LTC1408 LINER [Linear Technology], LTC1408 Datasheet - Page 14

• Manufacturer Part Number: LTC1408
• Description: Channel, 14-Bit, 600ksps Simultaneous Sampling ADC with Shutdown
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1408
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 LTC1408 noise and distortion. The small-signal band-
width 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 CHO
to limit the net input bandwidth to 30MHz. The 47pF ca-
pacitor 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 distor-
tion. 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 13pF of
input capacitance, will reduce the rated 50MHz input band-
width and increase acquisition time beyond 39ns.
14
ANALOG
ANALOG
INPUT
INPUT
*TIGHT TOLERANCE REQUIRED TO AVOID
APERTURE SKEW DEGRADATION
Figure 1. RC Input Filter
51Ω*
51Ω*
U
+
to ground and a 51Ω source resistor
U
10µF
47pF*
47pF*
11
1
2
3
4
5
CH0
CH0
V
GND
CH1
CH1
REF
LTC1408
+
–
+
–
W
1408 F01
U
INPUT RANGE
The analog inputs of the LTC1408 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 LTC1408 has an on-chip, temperature compensated,
bandgap reference that is factory trimmed near 2.5V to
obtain a precise 2.5V input span. The reference amplifier
output V
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
CC
–
at any input pair exceeds 2.5V (unipolar) or 1.25V
REF
, provided the differential swing is no greater than
, (Pin 23) must be bypassed with a capacitor
DD
3V REF
. An external reference at 2.55V will see a
Figure 2. External Reference
CC
. If the difference between the CH
10µF
REF
23
22
V
GND
pin can be overdriven
REF
LTC1408
1408 F02
1408fa
+