LTC2256-14_15 Datasheet, PDF (20/34 Pages) Linear Technology – 14-Bit, 65/40/25Msps Ultralow Power 1.8V ADCs

LTC2258-14
LTC2257-14/LTC2256-14
Applications Information
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 LTC2258-14/2257-14/2256-14 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).
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
0.1µF
++
200Ω 200Ω
25Ω
– ^{– 0.1µF}
25Ω
V_CM
0.1µF
A_IN+
12pF
LTC2258-14
A_IN–
12pF
225814 F07
1.25V
LTC2258-14
V_REF
1µF
5Ω
1.25V BANDGAP
REFERENCE
0.625V
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
SENSE
0.1µF
REFH
RANGE
DETECT
AND
CONTROL
BUFFER
INTERNAL ADC
HIGH REFERENCE
2.2µF
0.1µF
0.1µF
REFL
0.8x
DIFF AMP
INTERNAL ADC
LOW REFERENCE
Figure 8. Reference Circuit
225814 F08
1.25V
EXTERNAL
REFERENCE
V_REF
1µF
SENSE
LTC2258-14
1µF
225814 F09
Figure 9. Using an External 1.25V Reference
Figure 7. Front-End Circuit Using a High Speed
Differential Amplifier
20
For more information www.linear.com/LTC2258-14
225814fc