LTC2351-14 Linear Technology, LTC2351-14 Datasheet - Page 14

• Manufacturer Part Number: LTC2351-14
• Description: 1.5Msps Simultaneous Sampling ADC
• Manufacturer: Linear Technology

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LTC2351-14
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 LTC2351-14 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.
A simple 1-pole RC filter is sufficient for many applica-
tions. For example, Figure 1 shows a 47pF capacitor from
CHO
input bandwidth to 30MHz. The 47pF capacitor also acts
as a charge reservoir for the input sample-and-hold and
isolates the ADC input from sampling-glitch sensitive cir-
cuitry. High quality capacitors and resistors should be
used since these components can add distortion. NPO and
silvermica type dielectric capacitors have excellent linear-
ity. Carbon surface mount resistors can generate distor-
tion 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 am-
plitude unwanted signals are close in frequency to the
desired signal frequency a multiple pole filter is required.
High external source resistance, combined with 13pF of
input capacitance, will reduce the rated 50MHz input band-
width and increase acquisition time beyond 39ns.
14
+
to ground and a 51Ω source resistor to limit the net
ANALOG
ANALOG
INPUT
INPUT
*TIGHT TOLERANCE REQUIRED TO AVOID
APERTURE SKEW DEGRADATION
Figure 1. RC Input Filter
51Ω*
51Ω*
U
U
10µF
47pF*
47pF*
11
1
2
3
4
5
CH0
CH0
V
GND
CH1
CH1
REF
LTC2351-14
+
–
+
–
W
235114 F01
U
INPUT RANGE
The analog inputs of the LTC2351-14 may be driven fully
differentially with a single supply. Either input may swing
up to V
2.5V with BIP (Pin 29) Low, or ±1.25V with (BIP Pin 29)
High. 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
and CH
(bipolar), the output code will stay fixed at positive full-
scale, and if this difference goes below 0V (unipolar) or
–1.25V (bipolar), the output code will stay fixed at negative
full-scale.
INTERNAL REFERENCE
The LTC2351-14 has an on-chip, temperature compen-
sated, bandgap reference that is factory trimmed to 2.5V
to obtain a precise 2.5V input span. The reference ampli-
fier output V
tor to ground. The reference amplifier is stable with capaci-
tors 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
with an external reference as shown in Figure 2. The volt-
age of the external reference must be higher than the 2.5V
of the open-drain P-channel output of the internal refer-
ence. The recommended range for an external reference
is 2.55V to V
DC quiescent load of 0.75mA and as much as 3mA
during conversion.
CC
–
at any input pair exceeds 2.5V (unipolar) or 1.25V
, provided the differential swing is no greater than
3.5V to 18V
REF
DD
, (Pin 23) must be bypassed with a capaci-
. An external reference at 2.55V will see a
Figure 2. External Reference
LT1790-3
CC
. If the difference between the CH
10µF
REF
23
22
pin can be overdriven
V
GND
REF
LTC2351-14
235114 F02
235114f
+