Datasheet

LTC2228/LTC2227/LTC2226
18
222876fb
APPLICATIONS INFORMATION
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.
25Ω
25Ω
12pF
2.2μF
V
CM
LTC2228/27/26
222876 F04
–
–
+
+
CM
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
A
IN
+
A
IN
–
Figure 4. Differential Drive with an Amplifi er
25Ω
0.1μF
ANALOG
INPUT
V
CM
A
IN
+
A
IN
–
1k
12pF
222876 F05
2.2μF
1k
25Ω
0.1μF
LTC2228/27/26
Figure 5. Single-Ended Drive
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 fl ux 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.
25Ω
25Ω
12Ω
12Ω
0.1μF
A
IN
+
A
IN
–
8pF
2.2μF
V
CM
LTC2228/27/26
ANALOG
INPUT
0.1μF
0.1μF
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
222876 F06
Figure 6. Recommended Front-End Circuit for
Input Frequencies Between 70MHz and 170MHz
25Ω
25Ω
0.1μF
A
IN
+
A
IN
–
2.2μF
V
CM
LTC2228/27/26
ANALOG
INPUT
0.1μF
0.1μF
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
222876 F07
Figure 7. Recommended Front-End Circuit for
Input Frequencies Between 170MHz and 300MHz
25Ω
25Ω
0.1μF
A
IN
+
A
IN
–
2.2μF
V
CM
LTC2228/27/26
ANALOG
INPUT
0.1μF
0.1μF
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
222876 F08
6.8nH
6.8nH
Figure 8. Recommended Front-End Circuit for
Input Frequencies Above 300MHz