CS5532-AS datasheet(35/52 Pages) CIRRUS

35 / 52 page

CS5531/32/33/34

DS289PP5

sult in the loss of synchronization between the mi-

crocontroller and the ADC, and may prematurely

halt the calibration cycle.

For maximum accuracy, calibrations should be per-

formed for both offset and gain (selected by chang-

ing the G2-G0 bits of the channel-setup registers).

Note that only one gain range can be calibrated per

physical channel when the OGS bit in the Configu-

ration Register is set to ‘0’. Multiple gain ranges

can be calibrated for a single channel by manipulat-

ing the OGS bit and the OG1-OG0 bits of the se-

lected Setup (see Section 2.3.7 for more details). If

factory calibration of the user’s system is per-

formed using the system calibration capabilities of

the CS5531/32/33/34, the offset and gain register

contents can be read by the system microcontroller

and recorded in non-volatile memory. 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.

When the device is used without calibration, the

uncalibrated gain accuracy is about ±1 percent and

the gain tracking from range (2X to 64X) to range

is approximately ±0.3 percent.

Note that the gain from the offset register to the

output is 1.83007966 decimal, not 1. If a user wants

to adjust the calibration coefficients externally,

they will need to divide the information to be writ-

ten to the offset register by the scale factor of

1.83007966. (This discussion assumes that the gain

is also multiplied by the gain register before being

applied to the output conversion words).

2.5.8. 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 3% of the nominal full-

scale value. At this point, the gain register is ap-

proximately equal to 33.33 (decimal). While the

gain register can hold numbers all the way up to 64

-2-24, gain register settings above a decimal value

of 40 should not be used. With the converter’sin-

trinsic gain error, this minimum 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. Inversely, the

input full scale signal can be increased to a point in

which the modulator reaches its 1’s density limit of

86 percent, which under nominal conditions occurs

when the full scale input signal is 1.1 times the

nominal full scale value. With the chip’s intrinsic

gain error, this maximum full scale input signal

maybe higher or lower. In defining the maximum

FSCR, margin is again incorporated to accommo-

date the intrinsic gain error.

2.6. Performing Conversions

The CS5531/32/33/34 offers two distinctly differ-

ent conversion modes. The three sections that fol-

low detail the differences and provide examples

illustrating how to use the conversion modes with

the channel-setup registers.

2.6.1. Single Conversion Mode

Based on the information provided in the channel-

setup registers (CSRs), after the user transmits the

conversion command, a single, fully-settled con-

version is performed. The command byte includes

a pointer address to the Setup register to be used

during the conversion. Once transmitted, the serial

port enters data mode where it waits until the con-

version is complete. When the conversion data is

available, SDO falls to logic 0. Forty SCLKs are

then needed to read the conversion data word. The

first 8 SCLKs are used to clear the SDO flag. Dur-

ing the first 8 SCLKs, SDI must be logic 0. The last

32 SCLKs are needed to read the conversion result.

Note that the user is forced to read the conversion

in single conversion mode as SDO will remain low

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