise.
The encode inputs have a common mode range of 1.8V to
VDD. Each input may be driven from ground to VDD for
single-ended drive.
Maximum and Minimum Encode Rates
The maximum encode rate for the LTC1745 is 25Msps. For
the ADC to operate properly the encode signal should have
a 50% (
±5%) duty cycle. Each half cycle must have at least
19ns for the ADC internal circuitry to have enough settling
time for proper operation. Achieving a precise 50% duty
cycle is easy with differential sinusoidal drive using a
transformer or using symmetric differential logic such as
PECL or LVDS. When using a single-ended encode signal
asymmetric rise and fall times can result in duty cycles that
are far from 50%.
At sample rates slower than 25Msps the duty cycle can
vary from 50% as long as each half cycle is at least 19ns.
The lower limit of the LTC1745 sample rate is determined
by the 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 dis-
charge the capacitors. The specified minimum operating
frequency for the LTC1745 is 1Msps.
DIGITAL OUTPUTS
Digital Output Buffers
Figure 9 shows an equivalent circuit for a single output
buffer. Each buffer is powered by OVDD and OGND, iso-
lated from the ADC power and ground. The additional
N-channel transistor in the output driver allows operation
down to low voltages. The internal resistor in series with
the output makes the output appear as 50
to external
circuitry and may eliminate the need for external damping
resistors.
LTC1745
1745 F09
OVDD
VDD
0.1
F
43
TYPICAL
DATA
OUTPUT
OGND
OVDD
0.5V TO
VDD
PREDRIVER
LOGIC
DATA
FROM
LATCH
OE
Figure 9. Equivalent Circuit for a Digital Output Buffer
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