Datasheet

LTC1604
13
1604fa
APPLICATIONS INFORMATION
Input Range
The ±2.5V input range of the LTC1604 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
translation circuitry.
Some applications may require other input ranges. The
LTC1604 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 LTC1604 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 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 volt-
age 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 refer-
ence 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.
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 LTC1604 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 LTC1604 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
LTC1604
A
IN
+
A
IN
–
V
REF
REFCOMP
AGND
1604 F11
1
2
3
4
5
47μF
3000pF
100Ω
ANALOG INPUT
R2
12k
R3
16k
REFERENCE
AMP
47μF
REFCOMP
AGND
V
REF
R1
7.5k
3
4
5
2.500V
4.375V
LTC1604
1604 F12a
BANDGAP
REFERENCE
1
2
3
0.1μF10μF
ANALOG
INPUT
1604 F12b
LT1019A-2.5
V
OUT
V
IN
5V
A
IN
+
A
IN
–
V
REF
LTC1604
AGND
REFCOMP
5
4
+
Figure 11. RC Input Filter
Figure 12a. LTC1604 Reference Circuit
Figure 12b. Using the LT1019-2.5 as an External Reference