LTC2355-14 Linear Technology, LTC2355-14 Datasheet - Page 10

• Manufacturer Part Number: LTC2355-14
• Description: (LTC2355-12/-14) 3.5Msps Sampling ADCs
• Manufacturer: Linear Technology

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LTC2355-12/LTC2355-14
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 LTC2355-12/LTC2355-14 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 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 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.
10
51Ω
Figure 1. RC Input Filter
U
10µF
47pF
U
11
1
2
3
A
A
V
GND
IN
IN
REF
LTC2355-12/
LTC2355-14
+
–
W
2355 F01
IN
+
to ground and
U
INPUT RANGE
The analog inputs of the LTC2355-12/LTC2355-14 may be
driven fully differentially with a single supply. Each input
may swing up to 2.5V
internal reference, the noninverting input should never be
more than 2.5V more positive than the inverting input. 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
inputs exceeds 2.5V, the output 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 LTC2355-12/LTC2355-14 has an on-chip, tempera-
ture compensated, bandgap reference that is factory
trimmed to 2.5V to obtain a unipolar 0V to 2.5V input span.
The reference amplifier output V
bypassed with a capacitor to ground. The reference ampli-
fier is stable with capacitors of 1µF or greater. For the best
noise performance, a 10µF ceramic or a 10µF tantalum in
parallel with a 0.1µF ceramic is recommended. The V
pin can be overdriven with an external reference as shown
in Figure 2. The voltage of the external reference must be
higher than the 2.5V 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
Figure 2. Overdriving V
. An external reference at 2.55V will see a DC quiescent
3.5V TO 18V
DD
. If the difference between the A
LT1790-3
P-P
REF
10µF
Pin with an External Reference
individually. When using the
3V
REF
11
3
V
GND
, (Pin 3) must be
LTC2355-12/
LTC2355-14
REF
2355 F02
IN
+
and A
2355f
REF
IN
–