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LTC1282

APPLICATI

I FOR ATIO

Overdriving the Internal Reference

The VREF pin can be driven above its normal value with a

DAC or other means to provide input span adjustment.

Figure 6 shows an LT1006 op amp driving the reference

pin. The VREF pin must be driven to at least 1.25V to

prevent conflict with the internal reference. The reference

should be driven to no more than 1.44V in unipolar mode

or 2.88V for bipolar mode to keep the input span within the

single 3V or

±3V supplies.

Figure 6. Driving the VREF with the LT1006 Op Amp

natural binary with 1LSB = FS/4096 = 2.5V/4096 = 0.61mV.

Figure 9 shows the input/output transfer characteristics

for the LTC1282 in bipolar operation. The full scale for

LTC1282 in bipolar mode is still 2.5V and 1LSB = 0.61mV.

UNIPOLAR/BIPOLAR OPERATION AND ADJUSTMENT

Figure 8 shows the ideal input/output characteristics for

the LTC1282. The code transitions occur midway be-

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

1.5LSBs, 2.5LSBs, FS – 1.5LSBs). The output code is

Figure 7. Supplying a 2.5V Reference Voltage

to the LTC1282 with the LT1019A-2.5

Figure 7 shows a typical reference, the LT1019A-2.5

connected to the LTC1282 operating in bipolar mode. This

will provide an improved drift (due to the 5ppm/

°C of the

LT1019A-2.5) and a

±2.604V full scale.

INPUT VOLTAGE (V)

FS – 1LSB

LTC1282 • F8

111...111

111...110

111...101

111...100

000...000

000...001

000...010

000...011

LSB

UNIPOLAR

ZERO

1LSB = FS/4096

FS = 2.5V

Figure 8. LTC1282 Unipolar Transfer Characteristic

Figure 9. LTC1282 Bipolar Transfer Characteristic

Unipolar Offset and Full-Scale Adjustment

In applications where absolute accuracy is important,

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

10 shows the extra components required for full-scale

error adjustment. If both offset and full-scale adjust-

ments are needed, the circuit in Figure 11 can be used.

Offset should be adjusted before full scale. To adjust

INPUT VOLTAGE (V)

–1

LSB

LTC1282 • F09

011...111

011...110

000...001

000...000

100...000

100...001

100...010

LSB

BIPOLAR

ZERO

111...111

FS/2 – 1LSB

–FS/2

FS = 2.5V

1LSB = FS/4096

111...110

VREF(OUT) ≥ 1.25V

Ω

INPUT RANGE

±1.033VREF(OUT)

–

LT1006

µF

LTC1282 • F06

LTC1282

AIN

AGND

VREF

VDD

VSS

–3V

Ω

INPUT RANGE

±2.60V

LTC1282

AIN

AGND

VREF

µF

LTC1282 • F07

LT1019A-2.5

VIN

GND

VOUT

VDD

VSS

–3V

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