Datasheet

LTC2158-14
14
215814f
applicaTions inForMaTion
Figure 5. Recommended Front-End Circuit for
Input Frequencies from 150MHz 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 (Figures
3 and 5) should convert the signal to differential before
driving the A/D. The A/D cannot be driven single-ended.
Reference
The LTC2158-14 has an internal 1.25V voltage reference.
For a 1.32V input range with internal reference, connect
SENSE to V
DD
. For a 1.32V 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
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.
4.7Ω
4.7Ω
50Ω
50Ω
0.1µF
A
IN
+
A
IN
–
0.1µF
3pF
3pF
3pF
V
CM
LTC2158-14
215814 F06
INPUT
0.1µF
45Ω
45Ω
10Ω
100Ω
4.7Ω
4.7Ω
0.1µF
0.1µF
IN
0.1µF
0.1µF
T1: MABA
007159-000000
215814 F05
LTC2158-14
A
IN
+
A
IN
–
V
CM
Figure 7. Reference Circuit
Figure 8. Equivalent Encode Input Circuit
V
DD
LTC2158-14
215814 F08
1.2V
10k
ENC
+
ENC
–
SCALER/
BUFFER
V
REF
2.2µF
SENSE
1.25V
LTC2158-14
215814 F07
5Ω
ADC
REFERENCE
SENSE
DETECTOR