Cirrus Logic CS5530 - Manual (Page 21)

CS5530
DS742F3 21
2.4 Calibration
Calibration is used to set the zero and gain slope of
the ADC’s transfer function. The CS5530 provides
system calibration.
Note: After the ADC is reset, it is functional and can
perform measurements without being
calibrated (remember that the VRS bit in the
configuration register must be properly
configured). If the converter is operated
without calibraton, the converter will utilize
the initialized values of the on-chip registers
(Offset = 0.0; Gain = 1.0) to calculate output
words. Any initial offset and gain errors in the
internal circuitry of the chip will remain.
2.4.1 Calibration Registers
The CS5530 converter has an offset register that is
used to set the zero point of the converter’s transfer
function. As shown in Offset Register section, one
LSB in the offset register is 1.835007966 X 2
-24
proportion of the input span (bipolar span is 2 times
the unipolar span, gain register = 1.000...000 deci-
mal). The MSB in the offset register determines if
the offset to be trimmed is positive or negative (0
positive, 1 negative). Note that the magnitude of
the offset that is trimmed from the input is mapped
through the gain register. The converter can typi-
cally trim ±100 percent of the input span. As shown
in the Gain Register section, the gain register spans
from 0 to (64 - 2
-24
). The decimal equivalent mean-
ing of the gain register is
where the binary numbers have a value of either
zero or one (b
D29
is the binary value of bit D29).
While gain register settings of up to 64 - 2
-24
are
available, the gain register should never be set to
values above 40.
2.4.2 Gain Register
The gain register span is from 0 to (64-2
-24
). After Reset D24 is 1, all other bits are ‘0’.
2.4.3 Offset Register
One LSB represents 1.835007966 X 2
-24
proportion of the input span (bipolar span is 2 times unipolar span).
Offset and data word bits align by MSB. After reset, all bits are ‘0’.
The offset register is stored as a 32-bit, two’s complement number, where the last 8 bits are all 0.
D b
D29
2
5
b
D28
2
4
b
D27
2
3
… b
D0
2
24 –
) + + + + b
Di
2
24 – i + ( )
i 0 =
29

= =
Decimal Point
MSB D30 D29 D28 D27 D26 D25 D24 D23 D22 D21 D20 D19 D18 D17 D16
NU NU 2 5 2 4 2 3 2 2 2 1 2 0 2 -1 2 -2 2 -3 2 -4 2 -5 2 -6 2 -7 2 -8
0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 LSB
2
-9
2
-10
2
-11
2
-12
2
-13
2
-14
2
-15
2
-16
2
-17
2
-18
2
-19
2
-20
2
-21
2
22
2
-23
2
-24
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
MSB D30 D29 D28 D27 D26 D25 D24 D23 D22 D21 D20 D19 D18 D17 D16
Sign 2 -2 2 -3 2 -4 2 -5 2 -6 2 -7 2 -8 2 -9 2 -10 2 -11 2 -12 2 -13 2 -14 2 -15 2 -16
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 LSB
2
-17
2
-18
2
-19
2
-20
2
-21
2
-22
2
-23
2
-24 NU NU NU NU NU NU NU NU
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0