CS5522-AP datasheet(34/56 Pages) CIRRUS

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CS5521/22/23/24/28

DS317F2

The variables are defined below.

First calibration voltage

Second calibration voltage (greater than V0)

Result of any uncalibrated conversion

Ru0

Result of uncalibrated conversion V0

(24-bit integer or 2’s complement)

Ru1

Result of uncalibrated conversion of V1

(24-bit integer or 2’s complement)

Result of any conversion

Rc0

Desired calibrated result of converting V0

(24-bit integer or 2’s complement)

Rc1

Desired calibrated result of converting V1

(24-bit integer or 2’s complement)

Offset calibration register value

(24-bit 2’s complement)

Gain calibration register value

(24-bit integer)

2.3.3 Calibration Tips

Calibration steps are performed at the output word

rate selected by the WR2-WR0 bits of the configu-

ration register. Since higher word rates result in

conversion words with more peak-to-peak noise,

calibration should be performed at lower output

word rates.

Also, to minimize digital noise near

the device, the user should wait for each calibration

step to be completed before reading or writing to

the serial port.

For maximum accuracy, calibrations should be per-

formed for offset and gain (selected by changing

the G2-G0 bits of the desired Setup). Note that only

one gain range can be calibrated per physical chan-

nel. If factory calibration of the user’s system is

performed using the system calibration capabilities

of the CS5521/22/23/24/28, the offset and gain reg-

ister contents can be read by the system microcon-

troller and recorded in EEPROM. These same

calibration words can then be uploaded into the off-

set and gain registers of the converter when power

is first applied to the system, or when the gain range

is changed.

2.3.4 Limitations in Calibration Range

System calibration can be limited by signal head-

room in the analog signal path inside the chip as

discussed under the Analog Input section of this

data sheet. For gain calibration the full scale input

signal can be reduced to the point in which the gain

or FFFFFF (hexadecimal). Under nominal condi-

tions, this occurs with a full scale input signal equal

to about 1/4 the nominal full scale. With the con-

verter’s intrinsic gain error, this full scale input sig-

nal may be higher or lower. In defining the

minimum Full Scale Calibration Range (FSCR)

under ANALOG CHARACTERISTICS, margin is

retained to accommodate the intrinsic gain error.

Alternatively the input full scale signal can be in-

creased to a point in which the modulator reaches

its 1’s density limit of 80 percent, which under

nominal condition occurs when the full scale input

signal is 1.5 times the nominal full scale. With the

chip’s intrinsic gain error, this full scale input sig-

nal may be higher or lower. In defining the maxi-

mum FSCR, margin is again incorporated to

accommodate the intrinsic gain error. In addition,

for full scale inputs greater than the nominal full

scale value of the range selected, there is some volt-

age at which various internal circuits may saturate

due to limited amplifier headroom. This is most

likely to occur in the 100 mV range.

2.4 Performing Conversions and Reading

the Data Conversion FIFO

The CS5521/22/23/24/28 offers various modes of

performing conversions. The sections that follow

detail the differences between the conversion

modes. The sections also provide examples illus-

trating how to use the conversion modes with the

channel-setup registers and to acquire conversions

for further processing. While reading, note that the

CS5521/22 have a FIFO which is four words deep.

The CS5523/24 have a FIFO which is eight words

deep and the CS5528 has a FIFO which is sixteen

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