Datasheet

12
LTC1418
APPLICATIONS INFORMATION
WUU
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amplifier. 2.2V to ±15V supplies. Good AC performance,
input noise voltage = 12nV/√Hz (typ).
LT1630/LT1631: 30MHz, 10V/µs, Dual/Quad Rail-to-Rail
Input and Output Precision Op Amps. 3.5mA supply
current per amplifier. 2.7V to ±15V supplies. Best AC
performance, input noise voltage = 6nV/√Hz (typ),
THD = –86dB at 100kHz.
Input Filtering
The noise and the distortion of the input amplifier and
other circuitry must be considered since they will add to
the LTC1418 noise and distortion. The small-signal band-
width of the sample-and-hold circuit is 5MHz. 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 2000pF
capacitor from +A
IN
to ground and a 100Ω source resistor
to limit the input bandwidth to 800kHz. The 2000pF
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.
Input Range
The ±2.048V and 0V to 4.096V input ranges of the
LTC1418 are optimized for low noise and low distortion.
Most op amps also perform well over these ranges,
allowing direct coupling to the analog inputs and eliminat-
ing the need for special translation circuitry.
Some applications may require other input ranges. The
LTC1418 differential inputs and reference circuitry can
accommodate other input ranges often with little or no
additional circuitry. The following sections describe the
reference and input circuitry and how they affect the input
range.
INTERNAL REFERENCE
The LTC1418 has an on-chip, temperature compensated,
curvature corrected, bandgap reference which is factory
trimmed to 2.500V. It is internally connected to a reference
amplifier and is available at Pin 3. A 8k resistor is in series
with the output so that it can be easily overdriven in
applications where an external reference is required, see
Figure 8. The reference amplifier compensation pin
(REFCOMP, Pin 4) must be connected to a capacitor to
ground. The reference is stable with capacitors of 1µF or
greater. For the best noise performance, a 10µF in parallel
with a 0.1µF ceramic is recommended.
The V
REF
pin can be driven with a DAC or other means
to provide input span adjustment. The reference should
be kept in the range of 2.25V to 2.75V for specified
linearity.
LTC1418
A
IN
+
A
IN
–
V
REF
REFCOMP
AGND



ANALOG INPUT
100Ω




1418 F07
1
2
3
4
5
2000pF
10µF
Figure 7. RC Input Filter
ANALOG
INPUT
5V
1418 F08
10µF
0.1µF
V
IN
V
OUT
LT1460
1
2
3
4
5
LTC1418
5V
A
IN
+
A
IN
–
V
REF
REFCOMP
AGND
V
DD
Figure 8. Using the LT1460 as an External Reference