LTC1416 Linear Technology, LTC1416 Datasheet - Page 10

• Manufacturer Part Number: LTC1416
• Description: Low Power 14-Bit/ 400ksps Sampling ADC
• Manufacturer: Linear Technology

APPLICATIONS
LTC1416
Driving the Analog Input
The differential analog inputs of the LTC1416 are easy to
drive. The inputs may be driven differentially or as a single-
ended input (i.e., the A
A
wanted signal that is common mode to both inputs 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 LTC1416
inputs can be driven directly. As source impedance
increases so will acquisition time (see Figure 6). For
minimum acquisition time, with high source impedance, a
buffer amplifier should be used. The only 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 400ns for full
throughput rate).
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 (<100 ) at the closed-loop bandwidth
10
IN
–
inputs are sampled at the same instant. Any un-
Figure 6. Acquisition Time vs Source Resistance
0.01
0.1
10
1
10
U
100
SOURCE RESISTANCE ( )
IN
–
INFORMATION
input is grounded). The A
U
1k
W
10k
1416 F06
100k
U
IN
+
and
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 should be less than 100 .
The second requirement is that the closed-loop bandwidth
must be greater than 10MHz to ensure adequate small-
signal settling for full throughput rate. If slower op amps
are used, more settling time can be provided by increasing
the time between conversions.
The best choice for an op amp to drive LTC1416 will
depend on the application. Generally, applications fall into
two categories: AC applications where dynamic specifica-
tions 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 LTC1416. More detailed information is
available in the Linear Technology Databooks and the
LinearView
LT
amplifier. 5V to 15V supplies, excellent DC specifica-
tions.
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.
LT1227: 140MHz video current feedback amplifier. 10mA
supply current, 5V to 15V supplies, lowest distortion at
frequencies above 400kHz, low noise, best for AC applica-
tions.
LT1229/LT1230: Dual and quad 100MHz current feedback
amplifiers. 2V to 15V supplies, low noise, good AC
specs, 6mA supply current each amplifier.
LT1360: 50MHz voltage feedback amplifier. 3.8mA supply
current, good AC and DC specs, 5V to 15V supplies.
LT1363: 70MHz, 1000V/ s op amps. 6.3mA supply cur-
rent, good AC and DC specs.
LT1364/LT1365: Dual and quad 70MHz, 100V/ s op amps.
6.3mA supply current per amplifier.
LinearView is a trademark of Linear Technology Corporation.
®
1220: 30MHz unity-gain bandwidth voltage feedback
TM
CD-ROM.