ltc1407a-1 Linear Technology Corporation, ltc1407a-1 Datasheet - Page 13

• Manufacturer Part Number: ltc1407a-1
• Description: Serial 12-bit, 3msps Simultaneous Sampling Adcs With Shutdown
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT1818/LT1819: 400MHz, 2500V/µs, 9mA, Single/Dual
Voltage Mode Operational Amplifier.
LT6200: 165MHz GBWP, –85dBc distortion at 1MHz,
unity gain stable, rail-to-rail in and out, 15mA/amplifier,
0.95nV/√Hz.
LT6203: 100MHz GBWP, –80dBc distortion at 1MHz,
unity gain stable, rail-to-rail in and out, 3mA/amplifier,
1.9nV/√Hz.
LT6600: Amplifier/Filter Differential In/Out with 10MHz
Cutoff.
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 LTC1407-1/LTC1407A-1 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 band-
width. 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
source resistor to limit the net 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 circuitry. High quality capaci-
tors and resistors should be used since these components
ANALOG
ANALOG
1.5V DC
1.5V DC
INPUT
INPUT
V
V
CM
CM
*TIGHT TOLERANCE REQUIRED TO AVOID
Figure 1. RC Input Filter
APERTURE SKEW DEGRADATION
U
51Ω*
51Ω*
U
10µF
47pF*
47pF*
11
1
2
3
4
5
CH0
CH0
V
GND
CH1
CH1
LTC1407A-1
REF
LTC1407-1/
+
W
+
–
+
–
to ground and a 51Ω
14071 F01
U
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 13pF of
input capacitance, will reduce the rated 50MHz input band-
width and increase acquisition time beyond 39ns.
INPUT RANGE
The analog inputs of the LTC1407-1/LTC1407A-1 may be
driven fully differentially with a single supply. Either input
may swing up to 3V, provided the differential swing is no
greater than 1.25V. In the valid input range, each input of
each channel is always up to ±1.25V away from the other
input of each channel. The –1.25V to 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
DC external reference. The DC common mode level is
supplied by the previous stage that is already bounded by
single supply voltage of the system. The common mode
range of the inputs extends from ground to the supply
voltage V
inputs or the CH1
output code will stay fixed at zero and all ones, and if this
difference goes below –1.25V, the ouput code will stay
fixed at one and all zeros.
Figure 2. AC Coupling of AC Signals with 1kHz Low Cut
V
IN
DD
56pF
51Ω
. If the difference between the CH0
C3
R3
LTC1407-1/LTC1407A-1
+
C1, C2: FILM TYPE
C3: COG TYPE
C4: CERAMIC BYPASS
and CH1
1.6k
1.6k
1µF
C2
R2
R1
C1
1µF
–
inputs exceeds 1.25V, the
10µF
C4
4.09V
+
1
2
3
LTC1407A-1
CHO
CHO
V
LTC1407-1/
REF
+
–
+
14071 F02
and CH0
13
14071fa
–