LTC1412 Linear Technology, LTC1412 Datasheet - Page 11

• Manufacturer Part Number: LTC1412
• Description: 12-Bit/ 3Msps/ Sampling A/D Converter
• Manufacturer: Linear Technology

width of the sample-and-hold circuit is 40MHz. 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 500pF capacitor from A
to ground and a 100 source resistor to limit the input
bandwidth to 3.2MHz. The 500pF capacitor also acts as a
charge reservoir for the input sample-and-hold and iso-
lates the ADC input from sampling 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 linear-
ity. Carbon surface mount resistors can also generate
distortion from self heating and from damage that may
occur during soldering. Metal film surface mount resis-
tors are much less susceptible to both problems.
When high amplitude unwanted signals are close in
frequency to the desired signal frequency, a multiple pole
APPLICATIONS
– 5V
Figure 7b. 1MHz Fifth-Order Elliptic Lowpass Filter
V
IN
1
2
3
4
ANALOG INPUT
0.1 F
LTC1560-1
Figure 7a. RC Input Filter
U
500pF
100
10 F
8
7
6
5
INFORMATION
U
0.1 F
5V
1
2
3
4
5
10 F
A
A
V
REFCOMP
AGND
W
IN
IN
REF
LTC1412
+
–
1412 F07a
1
2
3
4
5
A
A
V
REFCOMP
AGND
IN
IN
REF
LTC1412
+
–
U
1412 F07b
IN
+
filter is required. Figure 7b shows a simple implementa-
tion using an LTC1560-1 fifth-order elliptic continuous
time filter.
Input Range
The 2.5V input range of the LTC1412 is optimized for low
noise and low distortion. Most op amps also perform best
over this same range, allowing direct coupling to the ana-
log inputs and eliminating the need for special translation
circuitry.
Some applications may require other input ranges. The
LTC1412 differential inputs and reference circuitry can ac-
commodate other input ranges often with little or no addi-
tional circuitry. The following sections describe the reference
and input circuitry and how they affect the input range.
Internal Reference
The LTC1412 has an on-chip, temperature compensated,
curvature corrected, bandgap reference that is factory
trimmed to 2.500V. It is connected internally to a reference
amplifier and is available at V
2k resistor is in series with the output so that it can be
easily overdriven by an external reference or other cir-
cuitry, see Figure 8b. The reference amplifier gains the
voltage at the V
internal reference voltage. This provides buffering be-
tween the V
reference amplifier compensation pin, REFCOMP (Pin 4)
must be bypassed with a capacitor to ground. The refer-
ence amplifier is stable with capacitors of 1 F or greater.
For the best noise performance, a 10 F ceramic or 10 F
tantalum in parallel with a 0.1 F ceramic is recommended.
4.0625V
2.500V
10 F
REF
Figure 8a. LTC1412 Reference Circuit
3
4
5
pin and the high speed capacitive DAC. The
V
REFCOMP
AGND
REF
REF
pin by 1.625 to create the required
64k
R3
REFERENCE
40k
R2
AMP
REF
(Pin 3), see Figure 8a. A
R1
2k
LTC1412
REFERENCE
BANDGAP
LTC1412
1412 F08a
11