LTC1604CG View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1604CG' PDF : 20 Pages View PDF
LTC1604
APPLICATIONS INFORMATION
100Ω
ANALOG INPUT
3000pF
47µF
1 AIN+
2 AIN–
LTC1604
3
VREF
4 REFCOMP
5 AGND
1604 F11
Figure 11. RC Input Filter
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 transla-
tion 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.
3 VREF
2.500V
R1
7.5k BANDGAP
REFERENCE
4.375V
4 REFCOMP
REFERENCE
AMP
47µF
5 AGND
R2
12k
R3
16k
LTC1604
1604 F12a
Figure 12a. LTC1604 Reference Circuit
5V
VIN
LT1019A-2.5
VOUT
+
ANALOG
INPUT
1 AIN+
2 AIN–
3
VREF
LTC1604
4
REFCOMP
10µF
0.1µF
5
AGND
1604 F12b
Figure 12b. Using the LT1019-2.5 as an External Reference
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 VREF (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 VREF pin by 1.75 to create the required
internal reference voltage. This provides buffering
between the VREF 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 recom-
mended.
The VREF 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 AIN+ – AIN– 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
13
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