CS5522-AP datasheet(32/56 Pages) CIRRUS

32 / 56 page

CS5521/22/23/24/28

DS317F2

tion of offset to occur in the 25 mV, 55 mV, and

100 mV ranges, the AIN- pin must be at the proper

common-mode-voltage as specified in ‘Common

Mode +Signal AIN+/-’ specification in the Analog

Input section (if AIN- = 0 V, NBV must be between

-1.8 V to -2.5 V). For self-calibration of offset in the

1.0 V, 2.5 V, and 5 V ranges, the inputs of the mod-

ulator are connected together and then routed to the

VREF- pin as shown in Figure 12.

For self-calibration of gain, the differential inputs

of the modulator are connected to VREF+ and

VREF- as shown in Figure 13. For any input range

other than the 2.5 V range, the converter’s gain er-

ror can not be completely calibrated out when using

self-calibration. This is due to the lack of an accu-

rate full scale voltage internal to the chips. The

2.5 V range is an exception because the external

reference voltage is 2.5 V nominal and is used as

the full scale voltage. In addition, when self-cali-

bration of gain is performed in the 25 mV, 55 mV,

and 100 mV input ranges, the instrumentation am-

plifier’s gain is not calibrated. These two factors

can leave the converters with a gain error of up to

±20% after self-calibration of gain. Therefore, a

system gain calibration is required to get better ac-

curacy, except for the 2.5 V range.

2.3.2 System Calibration

For the system calibration functions, the user must

supply the calibration signals to the converter which

represent ground and full scale. When a system offset

calibration is performed, a ground referenced signal

must

applied

the

converters.

See

Figures 14 and 15.

As shown in Figures 16 and 17, the user must input

a signal representing the positive full scale point to

AIN+

AIN-

OPEN

CLOSED

X20

Figure 11. Self Calibration of Offset (Low Ranges)

AIN+

AIN-

OPEN

X20

OPEN

CLOSED

VREF-

Figure 12. Self Calibration of Offset (High Ranges)

AIN+

AIN-

OPEN

X20

OPEN

CLOSED

VREF+

CLOSED

VREF-

Reference

Figure 13. Self Calibration of Gain (All Ranges)

X20

External

Connections

AIN+

AIN-

CM +

Figure 14. System Calibration of Offset (Low Ranges)

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