Datasheet

LTC2356-12/LTC2356-14
12
2356fb
GND
LTC2356-12/
LTC2356-14
LT1790-3
V
REF
10µF
3.5V TO 18V
11
3
3V
2356 F03
+
A
IN
+
C4
10µF
2356 F02
R2
1.6k
C2
1µF
C1
1µF
C1, C2: FILM TYPE
C3: COG TYPE
C4: CERAMIC BYPASS
R1
1.6k
1
2
3
LTC2356-12/
LTC2356-14
A
IN
–
V
REF
R3
51Ω
C3
56pF
V
IN
10µF
V
CM
1.5V DC
11
3
A
IN
–
LTC2356-12/
LTC2356-14
A
IN
+
47pF
2
1
51Ω
GND
V
REF
2356 F01
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 LTC2356-12/LTC2356-14 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
IN
+
to ground and 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.
Figure 1. RC Input Filter
inverting input. 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
DD
. If the difference
between the A
IN
+
and A
IN
–
inputs exceeds 1.25V, the
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.
Figure 2. AC Coupling of AC Signals with 1kHz
Low Cutoff Frequency
INPUT RANGE
The analog inputs of the LTC2356-12/LTC2356-14 may be
driven fully differentially with a single supply. Each input
may swing up to 2.5V
P-P
individually. When using the
internal reference, the non-inverting input should never be
more than 1.25V more positive or more negative than the
INTERNAL REFERENCE
The LTC2356-12/LTC2356-14 has an on-chip, temperature
compensated, bandgap reference that is factory trimmed
to 2.5V to obtain a bipolar ±1.25V input span. The refer-
ence amplifier output V
REF
, (Pin 3) must be bypassed
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 paral-
lel with a 0.1µF ceramic is recommended. The V
REF
pin
can be overdriven with an external reference as shown in
Figure 3. Overdriving V
REF
Pin with an External Reference