LTC2281 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2281' PDF : 24 Pages View PDF
LTC2281
APPLICATIONS INFORMATION
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER 25Ω
++
CM
––
25Ω
VCM
2.2μF
AIN+
LTC2281
12pF
AIN–
2281 F04
Figure 4. Differential Drive with an Amplifier
ANALOG
INPUT
0.1μF
1k 1k
25Ω
VCM
2.2μF
AIN+ LTC2281
25Ω
0.1μF
12pF
AIN–
2281 F05
Figure 5. Single-Ended Drive
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
bandwidth of most op amps will limit the SFDR at high
input frequencies.
Figure 5 shows a single-ended input circuit. The impedance
seen by the analog inputs should be matched. This circuit
is not recommended if low distortion is required.
The 25Ω resistors and 12pF capacitor on the analog
inputs serve two purposes: isolating the drive circuitry
from the sample-and-hold charging glitches and limiting
the wideband noise at the converter input.
For input frequencies above 70MHz, the input circuits of
Figure 6, 7 and 8 are recommended. The balun transformer
gives better high frequency response than a flux coupled
center tapped transformer. The coupling capacitors allow
the analog inputs to be DC biased at 1.5V. In Figure 8, the
series inductors are impedance matching elements that
maximize the ADC bandwidth.
14
0.1μF
ANALOG
INPUT
T1
0.1μF
12Ω
25Ω 0.1μF
25Ω
12Ω
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
VCM
2.2μF
AIN+
LTC2281
8pF
AIN–
2281 F06
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 70MHz and 170MHz
0.1μF
ANALOG
INPUT
T1
0.1μF
25Ω 0.1μF
25Ω
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
VCM
2.2μF
AIN+
LTC2281
AIN–
2281 F07
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
VCM
ANALOG
INPUT
0.1μF
8.2nH
2.2μF
AIN+
25Ω 0.1μF
T1
0.1μF
25Ω 8.2nH
AIN–
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
LTC2281
2281 F08
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
2281fb
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