Datasheet

1603f
13
LTC1603
APPLICATIONS INFORMATION
WUU
U
LTC1603
A
IN
+
A
IN
–
V
REF
REFCOMP
AGND
1603 F11
1
2
3
4
5
47µF
3000pF
100Ω
ANALOG INPUT
Figure 11. RC Input Filter
Input Range
The ±2.5V input range of the LTC1603 is optimized for low
noise and low distortion. Most op amps also perform well
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
LTC1603 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 LTC1603 has an on-chip, temperature compensated,
curvature corrected, bandgap reference that is factory
trimmed to 2.500V. It is connected internally to a refer-
ence amplifier and is available at V
REF
(Pin 3) (see Figure
12a). A 7.5k resistor is in series with the output so that it
can be easily overdriven by an external reference or other
circuitry (see Figure 12b). The reference amplifier gains
the voltage at the V
REF
pin by 1.75 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 22µF or
greater. For the best noise performance a 47µF ceramic
or 47µF tantalum in parallel with a 0.1µF ceramic is
recommended.
R2
12k
R3
16k
REFERENCE
AMP
47µF
REFCOMP
AGND
V
REF
R1
7.5k
3
4
5
2.500V
4.375V
LTC1603
1603 F12a
BANDGAP
REFERENCE
Figure 12a. LTC1603 Reference Circuit
1
2
3
0.1µF10µF
ANALOG
INPUT
1603 F12b
LT1019A-2.5
V
OUT
V
IN
5V
A
IN
+
A
IN
–
V
REF
LTC1603
AGND
REFCOMP
5
4
+
Figure 12b. Using the LT1019-2.5 as an External Reference
The V
REF
pin can be driven with a DAC or other means
shown in Figure 13. This is useful in applications where the
peak input signal amplitude may vary. The input span of
the ADC can then be adjusted to match the peak input
signal, maximizing the signal-to-noise ratio. The filtering
of the internal LTC1603 reference amplifier will limit
the bandwidth and settling time of this circuit. A settling
time of 20ms should be allowed for after a reference
adjustment.
Differential Inputs
The LTC1603 has a unique differential sample-and-hold
circuit that allows rail-to-rail inputs. The ADC will always
convert the difference of A
IN
+
– A
IN
–
independent of the
common mode voltage (see Figure 15a). The common
mode rejection holds up to extremely high frequencies
(see Figure 14a). The only requirement is that both inputs