CS5521 datasheet(46/56 Pages) CIRRUS

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CS5521/22/23/24/28

DS317F8

verter stays constant but the number of codes af-

fected is doubled because the code size has been

reduced by half.

The converter input ranges are specified with a

voltage reference of 2.5 V. The device can be op-

erated with the reference tied directly to the +5 V

supply. When this is done, the input span of the in-

put ranges is doubled; the 25 mV range actually be-

comes a 50 mV range. The gain register can be set

to 2.0 (shift contents left one bit) and the input

range will be scaled back to 25 mV. Since the gain

one could scale the input span on the 25 mV range

to accept an analog full-scale span of about

6.25 mV. This is useful for ratiometric bridge mea-

surement of low-level differential outputs.

The gain register can also be scaled manually to a

value lower than 1.0. It is not recommended to use

the devices with the gain register scaled lower than

0.6.

This can enable the converter to accept a

40 mV input signal on the 25 mV range when using

a voltage reference of 2.5 V. Caution though in

scaling the gain register below 1.0 on the 100 mV,

2.5 and 5 volt ranges as the analog signal path into

the converter may saturate before the expected

full-scale code output is produced by the converter.

Note that digital gain scaling will directly influence

the number of digital output codes affected by

noise. The effects can be analytically determined

by calculating the size of the codes (V/Count)

which result from a given gain scaling condition

and relating the amount of noise in the converter

relative to the determined code size. The evalua-

tion board for the converter is a useful tool to aid

the assessment of noise performance with various

voltage reference values, input range settings, and

gain register settings. The evaluation board sup-

ports noise analysis through data capture and noise

histogram analysis.

1.10 Getting Started

The CS5521/22/23/24/28 have many features.

From a software programmer’s perspective, what

should be done first? To begin, a 32.768 kHz crys-

tal takes approximately 500 ms to start-up. To ac-

commodate for this, it is recommended that a

software delay greater than 500 ms precede the

processor’s ADC initialization code before any

registers are accessed in the ADC. This delay time

is dependent on the start-up delay of the clock

source. If a CMOS clock source with no start-up

delay is being used to drive the ADC, then this de-

lay is not necessary.

Once the oscillator is started, the following se-

quence of instructions should be performed to

guarantee the converter begins proper operation:

1) After power is applied, initialize the serial port

using the serial port synchronization sequence.

2) Write a ‘1’ to the reset bit (RS) of the configu-

ration register to reset the converter.

3) Read the configuration register to determine if

the reset valid bit (RV) is set to ‘1’. If the RV

bit is not set, the configuration register should

be read again.

4) When the RV bit has been set to ‘1’, reset the

RS bit back to ‘0’ by writing 0x000000 to the

configuration register. Note that while the RS

bit is set to ‘1’ all other register bits in the ADC

will be reset to their default state, and the RS bit

must be set to ‘0’ for normal operation of the

converters.

Once the RS bit has been set to ‘0’, the ADC is

placed in the command state were it waits for a val-

id command to execute. The next step is to load the

configuration register and then the channel setup

registers with conditions that you have decided. If

you need to do a factory calibration, perform offset

and gain calibrations for each channel that is to be

used. Then off-load the offset and gain register

contents into EEPROM. These registers can then

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