®
®
ADS-118/118A
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 1�½ 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
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.
Table 1. Output Coding for Bipolar Operation
ADS-118
INPUT
RANGE
(±1V )
+0.99951V
+0.75000V
+0.50000V
0.00000V
–0.50000V
–0.75000V
–0.99951V
–1.00000V
OUTPUT CODING
OFFSET BINARY
MSB
LSB
1111 1111 1111
1110 0000 0000
1100 0000 0000
1000 0000 0000
0100 0000 0000
0010 0000 0000
0000 0000 0001
0000 0000 0000
ADS-118A
INPUT
RANGE
(±1.25V )
+1.2494V
+0.9375V
+0.6250V
0.0000V
–0.6250V
–0.9375V
–1.2494V
–1.2500V
BIPOLAR
SCALE
+FS –1 LSB
+3/4 FS
+1/2 FS
0
–1/2 FS
–3/4 FS
–FS +1 LSB
–FS
+15V
ZERO/
OFFSET
ADJUST
20kΩ
1.2MΩ
2kΩ
GAIN
ADJUST
SIGNAL
INPUT
GAIN
ADJUST
+15V
1.98kΩ
50Ω
To Pin19
of ADS-118A
–15V
Potentiometer is at 25Ω during the device's factory trim procedure.
SIGNAL
INPUT
50Ω
+15V (or +5V)
To Pin19
of ADS-118
–15V
ZERO/
OFFSET
ADJUST
20kΩ
To Pin17
of ADS-118A
–15V (or –5V)
Figure 2a. Optional ADS-118 External
Gain and Offset Adjust Circuits
Figure 2b. Optional ADS-118A
Gain and Offset Adjust Circuits
4
MURATA-PS [ MURATA POWER SOLUTIONS INC. ]
