Datasheet

LTC1403-1/LTC1403A-1
11
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APPLICATIONS INFORMATION
DRIVING THE ANALOG INPUT
The differential analog inputs of the LTC1403-1/LTC1403A-1
are easy to drive. The inputs may be driven differentially or
as a single-ended input (i.e., the A
IN
–
input is set to V
CM
).
Both differential analog inputs, A
IN
+
with A
IN
–
, are sampled
at the same instant. Any unwanted signal that is common
to both inputs of each input pair will be reduced by the
common mode rejection of the sample-and-hold circuit.
The inputs draw only one small current spike while charging
the sample-and-hold capacitors at the end of conversion.
During conversion, the analog inputs draw only a small
leakage current. If the source impedance of the driving
circuit is low, then the LTC1403-1/LTC1403A-1 inputs can
be driven directly. As source impedance increases, so will
acquisition time. For minimum acquisition time with high
source impedance, a buffer amplifier must be used. The
main requirement is that the amplifier driving the analog
input(s) must settle after the small current spike before
the next conversion starts (settling time must be 39ns
for full throughput rate). Also keep in mind while choos-
ing an input amplifier, the amount of noise and harmonic
distortion added by the amplifier.
CHOOSING AN INPUT AMPLIFIER
Choosing an input amplifier is easy if a few requirements are
taken into consideration. First, to limit the magnitude of the
voltage spike seen by the amplifier from charging the sampling
capacitor, choose an amplifier that has a low output imped-
ance (<100Ω) at the closed-loop bandwidth frequency. For
example, if an amplifier is used with a gain of 1 and has a
unity-gain bandwidth of 50MHz, then the output impedance
at 50MHz must be less than 100Ω. The second require-
ment is that the closed-loop bandwidth must be greater
than 40MHz to ensure adequate small-signal settling for
full throughput rate. If slower op amps are used, more
time for settling can be provided by increasing the time
between conversions. The best choice for an op amp to
drive the LTC1403-1/LTC1403A-1 will depend on the ap-
plication. Generally, applications fall into two categories:
AC applications where dynamic specifications are most
critical and time domain applications where DC accuracy
and settling time are most critical. The following list is
a summary of the op amps that are suitable for driving
the LTC1403-1/LTC1403A-1. (More detailed information
is available in the Linear Technology Databooks and our
website at www.linear.com.)
LTC
®
1566-1: Low Noise 2.3MHz Continuous Time Low-
Pass Filter.
LT1630: Dual 30MHz Rail-to-Rail Voltage FB Amplifier.
2.7V to ±15V supplies. Very high A
VOL
, 500µV offset and
520ns settling to 0.5LSB for a 4V swing. THD and noise
are –93dB to 40kHz and below 1LSB to 320kHz (A
V
= 1,
2V
P-P
into 1kΩ, V
S
= 5V), making the part excellent for
AC applications (to 1/3 Nyquist) where rail-to-rail perfor-
mance is desired. Quad version is available as LT1631.
LT1632: Dual 45MHz Rail-to-Rail Voltage FB Amplifier.
2.7V to ±15V supplies. Very high A
VOL
, 1.5mV offset and
400ns settling to 0.5LSB for a 4V swing. It is suitable for
applications with a single 5V supply. THD and noise are
–93dB to 40kHz and below 1LSB to 800kHz (A
V
= 1,
2V
P-P
into 1kΩ, V
S
= 5V), making the part excellent for
AC applications where rail-to-rail performance is desired.
Quad version is available as LT1633.
LT1813: Dual 100MHz 750V/µs 3mA Voltage Feedback
Amplifier. 5V to ±5V supplies. Distortion is –86dB to
100kHz and –77dB to 1MHz with ±5V supplies (2V
P-P
into 500Ω). Excellent part for fast AC applications with
±5V supplies.
LT1801: 80MHz GBWP, –75dBc at 500kHz, 2mA/Ampli-
fier, 8.5nV/√Hz.
LT1806/LT1807: 325MHz GBWP, –80dBc Distortion at
5MHz, Unity-Gain Stable, R-R In and Out, 10mA/Ampli-
fier, 3.5nV/√Hz.
LT1810: 180MHz GBWP, –90dBc Distortion at 5MHz,
Unity-Gain Stable, R-R In and Out, 15mA/Amplifier,
16nV/√Hz.
LT1818/LT1819: 400MHz, 2500V/µs,9mA, Single/Dual
Voltage Mode Operational Amplifier.
LT6200: 165MHz GBWP, –85dBc Distortion at 1MHz,
Unity-Gain Stable, R-R In and Out, 15mA/Amplifier,
0.95nV/√Hz.
LT6203: 100MHz GBWP, –80dBc Distortion at 1MHz,
Unity-Gain Stable, R-R In and Out, 3mA/Amplifier,
1.9nV/√Hz.
LT6600-10: Amplifier/Filter Differential In/Out with 10MHz
Cutoff.