LTC1417AIGN datasheet(13/32 Pages) LINER

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LTC1417

APPLICATIONS INFORMATION

capacitor from + AIN to ground and a 100Ω source resistor

to limit the input bandwidth to 1.6MHz. The 1000pF

capacitor also acts as a charge reservoir for the input

sample-and-hold and isolates the ADC input from sam-

pling glitch sensitive circuitry. High quality capacitors and

resistors should be used since these components can add

distortion. NPO and silver mica type dielectric capacitors

have excellent linearity. Carbon surface mount resistors can

also generate distortion from self heating and from damage

that may occur during soldering. Metal film surface mount

resistors are much less susceptible to both problems.

Input Range

The

±2.048V and 0V to 4.096V input ranges of the

LTC1417 are optimized for low noise and low distortion.

Most op amps also perform well over these ranges,

allowing direct coupling to the analog inputs and eliminat-

ing the need for special translation circuitry.

Some applications may require other input ranges. The

LTC1417 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 LTC1417 has an on-chip, temperature compensated,

curvature corrected, bandgap reference which is factory

trimmed to 2.500V. It is internally connected to a reference

amplifier and is available at Pin 3. An 8k resistor is in series

with the output so that it can be easily overdriven in

applications where an external reference is required, see

Figure 9. A capacitor must be connected between the

reference amplifier compensation pin (REFCOMP, Pin 4)

and ground. The reference is stable with capacitors of 1

µF

or greater. For the best noise performance, a 10

µF in

parallel with a 0.1

µF ceramic is recommended.

The VREF pin can be driven with a DAC or other means

to provide input span adjustment. The reference should

be kept in the range of 2.25V to 2.75V for specified linearity.

UNIPOLAR / BIPOLAR OPERATION AND ADJUSTMENT

Figure 10a shows the input/output characteristics for the

LTC1417. The code transitions occur midway between

successive integer LSB values (i.e., 0.5LSB, 1.5LSB,

2.5LSB, … FS – 1.5LSB). The output code is natural binary

with 1LSB = FS/16384 = 4.096V/16384 = 250

µV. Figure

10b shows the input/output transfer characteristics for the

bipolar mode in two’s complement format.

Figure 9. Using the LT1460 as an External Reference

ANALOG

INPUT

1417 F09

µF

2.5V

0.1

µF

VIN

VOUT

LT1460-2.5

LTC1417

AIN

–

VREF

REFCOMP

AGND

VDD

Figure 10a. LTC1417 Unipolar Transfer Characteristics

Figure 10b. LTC1417 Bipolar Transfer Characteristics

INPUT VOLTAGE (V)

FS – 1LSB

1417 F10a

111...111

111...110

111...101

111...100

000...000

000...001

000...010

000...011

LSB

UNIPOLAR

ZERO

1LSB =

16384

4.096V

16384

INPUT VOLTAGE (V)

–1

LSB

1417 F10b

011...111

011...110

000...001

000...000

100...000

100...001

111...110

LSB

BIPOLAR

ZERO

111...111

FS/2 – 1LSB

–FS/2

FS = 4.096V

1LSB = FS/16384

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