CS5521 datasheet(31/56 Pages) CIRRUS

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CS5521/22/23/24/28

DS317F8

1.3 Calibration

The CS5521/22/23/24/28 offer four different cali-

bration functions including self calibration and sys-

tem calibration. However, after the devices are

reset, the converter is functional and can perform

measurements without being calibrated. In this

case, the converter will utilize the initialized values

of the on-chip registers (Gain = 1.0, Offset = 0.0)

to calculate output words for the

±100 mV range.

Any initial offset and gain errors in the internal cir-

cuitry of the chip will remain.

The gain and offset registers, which are used for

both self and system calibration, are used to set the

zero and full-scale points of the converter’s transfer

function. One LSB in the offset register is 2-24 pro-

portion of the input span when the gain register is

set to 1.0 decimal (bipolar span is 2 times the uni-

polar span). The MSB in the offset register deter-

mines if the offset to be trimmed is positive or

negative (0 positive, 1 negative). The converter can

typically trim ±50 percent of the input span. The

gain register spans from 0 to (4 - 2-22). The decimal

equivalent meaning of the gain register is:

where the binary numbers have a value of either

zero or one (b0 corresponds to bit MSB-1, N=22).

Refer to Table 5 for details.

The offset and gain calibration steps each take one

conversion cycle to complete. At the end of the cal-

ibration step, SDO falls to indicate that the calibra-

tion has finished.

1.3.1 Self Calibration

The CS5521/22/23/24/28 offer both self-offset and

self-gain calibrations. For self calibration of offset

in the 25 mV, 55 mV, and 100 mv ranges, the con-

verters internally tie the inputs of the instrumenta-

tion amplifier together and route them to the AIN-

pin as shown in Figure 11 (in the CS5528 they are

routed to AGND). For proper self-calibration of

Table 5. Offset and Gain Registers

Offset Register

One LSB represents 2-24 proportion of the input span when gain register is set to 1.0 decimal (bipolar span is

2 times unipolar span)

Offset and data word bits align by MSB (bit MSB-4 of offset register changes bit MSB-4 of data)

Gain Register

The gain register span is from 0 to (4-2-22). After Reset the (MSB-1) bit is 1, all other bits are 0.

MSB

LSB

Sign

2-2

2-3

2-4

2-5

2-6

2-19

2-20

2-21

2-22

2-23

2-24

Reset (R)

000000

MSB

LSB

2-1

2-2

2-3

2-4

2-17

2-18

2-19

2-20

2-21

2-22

Reset (R)

000000

MSB

(

–

… b

– )

MSB

–



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