LTC1409 Datasheet, PDF (12/20 Pages) Linear Technology – 12-Bit, 800ksps Sampling A/D Converter with Shutdown

LTC1409
APPLICATI S I FOR ATIO
is required. Figure 7b shows a simple implementation using
a LTC1560 5th order elliptic continuous time filter.
Input Range
The ±2.5V input range of the LTC1409 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
LTC1409 differential inputs and reference circuitry can ac-
commodate other input ranges often with little or no addi-
tional circuitry. The following sections describe the reference
and input circuitry and how they affect the input range.
Internal Reference
The LTC1409 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 8a. A
4k 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 gains 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 0.1µF ceramic is recommended (see Figure 8b).
2.5V ^{3 VREF}
R1
4k _BANGAP
REFERENCE
4.0625V
4 ^REFCOMP _REFERENCE
AMP
R2
10µF
40k
R3
5 AGND ^64k
LTC1409
LTC1409 • F08a
Figure 8a. LTC1409 Reference Circuit
5V
V_IN
LT1019A-2.5
V_OUT
ANALOG INPUT
1
+A_IN
2
– A_IN
LTC1409
3
V_REF
4
REFCOMP
10µF
5
AGND
LTC1409 • F08b
Figure 8b. 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 9. 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 LTC1409 reference amplifier will limit the
bandwidth and settling time of this circuit. A settling time
of 5ms should be allowed for, after a reference adjust-
ment.
ANALOG INPUT
1
+A_IN
2
– A_IN
LTC1450
12-BIT
RAIL-TO-RAIL DAC
LTC1409
1.25V TO 3V
3
V_REF
4
REFCOMP
10µF
5
AGND
LTC1409 • F09
Figure 9.Driving V_REF with a DAC
Differential Inputs
The LTC1409 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. The common mode rejection
holds up to extremely high frequencies, see Figure 10a.
The only requirement is that both inputs can not exceed
the AV_DD or AV_SS power supply voltages. Integral
nonlinearity errors (INL) and differential nonlinearity er-
rors (DNL) are independent of the common mode voltage,
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