Datasheet

LTC2285
14
2285fb
APPLICATIONS INFORMATION
Figure 4 demonstrates the use of a differential amplifi er 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 25 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 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.
Figure 4. Differential Drive with an Amplifi er
Figure 5. Single-Ended Drive
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 70MHz and 170MHz
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
25Ω
25Ω
12pF
2.2µF
V
CM
2285 F04
–
–
+
+
CM
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
A
IN
+
A
IN
–
LTC2285
25Ω
0.1µF
ANALOG
INPUT
V
CM
A
IN
+
A
IN
–
1k
12pF
2285 F05
2.2µF
1k
25Ω
0.1µF
LTC2285
25Ω
25Ω
12Ω
12Ω
0.1µF
A
IN
+
A
IN
–
8pF
2.2µF
V
CM
ANALOG
INPUT
0.1µF
0.1µF
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
2285 F06
LTC2285
25Ω
25Ω
0.1µF
A
IN
+
A
IN
–
2.2µF
V
CM
ANALOG
INPUT
0.1µF
0.1µF
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
2285 F07
LTC2285
25Ω
25Ω
0.1µF
A
IN
+
A
IN
–
2.2µF
V
CM
ANALOG
INPUT
0.1µF
0.1µF
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
2285 F08
8.2nH
8.2nH
LTC2285