LTC1407/LTC1407A
11
1407fb
APPLICATIONS INFORMATION
DRIVING THE ANALOG INPUT
The differential analog inputs of the LTC1407/LTC1407A are
easy to drive. The inputs may be driven differentially or as
a single-ended input (i.e., the CH0
–
input is grounded). All
four analog inputs of both differential analog input pairs,
CH0
+
with CH0
–
and CH1
+
with CH1
–
, are sampled at the
same instant. Any unwanted signal that is common to
both inputs of each input pair will be reduced by the com-
mon 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 LTC1407/LTC1407A inputs can be
driven directly. As source impedance increases, so will
acquisition time. For minimum acquisition time with high
source impedance, a buffer amplifi er must be used. The
main requirement is that the amplifi er 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 choosing
an input amplifi er, the amount of noise and harmonic
distortion added by the amplifi er.
CHOOSING AN INPUT AMPLIFIER
Choosing an input amplifi er is easy if a few requirements
are taken into consideration. First, to limit the magnitude
of the voltage spike seen by the amplifi er from charging
the sampling capacitor, choose an amplifi er that has a low
output impedance (< 100) at the closed-loop bandwidth
frequency. For example, if an amplifi er is used in a gain
of 1 and has a unity-gain bandwidth of 50MHz, then the
output impedance at 50MHz must be less than 100.
The second requirement is that the closed-loop band-
width 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 LTC1407/LTC1407A depends
on the application. Generally, applications fall into two
categories: AC applications where dynamic specifi cations
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 driv-
ing the LTC1407/LTC1407A. (More detailed information
is available in the Linear Technology Databooks and on
the LinearView™ CD-ROM.)
LTC1566-1: Low Noise 2.3MHz Continuous Time Low-
pass Filter.
LT
®
1630: Dual 30MHz Rail-to-Rail Voltage FB Amplifi er.
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 1k, 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 Amplifi er.
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 1k, V
S
= 5V), making the part excellent for
AC applications where rail-to-rail performance is desired.
Quad version is available as LT1633.
LT1801: 80MHz GBWP, –75dBc at 500kHz, 2mA/ampli-
fi er, 8.5nV/√Hz.
LT1806/LT1807: 325MHz GBWP, –80dBc distortion at
5MHz, unity-gain stable, rail-to-rail in and out, 10mA/am-
plifi er, 3.5nV/√
Hz
.
LT1810: 180MHz GBWP, –90dBc distortion at 5MHz,
unity-gain stable, rail-to-rail in and out, 15mA/amplifi er,
16nV/√Hz.
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