LTC1409CSW datasheet(12/20 Pages) LINER

12 / 20 page

LTC1409

APPLICATI

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 VREF (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 VREF

pin by 1.625 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 1

µF or greater. For the best noise

performance, a 10

µF ceramic or 10µF tantalum in parallel

with 0.1

µF ceramic is recommended (see Figure 8b).

LTC1409

+AIN

ANALOG INPUT

–AIN

VREF

REFCOMP

AGND

LTC1409 • F08b

µF

VIN

VOUT

LT1019A-2.5

Figure 8b. Using the LT1019-2.5 as an External Reference

The VREF 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.

40k

64k

REFERENCE

AMP

µF

REFCOMP

AGND

VREF

BANGAP

REFERENCE

2.5V

4.0625V

LTC1409

LTC1409 • F08a

Figure 8a. LTC1409 Reference Circuit

LTC1409

+AIN

ANALOG INPUT

–AIN

VREF

REFCOMP

AGND

LTC1409 • F09

µF

LTC1450

12-BIT

RAIL-TO-RAIL DAC

1.25V TO 3V

Figure 9.Driving VREF 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 +AIN – (–AIN) 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 AVDD or AVSS power supply voltages. Integral

nonlinearity errors (INL) and differential nonlinearity er-

rors (DNL) are independent of the common mode voltage,

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