®
®
ADS-929
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-929'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-929, 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
+�½ 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�½ LSB's
(+4.999085V).
+15V
ZERO/
OFFSET
ADJUST
20k
Ω
200k
Ω
2k
Ω
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.
Table 1. Zero and Gain Adjust
INPUT VOLTAGE
RANGE
±5V
ZERO ADJUST
+�½ LSB
+305µV
GAIN ADJUST
+FS –1�½ LSB
+4.999085V
Table 2. Output Coding
OUTPUT CODING
MSB
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
INPUT RANGE
±5V
+4.99939
+3.75000
+2.50000
0.00000
–2.50000
–3.75000
–4.99939
–5.00000
BIPOLAR
SCALE
+FS –1 LSB
+3/4 FS
+1/2FS
0
–1/2FS
–3/4FS
–FS +1 LSB
–FS
–15V
SIGNAL
INPUT
GAIN
ADJUST
+15V
1.98k
Ω
50
Ω
To Pin 20
of ADS-929
Coding is offset binary; 1LSB = 610µV.
–15V
Figure 2. ADS-929 Calibration Circuit
18 BIT 1 (MSB)
+5V
4.7µF
+
0.1µF
14
DIGITAL
GROUND
13
17 BIT 2
12 BIT 3
11 BIT 4
10 BIT 5
9 BIT 6
8 BIT 7
7 BIT 8
6 BIT 9
5 BIT 10
4 BIT 11
3 BIT 12
2 BIT 13
1 BIT 14 (LSB)
15 EOC
START
16
CONVERT
–12V/–15V
4.7µF
+
4.7µF
+
+12V/+15V
–5V to +5V
0.1µF
24
ADS-929
ANALOG
19, 23 GROUND
0.1µF
22
ANALOG
20 INPUT
21 +10V REF. OUT
0.1µF
+
4.7µF
Figure 3. Typical ADS-929 Connection Diagram
4
MURATA-PS [ MURATA POWER SOLUTIONS INC. ]
