ADS-933

®

®

5

LED's to the digital outputs and performing adjustments 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-933, offset adjusting is normally accomplished

when the analog input is 0 minus ½ LSB (–42µV). See Table

2b for the proper bipolar output coding.

Gain adjusting is accomplished when the analog input is at

nominal full scale minus 1½ LSB's (+2.749874V).

Note: Connect pin 5 to ANALOG GROUND (pin 4) for

operation without zero/offset adjustment. Connect pin 6 to

pin 4 for operation without gain adjustment.

Zero/Offset Adjust Procedure

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

12) so that the converter is continuously converting.

2. For zero/offset adjust, apply –42µV to the ANALOG INPUT

(pin 3).

3. Adjust the offset potentiometer until the code flickers

between 1000 0000 0000 0000 and 0111 1111 1111 1111

with pin 35 tied high (complementary offset binary) or

between 0111 1111 1111 1111 and 1000 0000 0000 0000

with pin 35 tied low (offset binary).

4. Two's complement coding requires using BIT 1 (MSB) (pin

29). With pin 35 tied low, adjust the trimpot until the output

code flickers between all 0’s and all 1’s.

Gain Adjust Procedure

1. For gain adjust, apply +2.749874V to the ANALOG INPUT

(pin 3).

2. Adjust the gain potentiometer until all output bits are 0’s and

the LSB flickers between a 1 and 0 with pin 35 tied high

(complementary offset binary) or until all output bits are 1’s

and the LSB flickers between a 1 and 0 with pin 35 tied low

(offset binary).

3. Two's complement coding requires using BIT 1 (MSB)

(pin 29). With pin 35 tied low, adjust the gain trimpot until

the output code flickers equally between 0111 1111 1111

1111 and 0111 1111 1111 1110.

4. To confirm proper operation of the device, vary the applied

input voltage to obtain the output coding listed in Table 2b.

1111 1111 1111 1111

LSB "1" to "0"

1110 0000 0000 0000

1100 0000 0000 0000

1000 0000 0000 0000

0111 1111 1111 1111

0100 0000 0000 0000

0010 0000 0000 0000

0000 0000 0000 0001

LSB "0" to "1"

0000 0000 0000 0000

OFFSET BINARY

MSB

LSB

MSB

LSB

MSB

LSB

MSB

LSB

+FS –1 LSB

+FS –1 1/2 LSB

+3/4 FS

+1/2 FS

0

–1 LSB

–1/2 FS

–3/4 FS

–FS +1 LSB

–FS + 1/2 LSB

–FS

OUTPUT CODING

INPUT

RANGE

±2.75V

+2.749916

+2.749874

+2.062500

+1.375000

0.000000

–0.000084

–1.375000

–2.062500

–2.749916

–2.749958

–2.750000

0000 0000 0000 0000

LSB "0" to "1"

0001 1111 1111 1111

0011 1111 1111 1111

0111 1111 1111 1111

1000 000

000 0000

1011 1111 1111 1111

1101 1111 1111 1111

1111 1111 1111 1110

LSB "1" to "0"

1111 1111 1111 1111

COMP. OFF. BIN.

0111 1111 1111 1111

LSB "1" to "0"

0110 0000 0000 0000

0100 0000 0000 0000

0000 0000 0000 0000

1111 1111 1111 1111

1100 0000 0000 0000

1010 0000 0000 0000

1000 0000 0000 0001

LSB "0" to "1"

1000 0000 0000 0000

TWO'S COMP.

1000 0000 0000 0000

LSB "0" to "1"

1001 1111 1111 1111

1011 1111 1111 1111

1111 1111 1111 1111

0000 0000 0000 0000

0011 1111 1111 1111

0101 1111 1111 1111

0111 1111 1111 1110

LSB "1" to "0"

0111 1111 1111 1111

COMP. TWO'S COMP.

BIPOLAR

SCALE

Table 2b. Output Coding

Complementary Offset Binary

1

Offset Binary

0

Complementary Two’s Complement

1

(Using MSB, pin 29)

Two’s Complement

0

(Using MSB, pin 29)

OUTPUT FORMAT

PIN 35 LOGIC LEVEL

Table 2a. Setting Output Coding Selection (Pin 35)

Figure 2. Connection Diagram

CALIBRATION PROCEDURE

Connect the converter per Figure 2. Any offset/gain

calibration procedures should not be implemented until the

device is fully warmed up. To avoid interaction, adjust offset

before gain. The ranges of adjustment for the circuits in

Figure 2 are guaranteed to compensate for the ADS-933’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 is accomplished by connecting

ADS-933

20k

Ω

33

32

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

OVERFLOW

EOC

BIT 1 (MSB)

BIT 1 (MSB)

BIT2

BIT 3

BIT 4

BIT 5

BIT 6

BIT 7

BIT 8

BIT 9

BIT 10

BIT 11

BIT 12

BIT 13

BIT 14

BIT 15

BIT 16 (LSB)

ANALOG

GROUND

DIGITAL

GROUND

0.1µF

4.7µF

0.1µF

COMP. BITS

4.7µF

+3.2V

REF. OUT

FIFO READ

31

7, 30

35

1

9

+5V

DIGITAL

–5V

+5V

OFFSET

ADJUST

GAIN

ADJUST

5

6

3

0.1µF

4.7µF

2, 4, 36

37

0.1µF

4.7µF

38

20k

Ω

–5V

+5V

–5V

+5V ANALOG

12

START CONVERT

ANALOG INPUT

34

ENABLE

8

FIFO/DIR

10

FSTAT1

11

FSTAT2

+5V

+5V

+5V

–5V