LTC2236 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2236' PDF : 24 Pages View PDF
APPLICATIO S I FOR ATIO
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER 25Ω
++
CM
––
25Ω
VCM
2.2µF
AIN+
12pF
AIN–
LTC2238
LTC2237
LTC2236
223876 F04
Figure 4. Differential Drive with an Amplifier
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 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.
ANALOG
INPUT
0.1µF
1k 1k
25Ω
VCM
2.2µF
AIN+
12pF
LTC2238
LTC2237
LTC2236
25Ω
0.1µF
AIN–
223876 F05
Figure 5. Single-Ended Drive
For input frequencies above 70MHz, the input circuits of
Figure 6, 7 and 8 are recommended. The balun trans-
former gives better high frequency response than a flux
coupled center tapped transformer. The coupling capaci-
tors 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.
LTC2238/LTC2237/LTC2236
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+
8pF
AIN–
LTC2238
LTC2237
LTC2236
223876 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+
LTC2238
LTC2237
LTC2236
AIN–
223876 F07
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
VCM
0.1µF
ANALOG
INPUT
T1
6.8nH
25Ω 0.1µF
2.2µF
AIN+
0.1µF
25Ω 6.8nH
AIN–
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
LTC2238
LTC2237
LTC2236
223876 F08
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
223876fa
17
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