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LTC1604

APPLICATIONS INFORMATION

LTC1604

AIN

–

VREF

REFCOMP

AGND

1604 F11

µF

3000pF

100

Ω

ANALOG INPUT

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.

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 22

µF or

greater. For the best noise performance a 47

µF ceramic or

µF tantalum in parallel with a 0.1µF ceramic is recom-

mended.

12k

16k

REFERENCE

AMP

µF

REFCOMP

AGND

VREF

7.5k

2.500V

4.375V

LTC1604

1604 F12a

BANDGAP

REFERENCE

Figure 12a. LTC1604 Reference Circuit

0.1

µF

ANALOG

INPUT

1604 F12b

LT1019A-2.5

VOUT

VIN

AIN

–

VREF

LTC1604

AGND

REFCOMP

Figure 12b. Using the LT1019-2.5 as an External Reference

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

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