Datasheet

LTC1407-1/LTC1407A-1
14
14071fb
APPLICATIONS INFORMATION
LT1818/LT1819: 400MHz, 2500V/µs, 9mA, Single/Dual
Voltage Mode Operational Amplifi er.
LT6200: 165MHz GBWP, –85dBc distortion at 1MHz,
unity-gain stable, rail-to-rail in and out, 15mA/amplifi er,
0.95nV/√Hz.
LT6203: 100MHz GBWP, –80dBc distortion at 1MHz,
unity-gain stable, rail-to-rail in and out, 3mA/amplifi er,
1.9nV/√Hz.
LT6600: Amplifi er/Filter Differential In/Out with 10MHz
Cutoff.
INPUT FILTERING AND SOURCE IMPEDANCE
The noise and the distortion of the input amplifi er 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 pres-
ent at the analog inputs will be summed over this entire
bandwidth. Noisy input circuitry should be fi ltered prior
to the analog inputs to minimize noise. A simple 1-pole
RC fi lter is suffi cient for many applications. For example,
Figure 1 shows a 47pF capacitor from CHO
+
to ground
and a 51 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 capacitors and resistors should be used since these
Figure 1. RC Input Filter
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
fi lm 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 fi lter is required.
High external source resistance, combined with 13pF
of input capacitance, will reduce the rated 50MHz input
bandwidth 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
DD
. If the difference between the CH0
+
and CH0
–
inputs or the CH1
+
and CH1
–
inputs exceeds
1.25V, the output code will stay fi xed at zero and all ones,
and if this difference goes below –1.25V, the output code
will stay fi xed at one and all zeros.
Figure 2. AC Coupling of AC Signals with 1kHz Low Cut
LTC1407-1/
LTC1407A-1
CH0
+
CH0
–
V
REF
GND
14071 F01
1
2
11
3
10
µF
47pF*
51*
CH1
+
CH1
–
4
5
47pF*
*TIGHT TOLERANCE REQUIRED TO AVOID
APERTURE SKEW DEGRADATION
51Ω*
ANALOG
INPUT
ANALOG
INPUT
V
CM
1.5V DC
V
CM
1.5V DC
+
CHO
+
4.09V
C4
10
µF
14071 F02
R2
1.6k
C2
1
µF
C1
1
µF
C1, C2: FILM TYPE
C3: COG TYPE
C4: CERAMIC BYPASS
R1
1.6k
1
2
3
LTC1407-1/
LTC1407A-1
CHO
–
V
REF
R3
51Ω
C3
56pF
V
IN