LTC1418 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1418' PDF : 30 Pages View PDF
LTC1418
APPLICATIONS INFORMATION
LT1630/LT1631: 30MHz, 10V/µs, Dual/Quad Rail-to-Rail
Input and Output Precision Op Amps. 3.5mA supply cur-
rent per amplifier. 2.7V to ±15V supplies. Best AC perfor-
mance, 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
bandwidth 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 +AIN 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
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 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.
100Ω
ANALOG INPUT
1 AIN+
2000pF 2 AIN–
3 VREF LTC1418
4 REFCOMP
10μF
5 AGND
1418 F07
Figure 7. RC Input Filter
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 eliminating 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 refer-
ence 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 VREF 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.
5V
5V
VIN
VOUT
LT1460
ANALOG
INPUT
1 AIN+ VDD
2 AIN–
3 VREF
LTC1418
4 REFCOMP
10μF
0.1μF
5 AGND
1418 F08
Figure 8. Using the LT1460 as an External Reference
For more information www.linear.com/LTC1418
1418fa
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