參數(shù)資料
型號(hào)： LTC2248CUH#TRPBF
廠商： Linear Technology
文件頁(yè)數(shù)： 7/24頁(yè)
文件大小： 0K
描述： IC ADC 14BIT 65MSPS 3V 32-QFN
標(biāo)準(zhǔn)包裝： 2,500
位數(shù)： 14
采樣率（每秒）： 65M
數(shù)據(jù)接口： 并聯(lián)
轉(zhuǎn)換器數(shù)目： 1
功率耗散（最大）： 240mW
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： 32-WFQFN 裸露焊盤
供應(yīng)商設(shè)備封裝： 32-QFN 裸露焊盤（5x5）
包裝： 帶卷 (TR)
輸入數(shù)目和類型： 1 個(gè)單端，雙極; 1 個(gè)差分，雙極
LTC2248/LTC2247/LTC2246
15
224876fa
Input Drive Impedance
As with all high performance, high speed ADCs, the
dynamic performance of the LTC2248/LTC2247/LTC2246
can be influenced by the input drive circuitry, particularly
the second and third harmonics. Source impedance and
reactance can influence SFDR. At the falling edge of CLK,
the sample-and-hold circuit will connect the 4pF sampling
capacitor to the input pin and start the sampling period.
The sampling period ends when CLK rises, holding the
sampled input on the sampling capacitor. Ideally the input
circuitry should be fast enough to fully charge
the sampling capacitor 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 LTC2248/LTC2247/LTC2246 being
driven by an RF transformer with a center tapped second-
ary. The secondary center tap is DC biased with VCM,
setting the ADC input signal at its optimum DC level.
Terminating on the transformer secondary is desirable, as
this provides a common mode path for charging glitches
caused by the sample and hold. Figure 3 shows a 1:1 turns
ratio transformer. Other turns ratios can be used if the
source impedance seen by the ADC does not exceed 100
for each ADC input. A disadvantage of using a transformer
is the loss of low frequency response. Most small RF
transformers have poor performance at frequencies be-
low 1MHz.
Figure 4 demonstrates the use of a differential amplifier to
convert a single ended input signal into a differential input
signal. The advantage of this method is that it provides low
frequency input response; however, the limited gain band-
width of most op amps will limit the SFDR at high input
frequencies.
APPLICATIO S I FOR ATIO
WU
UU
Figure 5. Single-Ended Drive
Figure 3. Single-Ended to Differential Conversion
Using a Transformer
25
25
25
25
0.1
F
AIN
+
AIN
–
12pF
2.2
F
VCM
LTC2248/47/46
ANALOG
INPUT
0.1
FT1
1:1
T1 = MA/COM ETC1-1T
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
224876 F03
Figure 4. Differential Drive with an Amplifier
25
25
12pF
2.2
F
VCM
LTC2248/47/46
224876 F04
–
+
CM
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
AIN
+
AIN
–
25
0.1
F
ANALOG
INPUT
VCM
AIN
+
AIN
–
1k
12pF
224876 F05
2.2
F
1k
25
0.1
F
LTC2248/47/46
Figure 5 shows a single-ended input circuit. The imped-
ance seen by the analog inputs should be matched. This
circuit is not recommended if low distortion is required.
The 25
resistorsand12pFcapacitorontheanaloginputs
serve two purposes: isolating the drive circuitry from the
sample-and-hold charging glitches and limiting the
wideband noise at the converter input.
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