ADS-118/118A

®

®

4

SIGNAL

INPUT

GAIN

ADJUST

50

Ω

To Pin19

of ADS-118A

Potentiometer is at 25

Ω during the device's factory trim procedure.

20k

Ω

–15V (or –5V)

+15V (or +5V)

To Pin17

of ADS-118A

ZERO/

OFFSET

ADJUST

To Pin19

of ADS-118

–15V

SIGNAL

INPUT

GAIN

ADJUST

1.98k

Ω

50

Ω

+15V

2k

Ω

1.2M

Ω

20k

Ω

–15V

+15V

ZERO/

OFFSET

ADJUST

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 2a and 2b are guaran-

teed to compensate for the ADS-118'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.

For the ADS-118, 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

+½LSB (+244μV for ADS-118; +305μV for ADS-118A).

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

(+0.99927V for ADS-118; +1.249085V for ADS-118A).

Zero/Offset Adjust Procedure

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

(pin 16) so the converter is continuously converting.

2. Apply +244μV (ADS-118) or +305μV (ADS-118A) to the

Table 1. Output Coding for Bipolar Operation

+FS –1 LSB

+0.99951V

1111 1111 1111

+1.2494V

+3/4 FS

+0.75000V

1110 0000 0000

+0.9375V

+1/2 FS

+0.50000V

1100 0000 0000

+0.6250V

0

0.00000V

1000 0000 0000

0.0000V

–1/2 FS

–0.50000V

0100 0000 0000

–0.6250V

–3/4 FS

–0.75000V

0010 0000 0000

–0.9375V

–FS +1 LSB

–0.99951V

0000 0000 0001

–1.2494V

–FS

–1.00000V

0000 0000 0000

–1.2500V

ADS-118

OUTPUT CODING

ADS-118A

INPUT

INPUT

BIPOLAR

RANGE

OFFSET BINARY

RANGE

SCALE

(±1V )

MSB

LSB

(±1.25V )

ANALOG INPUT (pin 19).

3. Adjust the offset potentiometer until the output bits are

1000 0000 00000 and the LSB flickers between 0 and 1.

Gain Adjust Procedure

1. Apply +0.99927V (ADS-118) or +1.249085V (ADS-118A) to

the ANALOG INPUT (pin 19).

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 1.

Figure 2a. Optional ADS-118 External

Gain and Offset Adjust Circuits

Figure 2b. Optional ADS-118A

Gain and Offset Adjust Circuits