LT1460-2.5 LINER [Linear Technology], LT1460-2.5 Datasheet - Page 12

• Manufacturer Part Number: LT1460-2.5
• Description: Serial 12-Bit/14-Bit, 2.8Msps
• Manufacturer: LINER [Linear Technology]

LTC1403-1/LTC1403A-1
APPLICATIONS INFORMATION
INPUT FILTERING AND SOURCE IMPEDANCE
The noise and the distortion of the input amplifier and
other circuitry must be considered since they will add
to the LTC1403-1/LTC1403A-1 noise and distortion. The
small-signal bandwidth of the sample-and-hold circuit is
50MHz. Any noise or distortion products that are pres-
ent at the analog inputs will be summed over this entire
bandwidth. Noisy input circuitry should be filtered prior
to the analog inputs to minimize noise. A simple 1-pole
RC filter is sufficient for many applications. For example,
Figure 1 shows a 47pF capacitor from A
51Ω source resistor to limit the input bandwidth to 47MHz.
The 47pF capacitor also acts as a charge reservoir for the
input sample-and-hold and isolates the ADC input from
sampling-glitch sensitive circuitry. High quality capacitors
and resistors should be used since these components
can add distortion. NPO and silvermica type dielectric
capacitors have excellent linearity. Carbon surface mount
resistors can generate distortion from self heating and
from damage that may occur during soldering. Metal film
surface mount resistors are much less susceptible to both
problems. When high amplitude unwanted signals are close
in frequency to the desired signal frequency, a multiple
pole filter is required. High external source resistance,
combined with the 13pF of input capacitance, will reduce
the rated 50MHz bandwidth and increase acquisition time
beyond 39ns.
INPUT RANGE
The analog inputs of the LTC1403-1/LTC1403A-1 may be
driven fully differentially with a single supply. Each input
may swing up to 3V
range, each input is always up to 1.25V more positive or
more negative than the inverting input of each channel. The
12
1.5V DC
V
51
CM
Figure 1. RC Input Filter
47pF
10µF
P-P
individually. In the conversion
11
1
2
3
A
A
V
GND
IN
IN
REF
LTC1403A-1
LTC1403-1/
+
–
IN
1403A F01
+
to ground and a
±1.25V range is also ideally suited for AC-coupled signals
in single supply applications. Figure 2 shows how to AC
couple signals in a single supply system without needing
a mid-supply 1.5V external reference. The DC common
mode level is supplied by the previous stage that is already
bounded by the single supply voltage of the system. The
common mode range of the inputs extend from ground
to the supply voltage V
A
stay fixed at zero and all ones and if this difference goes
below –1.25V, the output code will stay fixed at one and
all zeros.
INTERNAL REFERENCE
The LTC1403-1/LTC1403A-1 has an on-chip, temperature
compensated, bandgap reference that is factory trimmed
near 2.5V to obtain ±1.25V input span. The reference
amplifier output V
a capacitor to ground. The reference amplifier is stable
with capacitors of 1µF or greater. For the best noise per-
formance, a 10µF ceramic or a 10µF tantalum in parallel
with a 0.1µF ceramic is recommended. The V
be overdriven with an external reference as shown in
Figure 3. The voltage of the external reference must be
higher than the 2.5V of the class A pull-up output of the
IN
+
Figure 2. AC Coupling of AC Signals with 1kHz Low Cut
and A
V
IN
IN
56pF
51
C3
R3
–
inputs exceeds 1.25V, the output code will
3V
REF
C1, C2: FILM TYPE
C3: COG TYPE
C4: CERAMIC BYPASS
REF
1.6k
1.6k
1µF
C2
R2
R1
, (Pin 3) must be bypassed with
10µF
DD
Figure 3
. If the difference between the
C1
1µF
11
10µF
3
C4
V
GND
LTC1403A-1
LTC1403-1/
REF
+
14031 F03
1
2
3
A
A
V
LTC1403A-1
LTC1403-1/
IN
IN
REF
+
–
REF
14031 F02
pin can
14031fc