LTC1401CS8 datasheet(10/20 Pages) LINER

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LTC1401

APPLICATIONS INFORMATION

is available at Pin 3 to provide up to 1mA current to an

external load. For minimum code transition noise, the

reference output should be decoupled with a capacitor to

filter wideband noise from the reference (10uF tantalum in

parallel with a 0.1uF ceramic is recommended). The VREF

pin can be driven with a DAC or other means to provide

input span adjustment. The VREF pin must be driven to at

least 1.25V to prevent conflict with the internal reference.

The reference should not be driven to more than 3V.

Figure 6 shows an LT1360 op amp driving the reference

pin. Figure 7 shows a typical reference (LT1634-1.25)

connected to the LTC1401. This will provide improved

drift (equal to the maximum 25ppm/

°C of the LT1634-

1.25) and a 2.1338V full scale.

UNIPOLAR OPERATION AND ADJUSTMENT

Figure 8 shows the ideal input/output characteristics for

the LTC1401. 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 = 2.048/4096 = 0.5mV.

Figure 7. Supplying a 2.5V Reference Voltage to

the LTC1401 with the LT1634-1.25

LTC1401 • F06

–

VREF(OUT) ≥ 1.25V

AIN

VREF

GND

µF

Ω

INPUT RANGE

1.707 • VREF(OUT)

LTC1401

LT1360

VCC

LTC1401 • F07

µF

Ω

INPUT RANGE 1.707 • VREF

(= 2.1338V)

LT1634-1.25

10V

VIN

VOUT

GND

AIN

VREF

GND

LTC1401

VCC

Figure 6. Driving the VREF with the LT1360 Op Amp

Figure 8. LTC1401 Unipolar Transfer Characteristics

INPUT VOLTAGE (V)

FS – 1LSB

LTC1401 • F08

111...111

111...110

111...101

111...100

000...000

000...001

000...010

000...011

LSB

UNIPOLAR

ZERO

1LSB = FS

4096

2.048

4096

Unipolar Offset and Full-Scale Error Adjustments

In applications where absolute accuracy is important, the

offset and full-scale errors can be adjusted to zero. Offset

error must be adjusted before full-scale error. Figure 9a

shows the extra components required for full scale error

adjustment. If both offset and full-scale adjustments are

needed, the circuit in Figure 9b can be used. For zero offset

error, apply 0.25mV (i.e., 0.5LSB) at the input and adjust

the offset trim until the LTC1401 output code flickers

between 0000 0000 0000 and 0000 0000 0001. For zero

full-scale error, apply an analog input of 2.04725V ( FS –

1.5LSB or last code transition ) at the input and adjust R5

until the LTC1401 output code flickers between 1111 1111

1110 and 1111 1111 1111.

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