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ADS-943

4

CALIBRATION PROCEDURE

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 circuits in Figures 2 and 3 are guaranteed to

compensate for the ADS-943's initial accuracy errors and may

not be able to compensate for additional system errors.

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.

Offset adjusting for the ADS-943 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 (+122µV).

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

(+1.99963V).

Zero/Offset Adjust Procedure

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

(pin 18) so the converter is continuously converting.

2. Apply +122µV to the ANALOG INPUT (pin 21).

3. Adjust the offset potentiometer until the output bits are

10 0000 0000 0000 and the LSB flickers between 0 and 1.

Gain Adjust Procedure

1. Apply +1.99963V to the ANALOG INPUT (pin 21).

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

the LSB flickers between 1 and 0.

3. To confirm proper operation of the device, vary the input

signal to obtain the output coding listed in Table 2.

Figure 3. Connection Diagram

0.1µF

0.1µF

4.7µF

4.7µF

22, 13

24

20

19

ADS-943

–5V

20k

Ω

0.1µF

4.7µF

+5V

14

–5V

+5V

21

23

18

1

2

3

4

5

6

7

8

9

10

11

12

15

16

17

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

A single +5V supply should be used for both the +5V analog and +5V digital.

If separate supplies are used, the difference between the two cannot exceed 100mV.

Œ

Œ

+

+

+

ZERO/

OFFSET

ADJUST

INPUT VOLTAGE

ZERO ADJUST

GAIN ADJUST

RANGE

+1/2 LSB

+FS –1½ LSB

±2V

+122µV

+1.99963V

Table 1. Gain and Zero Adjust

BIPOLAR

INPUT VOLTAGE

OFFSET BINARY

SCALE

(±2V RANGE)

MSB

LSB

+FS – 1 LSB

+1.99976

11 1111 1111 1111

+3/4FS

+1.50000

11 1000 0000 0000

+1/2FS

+1.00000

11 0000 0000 0000

0

0.00000

10 0000 0000 0000

–1/2 FS

–1.00000

01 0000 0000 0000

–3/4 FS

–1.50000

00 1000 0000 0000

–FS +1 LSB

1.99976

00 0000 0000 0001

–FS

–2.00000

00 0000 0000 0000

Table 2. Output Coding for Bipolar Operation