Datasheet

LTC2258-12
LTC2257-12/LTC2256-12
20
225812fd
For more information www.linear.com/LTC2258-12
applicaTions inForMaTion
25Ω
25Ω
200Ω
200Ω
0.1µF
A
IN
+
A
IN
–
12pF
12pF
0.1µF
V
CM
LTC2258-12
225812 F07
–
–
+
+
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
0.1µF
Figure 7. Front-End Circuit Using a High Speed
Differential Amplifier
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 (Figures 4
to 6) should convert the signal to differential before driv
-
ing the A/D.
Reference
The LTC2258-12/2257-12/2256-12 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. The 0.1µF capacitor between REFH
and REFL should be as close to the pins as
possible (
not
on the back side of the circuit board).
V
REF
REFH
SENSE
TIE TO V
DD
FOR 2V RANGE;
TIE TO GND FOR 1V RANGE;
RANGE = 1.6 • V
SENSE
FOR
0.65V < V
SENSE
< 1.300V
1.25V
REFL
0.1µF2.2µF
INTERNAL ADC
HIGH REFERENCE
BUFFER
5Ω
0.8x
DIFF AMP
INTERNAL ADC
LOW REFERENCE
1.25V BANDGAP
REFERENCE
0.625V
RANGE
DETECT
AND
CONTROL
1µF
0.1µF
0.1µF
225812 F08
LTC2258-12
Figure 8. Reference Circuit
SENSE
1.25V
EXTERNAL
REFERENCE
1µF
1µF
V
REF
225812 F09
LTC2258-12
Figure 9. Using an External 1.25V Reference