LTC2355-12_09 LINER [Linear Technology], LTC2355-12_09 Datasheet - Page 11

• Manufacturer Part Number: LTC2355-12_09
• Description: Serial 12-Bit/14-Bit, 3.5Msps Sampling ADCs with Shutdown
• Manufacturer: LINER [Linear Technology]

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 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 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.
51
Figure 1. RC Input Filter
47pF
10µF
11
1
2
3
A
A
V
GND
IN
IN
REF
LTC2355-12/
LTC2355-14
+
–
2355 F01
IN
+
to ground and a
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 sup-
ply voltage V
A
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, temperature
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
with a capacitor to ground. The reference amplifier 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
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 recom-
mended range for an external reference is 2.55V to V
An external reference at 2.55V will see a DC quiescent load
of 0.75mA and as much as 3mA during conversion.
IN
Figure 2. Overdriving V
–
inputs exceeds 2.5V, the output code will stay fixed
3.5V TO 18V
LTC2355-12/LTC2355-14
DD
. If the difference between the A
LT1790-3
REF
P-P
10µF
Pin with an External Reference
individually. When using the
REF
3V
, (Pin 3) must be bypassed
11
3
V
GND
LTC2355-12/
LTC2355-14
REF
2355 F02
REF
pin can be
IN

+
2355fa
and
DD
.