ADS-944

4

®

®

For the ADS-944, 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 (+76.3µV).

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

(+1.249771) .

Note: Due to inherent system noise, the averaging of several

conversions may be needed to accurately adjust both offset

and gain to 1LSB of accuracy.

Zero/Offset Adjust Procedure

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

(pin 32) so the converter is continuously converting.

2. Apply +76.3µV to the ANALOG INPUT (pin 3).

3. Adjust the offset potentiometer until the output bits are

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

with pin 8 tied low (offset binary) or between 01 1111 1111

1111 and 01 1111 1111 1110 with pin 8 tied high

(complementary offset binary).

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

(pin 31). With pin 8 tied low, adjust the trimpot until the

code flickers between 00 0000 0000 0000 and 00 0000

0000 0001.

Gain Adjust Procedure

1. Apply +1.249771V to the ANALOG INPUT (pin 3).

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

the LSB flickers between 1 and 0 with pin 8 tied low (offset

binary) or until all bits are 0's and the LSB flickers between

1 and 0 with pin 8 tied high (complementary offset binary).

3. Two's complement coding requires using pin 31. With pin 8

tied low, adjust the gain trimpot until the output code flickers

equally between 01 1111 1111 1110 and 01 1111 1111 1111.

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

input voltage to obtain the output coding listed in Table 1.

TECHNICAL NOTES CONT.

5. Applying a start convert pulse while a conversion is in

progress (EOC = logic "1") initiates a new and inaccurate

conversion cycle. Data for the interrupted and subsequent

conversions will be invalid.

6. A passive bandpass filter is used at the input of the A/D for

all production testing.

7. Though the ADS-944's digital outputs are capable of

driving multiple LSTTL or HCT loads, we recommend the

output bits and the EOC line each drive only a single gate.

These gates should be located as close to the unit as

possible. If they can not, 33

Ω resistors placed in series

with each output can aid in isolating pc run inductances.

The ADS-944 digital outputs should not be connected

directly to noisy digital busses.

8. Do not enable/disable or complement the output bits during

the conversion process (from the falling edge of START

CONVERT to the falling edge of EOC).

CALIBRATION PROCEDURE

(Refer to Figure 2 and Table 1)

Note: Connect pin 5 to ANALOG GROUND (pin 6) for

operation without zero/offset adjustment. Connect pin 7 to

ANALOG GROUND (pin 6) for operation without gain

adjustment.

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 in Figure 2 are guaranteed to

compensate for the ADS-944'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.

Figure 2. ADS-944

Connection Diagram

ADS-944

20k

Ω

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

7

16

9

3

BIT 1 (MSB)

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)

OVERFLOW

EOC

OUTPUT ENABLE

ANALOG INPUT

ANALOG

SUPPLY

DIGITAL

SUPPLY

10

15

OFFSET

ADJUST

5

0.1µF

4.7µF

4, 6

14

0.1µF

4.7µF

1

–5.2V

+5V

32

START CONVERT

2

COMP. BITS

0.1µF

4.7µF

0.1µF

4.7µF

–5.2V

+5V

2

0.1µF

4.7µF

11

13

0.1µF

4.7µF

12

–15V

+15V

DIGITAL

SUPPLY

ANALOG

SUPPLY

0.1µF

7

GAIN

ADJUST

20k

Ω

–15V

+15V

–15V

+15V