input (CPARASITIC
參數資料
型號: LTC2234IUK#PBF
廠商: Linear Technology
文件頁數: 6/24頁
文件大?。?/td> 0K
描述: IC ADC 10BIT 135MSPS SAMPL 48QFN
標準包裝: 52
位數: 10
采樣率(每秒): 135M
數據接口: 并聯(lián)
轉換器數目: 1
功率耗散(最大): 680mW
電壓電源: 單電源
工作溫度: -40°C ~ 85°C
安裝類型: 表面貼裝
封裝/外殼: 48-WFQFN 裸露焊盤
供應商設備封裝: 48-QFN-EP(7x7)
包裝: 管件
輸入數目和類型: 1 個單端,雙極; 1 個差分,雙極
產品目錄頁面: 1349 (CN2011-ZH PDF)
LTC2234
14
2234fa
NMOS transistors. The capacitors shown attached to each
input (CPARASITIC) are the summation of all other capaci-
tance associated with each input.
During the sample phase when ENC is low, the transistors
connect the analog inputs to the sampling capacitors and
they charge to, and track the differential input voltage.
When ENC transitions from low to high, the sampled input
voltage is held on the sampling capacitors. During the hold
phase when ENC is high, the sampling capacitors are
disconnected from the input and the held voltage is passed
to the ADC core for processing. As ENC transitions from
high to low, the inputs are reconnected to the sampling
capacitors to acquire a new sample. Since the sampling
capacitors still hold the previous sample, a charging glitch
proportional to the change in voltage between samples will
be seen at this time. If the change between the last sample
and the new sample is small, the charging glitch seen at
the input will be small. If the input change is large, such as
the change seen with input frequencies near Nyquist, then
a larger charging glitch will be seen.
Single-Ended Input
For cost sensitive applications, the analog inputs can be
driven single-ended. With a single-ended input the har-
monic distortion and INL will degrade, but the SNR and
DNL will remain unchanged. For a single-ended input, AIN+
should be driven with the input signal and AIN– should be
connected to 1.6V or VCM.
Common Mode Bias
For optimal performance the analog inputs should be
driven differentially. Each input should swing
±0.5V for
the 2V range or
±0.25V for the 1V range, around a
common mode voltage of 1.6V. The VCM output pin (Pin
44) may be used to provide the common mode bias level.
VCM can be tied directly to the center tap of a transformer
to set the DC input level or as a reference level to an op amp
differential driver circuit. The VCM pin must be bypassed to
ground close to the ADC with a 2.2
F or greater capacitor.
Input Drive Impedance
As with all high performance, high speed ADCs, the
dynamic performance of the LTC2234 can be influenced
by the input drive circuitry, particularly the second and
third harmonics. Source impedance and input reactance
can influence SFDR. At the falling edge of ENC, the sample-
and-hold circuit will connect the 1.6pF sampling capacitor
to the input pin and start the sampling period. The sam-
pling period ends when ENC rises, holding the sampled
input on the sampling capacitor. Ideally the input circuitry
should be fast enough to fully charge the sampling capaci-
tor during the sampling period 1/(2FENCODE); however,
this is not always possible and the incomplete settling may
degrade the SFDR. The sampling glitch has been designed
to be as linear as possible to minimize the effects of
incomplete settling.
For the best performance, it is recommended to have a
source impedance of 100
or less for each input. The
source impedance should be matched for the differential
inputs. Poor matching will result in higher even order
harmonics, especially the second.
Input Drive Circuits
Figure 3 shows the LTC2234 being driven by an RF
transformer with a center tapped secondary. The second-
ary center tap is DC biased with VCM, setting the ADC input
25
25
25
25
0.1
F
AIN
+
AIN
12pF
2.2
F
VCM
LTC2234
ANALOG
INPUT
0.1
FT1
1:1
T1 = MA/COM ETC1-1T
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
2234 F03
Figure 3. Single-Ended to Differential Conversion
Using a Transformer
APPLICATIO S I FOR ATIO
WU
UU
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