ADS -927
4
CALIBRATION PROCEDURE
(Refer to Figures 2 and 3)
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 circuit of Figure 2 are guaranteed to
compensate for the ADS-927's initial accuracy errors and may
not be able to compensate for additional system errors.
All fixed resistors in Figure 2 should be metal-film types, and
multiturn potentiometers should have TCR’s of 100ppm/°C or
less to minimize drift with temperature.
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-927, 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
+LSB (+305μV).
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 LSB's
(+4.999085V).
Zero/Offset Adjust Procedure
1. Apply a train of pulses to the START CONVERT input
(pin 16) so the converter is continuously converting. If
using LED's on the outputs, a 200kHz conversion rate will
reduce flicker.
2. Apply +305μV to the ANALOG INPUT (pin 20).
3. Adjust the offset potentiometer until the output bits are
a 1 and all 0's and the LSB flickers between 0 and 1.
Gain Adjust Procedure
1. Apply +4.999085V to the ANALOG INPUT (pin 20).
2. Adjust the gain potentiometer until the output bits are all 1's
and the LSB flickers between 1 and 0.
INPUT VOLTAGE
RANGE
ZERO ADJUST
+ LSB
GAIN ADJUST
+FS –1 LSB
±5V
+305μV
+4.999085V
Table 1. Zero and Gain Adjust
OUTPUT CODING
MSB
INPUT RANGE
±5V
BIPOLAR
SCALE
LSB
11 1111 1111 1111
11 1000 0000 0000
11 0000 0000 0000
10 0000 0000 0000
01 0000 0000 0000
00 1000 0000 0000
00 0000 0000 0001
00 0000 0000 0000
+4.99939
+3.75000
+2.50000
0.00000
–2.50000
–3.75000
–4.99939
–5.00000
+FS –1 LSB
+3/4 FS
+1/2FS
0
–1/2FS
–3/4FS
–FS +1 LSB
–FS
Table 2. Output Coding
Figure 3. Typical ADS-927 Connection Diagram
Figure 2. ADS-927 Calibration Circuit
Coding is offset binary; 1LSB = 610μV.
To Pin 20
–15V
SINPUT
AGAIN
1.98k
50
+15V
2k
1.2M
20k
–15V
+15V
OZERO/
13
ADS-927
14
20
16
18
17
12
11
10
9
8
7
6
5
4
3
2
1
15
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
ANALOG
INPUT
START
CONVERT
19, 23
22
24
0.1μF
4.7μF
+5V
0.1μF
4.7μF
0.1μF
4.7μF
+
+
–12V/–15V
+12V/+15V
+
0.1μF
+
4.7μF
21 +10V REF. OUT
DIGITAL
GROUND
0 to +10V
ANALOG
GROUND