LTC1414 Linear Technology, LTC1414 Datasheet - Page 10

• Manufacturer Part Number: LTC1414
• Description: 14-Bit/ 2.2Msps/ Sampling A/D Converter
• Manufacturer: Linear Technology

APPLICATIONS
LTC1414
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
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 100 . The
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 settling time can be provided by increasing
the time between conversions.
The best choice for an op amp to drive the LTC1414 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 LTC1414. More detailed information is
available in the Linear Technology Databooks and on the
LinearView
LT
6mA supply current. 5V to 15V supplies. Low noise.
Good for AC applications.
LT1227: 140MHz Video Current Feedback Amplifier. 10mA
supply current. 5V to 15V supplies. Low noise. Best for
10
AC applications.
LT1229/LT1230: Dual and Quad 100MHz Current Feed-
back Amplifiers. 2V to 15V supplies. Low noise. Good
AC specifications, 6mA supply current each amplifier.
LT1360: 50MHz Voltage Feedback Amplifier. 3.8mA sup-
ply current. Good AC and DC specs. 5V to 15V supplies.
70ns settling to 0.5LSB.
LT1363: 70MHz, 1000V/ s Op Amps. 6.3mA supply cur-
rent. Good AC and DC specifications. 60ns settling to
0.5LSB.
LT1364/LT1365: Dual and Quad 70MHz, 1000V/ s Op
Amps. 6.3mA supply current per amplifier. 60ns settling
to 0.5LSB.
LinearView is a trademark of Linear Technology Corporation.
®
1223: 100MHz Video Current Feedback Amplifier.
TM
CD-ROM.
U
INFORMATION
U
W
U
AC Coupled Inputs
In applications where only the AC component of the analog
input is important, it may be desirable to AC couple the
input. This is easily accomplished by DC biasing the
LTC1414 analog input with a resistor to ground and using
a coupling capacitor to the input. Figure 7 shows a simple
AC coupled input circuit for the LTC1414 using only two
additional components. C1 is a 10 F ceramic capacitor
and R1 is a 1000 resistor to ground. R1 and C1 form a
highpass filter with a lower cut off frequency of 1/2 (C1)R1
or 15.9Hz.
Differential Drive
In some applications the ADC drive circuitry is differential.
The differential drive can be applied directly to the LTC1414
without any special translation circuitry. Differential drive
can be advantageous at high frequencies (>1MHz) since it
provides improved THD and SFDR. Transformers can be
used to provide AC coupling, input scaling and single
ended to differential conversion as shown in Figure 8. The
resistor R
impedance on the primary. The input impedance of the
primary R
by the equation
For example, if a Minicircuits T4-6T transformer is used,
the turns ratio is 2; if R
The center tap of the secondary will set the common
mode voltage and should be grounded for optimal AC
performance.
R
P
= R
ANALOG INPUT
S
S
P
/n
will be related to the secondary load resistor R
across the secondary will determine the input
2
Figure 7. AC Coupled Input
10 F
C1
S
is 200 then R
R1
1k
1 F
10 F
1
2
3
4
5
A
A
V
REFCOMP
AGND
IN
IN
REF
+
–
LTC1414
P
is equal to 50 .
LTC1414 • F07
S