Datasheet
LTC2188
20
2188f
applicaTions inForMaTion
Figure 5. Recommended Front-End Circuit for Input
Frequencies from 150MHz to 250MHz
Figure 6. Recommended Front-End Circuit for Input
Frequencies Above 250MHz
Amplifier Circuits
Figure 7 shows the analog input being driven by a high
speed differential amplifier. The output of the amplifier is
AC-coupled to the A/D so the amplifier’s output common
mode voltage can be optimally set to minimize distortion.
At very high frequencies an RF gain block will often have
lower distortion than a differential amplifier. If the gain
block is single-ended, then a transformer circuit (Figure 4
to Figure 6) should convert the signal to differential before
driving the A/D.
Figure 4. Recommended Front-End Circuit for Input
Frequencies from 5MHz to 150MHz
Reference
The LTC2188 has an internal 1.25V voltage reference. For
a 2V input range using the internal reference, connect
SENSE to V
DD
. For a 1V input range using the internal
reference, connect SENSE to ground. For a 2V input range
with an external reference, apply a 1.25V reference voltage
to SENSE (Figure 9).
The input range can be adjusted by applying a voltage to
SENSE that is between 0.625V and 1.30V. The input range
will then be 1.6 • V
SENSE
.
The V
REF
, REFH and REFL pins should be bypassed as
shown in Figure 8. A low inductance 2.2µF interdigitated
capacitor is recommended for the bypass between REFH
and REFL. This type of capacitor is available at a low cost
from multiple suppliers.
25Ω
12Ω
12Ω
25Ω
50Ω
0.1µF
A
IN
+
A
IN
–
8.2pF
0.1µF
V
CM
ANALOG
INPUT
0.1µF
0.1µF
T1
T2
T1: MA/COM MABA-007159-000000
T2: COILCRAFT WBC1-1TL
RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE
2188 F04
LTC2188
25Ω
25Ω
50Ω
0.1µF
A
IN
+
A
IN
–
1.8pF
0.1µF
V
CM
ANALOG
INPUT
0.1µF
0.1µF
T1
T2
T1: MA/COM MABA-007159-000000
T2: COILCRAFT WBC1-1TL
RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE
2188 F05
LTC2188
25Ω
25Ω
50Ω
0.1µF
4.7nH
4.7nH
A
IN
+
A
IN
–
0.1µF
V
CM
T1: MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
2188 F06
LTC2188
ANALOG
INPUT
0.1µF
0.1µF
T1
25Ω
25Ω
200Ω
200Ω
0.1µF
A
IN
+
A
IN
–
0.1µF
12pF
12pF
V
CM
LTC2188
2188 F07
–
–
+
+
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
0.1µF
Figure 7. Front-End Circuit Using a High Speed
Differential Amplifier