ADS-929SLASH883 datasheet(4/8 Pages) ETC

4 / 8 page

ADS-929

CALIBRATION PROCEDURE

(Refer to Figures 2 and 3)

Any offset and/or gain calibration procedures should not be

implemented until devices are fully warmed up. To avoid

interaction, offset must be adjusted before gain. The ranges of

adjustment for the circuit of Figure 2 are guaranteed to

compensate for the ADS-929's initial accuracy errors and may

not be able to compensate for additional system errors.

All fixed resistors in Figure 2 should be metal-film types, and

multiturn potentiometers should have TCR’s of 100ppm/°C or

less to minimize drift with temperature.

A/D converters are calibrated by positioning their digital outputs

exactly on the transition point between two adjacent digital

output codes. This can be accomplished by connecting LED's

to the digital outputs and adjusting until certain LED's "flicker"

equally between on and off. Other approaches employ digital

comparators or microcontrollers to detect when the outputs

change from one code to the next.

For the ADS-929, offset adjusting is normally accomplished at

the point where the MSB is a 1 and all other output bits are 0's

and the LSB just changes from a 0 to a 1. This digital output

transition ideally occurs when the applied analog input is

+½ LSB (+305µV).

Gain adjusting is accomplished when all bits are 1's and the

LSB just changes from a 1 to a 0. This transition ideally occurs

when the analog input is at +full scale minus 1½ LSB's

(+4.999085V).

INPUT VOLTAGE

ZERO ADJUST

GAIN ADJUST

RANGE

+½ LSB

+FS –1½ LSB

±5V

+305µV

+4.999085V

Table 1. Zero and Gain Adjust

OUTPUT CODING

INPUT RANGE

BIPOLAR

MSB

LSB

±5V

SCALE

11 1111 1111 1111

+4.99939

+FS –1 LSB

11 1000 0000 0000

+3.75000

+3/4 FS

11 0000 0000 0000

+2.50000

+1/2FS

10 0000 0000 0000

0.00000

01 0000 0000 0000

–2.50000

–1/2FS

00 1000 0000 0000

–3.75000

–3/4FS

00 0000 0000 0001

–4.99939

–FS +1 LSB

00 0000 0000 0000

–5.00000

–FS

Table 2. Output Coding

Figure 3. Typical ADS-929 Connection Diagram

Zero/Offset Adjust Procedure

1. Apply a train of pulses to the START CONVERT input

(pin 16) so the converter is continuously converting. If using

LED's on the outputs, a 200kHz conversion rate will reduce

flicker.

2. Apply +305µV to the ANALOG INPUT (pin 20).

3. Adjust the offset potentiometer until the output bits are

a 1 and all 0's and the LSB flickers between 0 and 1.

Gain Adjust Procedure

1. Apply +4.999085V to the ANALOG INPUT (pin 20).

2. Adjust the gain potentiometer until the output bits are all 1's

and the LSB flickers between 1 and 0.

Figure 2. ADS-929 Calibration Circuit

Coding is offset binary; 1LSB = 610µV.

ADS-929

BIT 1 (MSB)

BIT 2

BIT 3

BIT 4

BIT 5

BIT 6

BIT 7

BIT 8

BIT 9

BIT 10

BIT 11

BIT 12

BIT 13

BIT 14 (LSB)

EOC

ANALOG

INPUT

START

CONVERT

19, 23

0.1µF

4.7µF

+5V

0.1µF

4.7µF

0.1µF

4.7µF

–12V/–15V

+12V/+15V

0.1µF

4.7µF

21 +10V REF. OUT

–5V to +5V

DIGITAL

GROUND

ANALOG

GROUND

To Pin 20

of ADS-929

–15V

SIGNAL

INPUT

GAIN

ADJUST

1.98k

Ω

+15V

Ω

200k

Ω

20k

Ω

–15V

+15V

ZERO/

OFFSET

ADJUST

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