Datasheet
LTC2157-12/
LTC2156-12/LTC2155-12
18
21576512f
APPLICATIONS INFORMATION
Figure 5. Recommended Front-End Circuit for
Input Frequencies from 150MHz Up to 900MHz
Figure 6. Front-End Circuit Using a High
Speed Differential Amplifier
Amplifier Circuits
Figure 6 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 (Figures3 and
5) should convert the signal to differential before driving
the A/D. The A/D cannot be driven single ended.
Reference
The LTC2157-12/LTC2156-12/LTC2155-12 has an internal
1.25V voltage reference. For a 1.5V input range with in-
ternal reference, connect SENSE to V
DD
. For a 1.5V input
range with an external reference, apply a 1.25V reference
voltage to SENSE (Figure 7).
Encode Input
The signal quality of the encode inputs strongly affects
the A/D noise performance. The encode inputs should
be treated as analog signals—do not route them next to
digital traces on the circuit board.
The encode inputs are internally biased to 1.2V through
10k equivalent resistance (Figure 8). If the common mode
of the driver is within 1.1V to 1.5V, it is possible to drive
Figure 7. Reference Circuit
Figure 8. Equivalent Encode Input Circuit
45Ω
100Ω
4.7Ω
4.7Ω
45Ω
10Ω
0.1µF
0.1µF
IN
0.1µF
0.1µF
T1: MABA
007159-000000
21576514 F05
LTC2157-12
A
IN
+
A
IN
–
V
CM
4.7Ω
4.7Ω
50Ω
50Ω
0.1µF
A
IN
+
A
IN
–
0.1µF
3pF
3pF
3pF
V
CM
LTC2157-12
21576512 F06
INPUT
0.1µF
SCALER/
BUFFER
V
REF
2.2µF
SENSE
1.25V
21576512 F07
5Ω
ADC
REFERENCE
SENSE
DETECTOR
V
DD
LTC2157-12
21576512 F08
1.2V
10k
ENC
+
ENC
–
the encode inputs directly. Otherwise a transformer or
coupling capacitors are needed (Figures 9 and 10). The
maximum (peak) voltage of the input signal should never
exceed V
DD
+0.1V or go below –0.1V.