MDC_ADS-949.B01 Page 4 of 8

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ADS 949

14 Bit, 12.8MHz

Sampling A/D Converters

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-949’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-949 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 (+153µV).

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 (+2.49954V).

Zero/Offset Adjust Procedure

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

(pin 7) so the converter is continuously converting.

2. Apply +153µV to the ANALog iNPUT (pin 1).

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 +2.49954V to the ANALog iNPUT (pin 1).

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.

INPUT VOLTAGE

ZERO ADJUST

GAIN ADJUST

RANGE

+½ LSB

+FS –1½ LSB

±2.5V

+153µV

+2.49954V

Table 1. Gain and Zero Adjust

Figure 2. Typical ADS-949 Bipolar Connection Diagram

0.1µF

4.7µF

27

+5VD

29

AGND

3

–5V

ADS-949

20k

�

4.7µF

+5VA

26

DGND

–5V

+5V

1VIN A

4 OFFSET ADJUST

7 START CONVERT

23

22

21

20

19

18

17

16

15

14

13

12

11

10

8

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

BIPOLAR

ANALOG

INPUT

START

CONVERT

➀

Bypass Pins 5, 6, 32, with a 4.7µF to Analog Ground.

Note: The Voltage Value at Pin 32 (Range) sets the input voltage range of the ADS-949

eg: If Pin 6 (2.5V Reference Out) is tied to the Range Pin 32 (20k Pot is shorted), then

the input range of the ADS-949 becomes ±2.5V

If the 20k Pot is set at midrange then the input range of the ADS-949 becomes ±1.25V

+

ZERO/

OFFSET

ADJUST

–5V

+5V

31 GAIN ADJUST

GAIN

ADJUST

20K

–5V

2 VIN B

9 ENABLE

20k

+15V

28

+15V

+5VD

4.7µF

+

30

+5VA

0.1µF

0.1µF

4.7µF

+

0.1µF

+