LTC1409 LINER [Linear Technology], LTC1409 Datasheet - Page 11

• Manufacturer Part Number: LTC1409
• Description: 12-Bit, 800ksps Sampling A/D Converter with Shutdown
• Manufacturer: LINER [Linear Technology]

bandwidth must be greater than 20MHz 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 LTC1409 will
depend 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 driving the LTC1409, more detailed informa-
tion is available in the Linear Technology databooks and
the LinearView
LT
back amplifier.
specifications, 90ns settling to 0.5LSB.
LT1223: 100MHz video current feedback amplifier. 6mA
supply current. 5V to 15V supplies. Low distortion up
to and above 400kHz. Low noise. Good for AC
applications.
LT1227: 140MHz video current feedback amplifier. 10mA
supply current 5V to 15V supplies. Lowest distortion
at frequencies above 400kHz. Low noise. Best for AC
applications.
LT1229/LT1230: Dual and quad 100MHz current feed-
back amplifiers. 2V to 15V supplies. Low noise. Good
AC specs. 6mA supply current for each amplifier.
LT1360: 37MHz voltage feedback amplifier. 3.8mA sup-
ply current. Good AC/DC specs. 5V to 15V supplies.
70ns settling to 0.5LSB.
LT1363: 50MHz, 450V/ s op amps. 6.3mA supply cur-
rent. Good AC/DC specs. 60ns settling to 0.5LSB.
LT1364/LT1365: Dual and quad 50MHz, 450V/ s op
amps. 6.3mA supply current per amplifier. 60ns settling
to 0.5LSB.
Input Filtering
The noise and the distortion of the input amplifier and
other circuitry must be considered since they will add to
the LTC1409 noise and distortion. The small-signal band-
A
LinearView is a trademark of Linear Technology Corporation.
PPLICATI
®
1220: 30MHz unity-gain bandwidth voltage feed-
TM
O
CD-ROM.
5V to 15V supplies. Excellent DC
U
S
I FOR ATIO
U
W
U
width of the sample-and-hold circuit is 20MHz. Any noise
or distortion products that are present at the analog inputs
will be summed over this entire bandwidth. Noisy input
circuitry should be filtered prior to the analog inputs to
minimize noise. A simple 1-pole RC filter is sufficient for
many applications. For example, Figure 7 shows a 1000pF
capacitor from + A
to limit the input bandwidth to 1.6MHz. The 1000pF
capacitor also acts as a charge reservoir for the input
sample-and-hold and isolates the ADC input from sam-
pling glitch sensitive circuitry. High quality capacitors and
resistors should be used since these components can add
distortion. NPO and silver mica type dielectric capacitors
have excellent linearity. Carbon surface mount resistors
can also generate distortion from self heating and from
damage that may occur during soldering. Metal film
surface mount resistors are much less susceptible to both
problems.
When high amplitude unwanted signals are close in fre-
quency to the desired signal frequency, a multiple pole filter
–5V
V
IN
Figure 7b. 500kHz 5th Order Elliptic Lowpass Filter
ANALOG INPUT
1
2
3
4
0.1 F
LTC1560-1
1000pF
Figure 7a. RC Input Filter
IN
10 F
8
7
6
5
50
to ground and a 100 source resistor
5V
0.1 F
1
2
3
4
5
+A
–A
V
REFCOMP
AGND
REF
IN
IN
LTC1409
10 F
LTC1409 • F07b
1
2
3
4
5
+A
–A
V
REFCOMP
AGND
REF
IN
LTC1409
IN
LTC1409
LTC1409 • F07
11