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ADS-949GC Datasheet PDF : 8 Pages
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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.
Table 1. Gain and Zero Adjust
INPUT VOLTAGE
RANGE
±2.5V
ZERO ADJUST
+½ LSB
+153µV
GAIN ADJUST
+FS –1½ LSB
+2.49954V
–5V
+15V
4.7µF 4.7µF
+
+
+5VA
4.7µF
+
+5VD
4.7µF
+
0.1µF 0.1µF
0.1µF
0.1µF
+5V
GAIN
ADJUST
20K
–5V
BIPOLAR
ANALOG
INPUT
+5V
3
28 29 30 27 26
–5V
+15V AGND +5VA +5VD DGND
5 RANGE REF ➀
2 VIN B
1VIN A
31 GAIN ADJUST
ADS-949
ZERO/
OFFSET
ADJUST
4 OFFSET ADJUST
20k�
20k
–5V
START
CONVERT
32 RANGE ➀
6 2.5V REF ➀
7 START CONVERT
9 ENABLE
23 BIT 1 (MSB)
22 BIT 2
21 BIT 3
20 BIT 4
19 BIT 5
18 BIT 6
17 BIT 7
16 BIT 8
15 BIT 9
14 BIT 10
13 BIT 11
12 BIT 12
11 BIT 13
10 BIT 14 (LSB)
8 EOC
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
Figure 2. Typical ADS-949 Bipolar Connection Diagram
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MDC_ADS-949.B01 Page 4 of 8
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