11

LTC1279

the internal reference so driving the reference is not

recommended, since the input span will exceed the supply

and codes will be lost at the full scale.) Figure 7 shows a

typical reference, the LT1019A-2.5 connected to the

LTC1279. This will provide an improved drift (equal to the

LT1019A-2.5’s maximum of 5ppm/

°C) and a ±2.582V full

scale.

UNIPOLAR/BIPOLAR OPERATION AND ADJUSTMENT

Figure 8a shows the ideal input/output characteristics for

the LTC1279. 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 naturally

binary with 1LSB = FS/4096 = 5V/4096 = 1.22mV. Figure

8b shows the input/output transfer characteristics for the

bipolar mode in two’s complement format.

op amps are used, more settling time can be provided by

increasing the time between conversions. Suitable de-

LT

Internal Reference

The LTC1279 has an on-chip, temperature compensated,

curvature corrected, bandgap reference that is factory

trimmed to 2.42V. It is internally connected to the DAC and

is available at pin 2 to provide up to 800

µA 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 (10

µF tantalum in parallel with a

0.1

µF ceramic).

pin must be driven to at least 2.45V to prevent conflict with

the internal reference. The reference should be driven to

no more than 4.8V to keep the input span within the

±5V

supplies.

the unipolar mode, the input span is already 0V to 5V with

Figure 8b. LTC1279 Bipolar Transfer Characteristics

Figure 7. Supplying a 2.5V Reference Voltage

to the LTC1279 with the LT1019A-2.5

Figure 8a. LTC1279 Unipolar Transfer Characteristics

3

Ω

INPUT RANGE

–

+

LT1006

5V

–5V

LTC1279

AGND

10

µF

1279 F06

3

Ω

INPUT RANGE

±2.58V

(=

10

µF

1279 F07

LT1019A-2.5

GND

5V

5V

–5V

LTC1279

AGND

INPUT VOLTAGE (V)

0V

–1

LSB

1279 F08b

011...111

011...110

000...001

000...000

100...000

100...001

111...110

1

LSB

BIPOLAR

ZERO

111...111

FS/2 – 1LSB

–FS/2

FS = 5V

1LSB = FS/4096

INPUT VOLTAGE (V)

0V

FS – 1LSB

1279 F08a

111...111

111...110

111...101

111...100

000...000

000...001

000...010

000...011

1

LSB

UNIPOLAR

ZERO

1LSB = FS

4096

5V

4096

=