
21
21421014fa
LTC2142-14/
LTC2141-14/LTC2140-14
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 LTC2142-14/LTC2141-14/LTC2140-14 has an internal
1.25V voltage reference. For a 2V input range using the
internal reference, connect SENSE to VDD. 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 VSENSE.
The VREF, 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
25Ω
50Ω
0.1μF
AIN
+
AIN
–
8.2pF
0.1μF
VCM
ANALOG
INPUT
0.1μF
T1
T2
T1: MA/COM MABA-007159-000000
T2: COILCRAFT WBC1-1TL
RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE
21421014 F04
LTC2142
25Ω
50Ω
0.1μF
AIN
+
AIN
–
1.8pF
0.1μF
VCM
ANALOG
INPUT
0.1μF
T1
T2
T1: MA/COM MABA-007159-000000
T2: COILCRAFT WBC1-1TL
RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE
21421014 F05
LTC2142
25Ω
50Ω
0.1μF
4.7nH
AIN
+
AIN
–
0.1μF
VCM
T1: MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
21421014 F06
LTC2142
ANALOG
INPUT
0.1μF
T1
25Ω
200Ω
0.1μF
AIN
+
AIN
–
0.1μF
12pF
VCM
LTC2142
21421014 F07
–
+
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
0.1μF
Figure 7. Front-End Circuit Using a High Speed
Differential Amplifier