LTC1403 Linear Technology, LTC1403 Datasheet - Page 10

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

APPLICATIO S I FOR ATIO
LTC1403/LTC1403A
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/LTC1403A 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
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 sam-
pling-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 dam-
age that may occur during soldering. Metal film surface
mount resistors are much less susceptible to both prob-
lems. When high amplitude unwanted signals are close in
frequency to the desired signal frequency, a multiple pole
filter is required. High external source resistance, com-
bined with the 13pF of input capacitance, will reduce the
rated 50MHz bandwidth and increase acquisition time
beyond 39ns.
10
51
Figure 1. RC Input Filter
U
10 F
47pF
U
11
1
2
3
A
A
V
GND
IN
IN
IN
REF
LTC1403A
+
–
LTC1403/
+
to ground and a 51
W
1403A F01
U
INPUT RANGE
The analog inputs of the LTC1403/LTC1403A may be
driven fully differentially with a single supply. Each input
may swing up to 3V
range, the noninverting input of each channel is always up
to 2.5V more positive than the inverting input of each
channel. The 0V to 2.5V range is also ideally suited for
single-ended input use with single supply applications.
The common mode range of the inputs extend from
ground to the supply voltage V
tween the A
code will stay fixed at all ones and if this difference goes
below 0V, the ouput code will stay fixed at all zeros.
INTERNAL REFERENCE
The LTC1403/LTC1403A has an on-chip, temperature com-
pensated, bandgap reference that is factory trimmed near
2.5V to obtain 2.5V input span. The reference amplifier
output V
ground. The reference amplifier is stable with capacitors
of 1 F or greater. For the best noise performance, a 10 F
ceramic or a 10 F tantalum in parallel with a 0.1 F ceramic
is recommended. The V
external reference as shown in Figure 2. The voltage of the
external reference must be higher than the 2.5V of the
class A pull-up output of the internal reference. The
recommended range for an external reference is 2.55V to
V
load of 0.75mA and as much as 3mA during conversion.
DD
. An external reference at 2.55V will see a DC quiescent
REF
, (Pin 3) must be bypassed with a capacitor to
IN
+
and A
3V
REF
IN
P-P
10 F
–
REF
inputs exceeds 2.5V, the output
Figure 2
individually. In the conversion
pin can be overdriven with an
11
3
DD
V
GND
. If the difference be-
REF
LTC1403A
LTC1403/
1403A F02
1403af