ADS-118A ETC [List of Unclassifed Manufacturers], ADS-118A Datasheet - Page 4

• Manufacturer Part Number: ADS-118A
• Description: 12-Bit, 5MHz, Low-Power Sampling A/D Converters
• Manufacturer: ETC [List of Unclassifed Manufacturers]

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 guaranteed
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
2. Apply +244µV (ADS-118) or +305µV (ADS-118A) to the
OFFSET
ADJUST
ZERO/
(pin 16) so the converter is continuously converting.
ANALOG INPUT (pin 19).
SIGNAL
INPUT
–15V
+15V
20k
Figure 2a. Optional ADS-118 External
ADJUST
GAIN
Gain and Offset Adjust Circuits
50
1.98k
1.2M
+15V
–15V
2k
To Pin19
of ADS-118
4
3. Adjust the offset potentiometer until the output bits are
Gain Adjust Procedure
1. Apply +0.99927V (ADS-118) or +1.249085V (ADS-118A) to
2. Adjust the gain potentiometer until all output bits are 1's
3. To confirm proper operation of the device, vary the input
+FS –1 LSB
–FS +1 LSB
BIPOLAR
1000 0000 00000 and the LSB flickers between 0 and 1.
the ANALOG INPUT (pin 19).
and the LSB flickers between 1 and 0.
signal to obtain the output coding listed in Table 1.
+3/4 FS
+1/2 FS
–1/2 FS
–3/4 FS
SCALE
–FS
0
Potentiometer is at 25 during the device's factory trim procedure.
Table 1. Output Coding for Bipolar Operation
SIGNAL
INPUT
+0.99951V
+0.75000V
+0.50000V
–0.50000V
–0.75000V
–0.99951V
–1.00000V
Gain and Offset Adjust Circuits
ADS-118
RANGE
0.00000V
Figure 2b. Optional ADS-118A
INPUT
OFFSET
ADJUST
(±1V )
ZERO/
–15V (or –5V)
+15V (or +5V)
ADJUST
GAIN
50
20k
OUTPUT CODING
OFFSET BINARY
MSB
1111 1111 1111
1110 0000 0000
1100 0000 0000
1000 0000 0000
0100 0000 0000
0010 0000 0000
0000 0000 0001
0000 0000 0000
To Pin17
of ADS-118A
®
LSB
To Pin19
of ADS-118A
ADS-118A
+1.2494V
+0.9375V
+0.6250V
–0.6250V
–0.9375V
–1.2494V
–1.2500V
(±1.25V )
RANGE
0.0000V
INPUT
®