LTC1403-1 LINER [Linear Technology], LTC1403-1 Datasheet - Page 11

• Manufacturer Part Number: LTC1403-1
• Description: Serial 12-Bit/14-Bit, 2.8Msps Sampling ADCs with Shutdown
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
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 present
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
a 51
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 ca-
pacitors and resistors should be used since these compo-
nents 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 resis-
tance, combined with the 13pF of input capacitance, will
reduce the rated 50MHz bandwidth and increase acquisi-
tion 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.
source resistor to limit the input bandwidth to
1.5V DC
V
51
CM
Figure 1. RC Input Filter
U
47pF
10 F
P-P
individually. In the conversion
U
11
1
2
3
A
A
V
GND
IN
IN
REF
LTC1403A-1
LTC1403-1/
+
–
W
1403A F01
IN
+
to ground and
U
The 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
between the A
output code will 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
capacitor to ground. The reference amplifier is stable with
capacitors of 1 F or greater. For the best noise perfor-
mance, a 10 F ceramic or a 10 F tantalum in parallel with
a 0.1 F ceramic is recommended. The V
overdriven with an external reference as shown in Fig-
ure 3. The voltage of the external reference must be higher
than the 2.5V of the class A pull-up output of the internal
Figure 2. AC Coupling of AC Signals with 1kHz Low Cut
V
IN
56pF
51
C3
R3
LTC1403-1/LTC1403A-1
IN
3V
+
REF
C1, C2: FILM TYPE
C3: COG TYPE
C4: CERAMIC BYPASS
REF
and A
1.6k
1.6k
1 F
C2
R2
R1
, (Pin 3) must be bypassed with a
10 F
Figure 3
IN
–
C1
1 F
inputs exceeds 1.25V, the
11
10 F
3
C4
V
GND
LTC1403A-1
LTC1403-1/
+
REF
DD
1
2
3
14031 F03
. If the difference
LTC1403A-1
A
A
V
LTC1403-1/
IN
IN
REF
REF
+
–
14031 F02
pin can be
11
14031f