LTC1402
U
W U U
APPLICATIONS INFORMATION
DRIVING THE ANALOG INPUT
second requirement 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 LTC1402 will depend on the
application. Generally, applications fall into two catego-
ries: 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 LTC1402. More detailed information is avail-
able in the Linear Technology Databooks and on the
LinearViewTM CD-ROM.
LT®1206: 60MHz Current Feedback Amplifier with Shut-
down Pin (Amplifier Draws 200µA While in Shutdown).
±5V to ±15V supplies. Distortion is –80dB to 1MHz
(2VP-P into 30Ω). Good for AC applications. Dual avail-
able with shutdown as LT1207. Output swings to within
2VBE of the supply rails.
The differential analog inputs of the LTC1402 are easy to
drive.Theinputsmaybedrivendifferentiallyorasasingle-
ended input (i.e., the AIN– input is grounded). The AIN+ and
AIN– inputsaresampledatthesameinstant.Anyunwanted
signalthatiscommontobothinputswillbereducedbythe
common mode rejection of the sample-and-hold circuit.
The inputs draw only one small current spike while charg-
ing the sample-and-hold capacitors at the end of conver-
sion. During conversion, the analog inputs draw only a
small leakage current. If the source impedance of the
driving circuit is low, then the LTC1402 inputs can be
driven directly. As source impedance increases, so will
acquisition time (see Figure 1). For minimum acquisition
time with high source impedance, a buffer amplifier must
be used. The only requirement is that the amplifier driving
the analog input(s) must settle after the small current
spikebeforethenextconversionstarts(settlingtimemust
be 50ns for full throughput rate).
1500
1400
1300
LT1223: 100MHz Video Current Feedback Amplifier. 6mA
supply current. ±5V to ±15V supplies. Low distortion at
frequencies above 400kHz. Low noise. Good for AC appli-
cations.
±5V
1200
1100
1000
900
800
700
5V
LT1227:140MHzVideoCurrentFeedbackAmplifier.10mA
supply current; has shutdown pin (draws 120µA while in
shutdown). ±5V to ±15V supplies. Lowest distortion
(–92dB)atfrequenciesabove400kHz. Lownoise. Bestfor
AC applications.
600
500
400
300
200
100
0
10
100
1k
10k
100k
SOURCE RESISTANCE (Ω)
LT1229/LT1230: Dual and Quad 100MHz Current Feed-
back Amplifiers. ±2V to ±15V supplies. Low noise. Good
AC specifications, 6mA supply current each amplifier.
1402 F01
Figure 1. Acquisition Time vs Source Resistance
in Bipolar and Unipolar Modes
LT1360: 50MHz Voltage Feedback Amplifier. 3.8mA sup-
ply current. ±5V to ±15V supplies. Good AC and DC
specifications. 70ns settling to 0.5LSB.
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 impedance (<100Ω) at the closed-loop bandwidth
frequency. For example, if an amplifier 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 100Ω. The
LT1363: 70MHz, 1000V/µs Op Amps. 6.3mA supply cur-
rent. Good AC and DC specifications. 60ns settling to
0.5LSB.
LT1364/LT1365: Dual and Quad 70MHz, 1000V/µs Op
Amps. 6.3mA supply current per amplifier. 60ns settling
to 0.5LSB.
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