Datasheet

11
LTC1414
APPLICATIONS INFORMATION
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Input Range
The ±2.5V input range of the LTC1414 is optimized for low
noise and low distortion. Most op amps also perform best
over this same range, allowing direct coupling to the
analog inputs and eliminating the need for special transla-
tion circuitry.
Some applications may require other input ranges. The
LTC1414 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 LTC1414 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
REF
(Pin 3), see Figure 10. A
2k resistor is in series with the output so that it can be
easily overdriven by an external reference or other cir-
cuitry. The reference amplifier multiplies the voltage at the
V
REF
pin by 1.625 to create the required internal reference
voltage. This provides buffering between the V
REF
pin and
the high speed capacitive DAC. The reference amplifier
compensation pin, REFCOMP (Pin 4) must be bypassed
with a capacitor to ground. The reference amplifier is
stable with capacitors of 1µF or greater. For the best noise
performance, a 10µF ceramic or 10µF tantalum in parallel
with a 0.1µF ceramic is recommended.
Figure 8. If a Transformer Coupled Input is Required,
this Circuit Provides a Simple Solution
LTC1414
A
IN
+
INPUT
A
IN
–
V
REF
REFCOMP
AGND
LTC1414 • F09
1
2
3
4
5
500pF
100Ω
10µF
Figure 9. An RC Filter Reduces the ADC’s 40MHz
Bandwidth to 3.2MHz and Filters Out Wideband Noise
Which May Be Present in the Input Signal
Input Filtering
The noise and the distortion of the input amplifier and
other circuitry must be considered since they will add to
the LTC1414 noise and distortion. The small-signal band-
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 9 shows a 500pF capacitor from A
IN
+
to ground and a 100Ω 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 cir-
cuitry. High quality capacitors and resistors should be
used since poor quality components can add distortion.
NPO and silver mica type dielectric capacitors have excel-
lent 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.
Figure 10. LTC1414 Reference Circuit
R2
40k
R3
64k
REFERENCE
AMP
10µF
REFCOMP
AGND
V
REF
R1
2k
3
4
5
2.500V
4.0625V
LTC1414
1414 F10
+
–
BANDGAP
REFERENCE
LTC1414
A
IN
+
ANALOG
INPUT
A
IN
–
V
REF
REFCOMP
AGND
LTC1414 • F08
1
2
3
4
5
R1
50Ω
10µF
1µF
R2
50Ω
C1
500pF
R
S
1:N
R
P