LTC1282ACSW datasheet(14/24 Pages) LINER

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LTC1282

APPLICATI

I FOR ATIO

offset, apply 0.305mV (i.e., 0.5LSB) at V1 and adjust the

op amp offset voltage until the LTC1282 output code

flickers between 0000 0000 0000 and 0000 0000 0001.

For zero full-scale error, apply an analog input of 2.49909V

(i.e., FS – 1.5LSBs or last code transition) at the input and

adjust the full-scale trim until the LTC1282 output code

flickers between 1111 1111 1110 and 1111 1111 1111.

Figure 10. Full-Scale Adjust Circuit

Figure 11. Unipolar Offset and Full-Scale Adjust Circuit

Figure 12. Bipolar Offset and Full-Scale Adjust Circuit

error adjustment is achieved by trimming the offset ad-

justment of Figure 12 while the input voltage is 0.5LSB

below ground. This is done by applying an input voltage of

– 0.305mV (– 0.5LSB for LTC1282) to the input in Figure

12 and adjusting R8 until the ADC output code flickers

between 0000 0000 0000 and 1111 1111 1111. For full-

scale adjustment, an input voltage of 1.24909V (FS –

1.5LSBs for LTC1282) is applied to the input and R5 is

adjusted until the output code flickers between 0111 1111

1110 and 0111 1111 1111.

BOARD LAYOUT AND BYPASSING

The LTC1282 is easy to use. To obtain the best perfor-

mance from the device, a printed circuit board is recom-

mended. Layout for the printed circuit board should en-

sure that digital and analog signal lines are separated as

much as possible. In particular, care should be taken not

to run any digital track alongside an analog signal track or

underneath the ADC. The analog input should be screened

by AGND.

High quality tantalum and ceramic bypass capacitors

should be used at the VDD and VREF pins as shown in Figure

13. In bipolar mode, a 0.1

µF ceramic provides adequate

bypassing for the VSS pin. The capacitors must be located

as close to the pins as possible. The traces connecting the

pins and the bypass capacitors must be kept short and

should be made as wide as possible.

Bipolar Offset and Full-Scale Adjustment

Bipolar offset and full-scale errors are adjusted in a

similar fashion to the unipolar case. Figure 10 shows the

extra components required for full-scale error adjust-

ment. If both offset and full-scale adjustments are needed,

the circuit in Figure 12 can be used. Again, bipolar offset

must be adjusted before full-scale error. Bipolar offset

LTC1282

AIN

AGND

LTC1282 • F10

100

Ω

FULL-SCALE

ADJUST

10k

Ω

–

ADDITIONAL PINS OMITTED FOR CLARITY

±20LSB TRIM RANGE

AIN

LTC1282 • F11

10k

100k

10k

Ω

ANALOG

INPUT

0V TO 2.5V

100k

10k

OFFSET

ADJUST

400

Ω

4.3k

FULL-SCALE

ADJUST

100k

–

LTC1282

AIN

LTC1282 • F12

10k

100k

10k

ANALOG

INPUT

±1.25V

100k

20k

OFFSET

ADJUST

200

Ω

4.3k

FULL-SCALE

ADJUST

100k

–

LTC1282

–5V

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