12

LTC1279

S

APPLICATI

I FOR ATIO

Unipolar Offset and Full-Scale Error Adjustments

In applications where absolute accuracy is important,

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 0.61mV (i.e., 0.5LSB) at the input and adjust

the offset trim until the LTC1279 output code flickers

between 0000 0000 0000 and 0000 0000 0001. For zero

full-scale error apply an analog input of 4.99817V (i.e.,

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

R5 until the LTC1279 output code flickers between 1111

1111 1110 and 1111 1111 1111.

1279 F09a

R4

100

Ω

FULL-SCALE

ADJUST

R3

10k

R2

10k

R1

50

Ω

V1

–

+

A1

ADDITIONAL PINS OMITTED FOR CLARITY

±20LSB TRIM RANGE

LTC1279

AGND

Figure 9a. Full-Scale Adjust Circuit

1279 F09b

R2

10k

R4

100k

R1

10k

10k

5V

R9

20

Ω

ANALOG

INPUT

0V TO 5V

R3

100k

5V

R8

10k

OFFSET

ADJUST

R6

400

Ω

R5

4.3k

FULL-SCALE

ADJUST

R7

100k

–

+

LTC1279

Figure 9b. LTC1279 Unipolar Offset and Full-Scale Adjust Circuit

1279 F09c

R2

10k

R4

100k

R1

10k

ANALOG

INPUT

R3

100k

5V

R8

20k

OFFSET

ADJUST

R6

200

Ω

R5

4.3k

FULL-SCALE

ADJUST

R7

100k

–

+

–5V

LTC1279

Figure 9c. LTC1279 Bipolar Offset and Full-Scale Adjust Circuit

Bipolar Offset and Full-Scale Error Adjustments

Bipolar offset and full-scale errors are adjusted in a similar

fashion to the unipolar case. Again, bipolar offset must be

adjusted before full-scale error. Bipolar offset error ad-

justment is achieved by trimming the offset of the op amp

driving the analog input of the LTC1279 while the input

voltage is 0.5LSB below ground. This is done by applying

an input voltage of – 0.61mV (– 0.5LSB) to the input in

Figure 9c and adjusting the R8 until the ADC output code

flickers between 0000 0000 0000 and 1111 1111 1111.

For full scale adjustment, an input voltage of 2.49817V

(FS – 1.5LSBs) 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

Wire wrap boards are not recommended for high resolu-

tion or high speed A/D converters. To obtain the best

performance from the LTC1279, a printed circuit board is

required. The printed circuit board’s layout should ensure

that digital and analog signal lines are separated as much

as possible. In particular, care should be taken not to run

any digital trace alongside an analog signal trace or

underneath the ADC. The analog input should be screened

by AGND.

High quality tantalum and ceramic bypass capacitors

Figure 10. For the bipolar mode, a 0.1

µF ceramic provides