LTC2284 Datasheet, PDF (14/24 Pages) Linear Technology – Dual 14-Bit, 105Msps Low Power 3V ADC

LTC2284
APPLICATIO S I FOR ATIO
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER 25Ω
++
CM
––
25Ω
V_CM
2.2µF
A_IN+
LTC2284
12pF
A_IN–
2284 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.
ANALOG
INPUT
0.1µF
1k 1k
25Ω
V_CM
2.2µF
A_{IN+ LTC2284}
25Ω
0.1µF
12pF
A_IN–
2284 F05
Figure 5. Single-Ended Drive
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 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.
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
V_CM
2.2µF
A_IN+
LTC2284
8pF
A_IN–
2284 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
V_CM
2.2µF
A_IN+
LTC2284
A_IN–
2284 F07
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
V_CM
0.1µF
ANALOG
INPUT
T1
6.8nH
25Ω 0.1µF
2.2µF
A_IN+
0.1µF
25Ω _6.8nH
T1 = MA/COM, ETC 1-1-13
A_IN–
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
LTC2284
2284 F08
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
2284fa
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