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LTC1279ISW

12-Bit, 600ksps Sampling A/D Converter with Shutdown

Linear Technology 

LTC1279

APPLICATIONS INFORMATION

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

increasing the time between conversions. Suitable de-

vices capable of driving the ADC’s AIN input include the

LT®1360, LT1220, LT1223 and LT1224 op amps.

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 800µA 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 (10µF tantalum in parallel with a

0.1µF ceramic).

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

provide input span adjustment in bipolar mode. The VREF

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.

Figure 6 shows an LT1006 op amp driving the VREF pin. (In

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

INPUT RANGE

±1.033 × VREF(OUT)

5V

+

LT1006

–

VREF(OUT) ≥ 2.45V

3Ω

AIN

VREF LTC1279

AGND

10µF

–5V

1279 F06

Figure 6. Driving the VREF with the LT1006 Op Amp

INPUT RANGE

± 2.58V

(= ±1.033 × VREF)

5V

VIN

VOUT

LT1019A-2.5

GND

3Ω

10µF

5V

AIN

VREF LTC1279

AGND

–5V

1279 F07

Figure 7. Supplying a 2.5V Reference Voltage

to the LTC1279 with the LT1019A-2.5

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.

111...111

111...110

111...101

111...100

1LSB

=

FS

4096

=

5V

4096

UNIPOLAR

ZERO

000...011

000...010

000...001

000...000

0V 1

LSB

INPUT VOLTAGE (V)

FS – 1LSB

1279 F08a

Figure 8a. LTC1279 Unipolar Transfer Characteristics

011...111

011...110

000...001

000...000

111...111

111...110

BIPOLAR

ZERO

100...001

100...000

FS = 5V

1LSB = FS/4096

– FS/2

–1 0V 1

LSB LSB

INPUT VOLTAGE (V)

FS/2 – 1LSB

1279 F08b

Figure 8b. LTC1279 Bipolar Transfer Characteristics

11

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