CS5550 datasheet(10/24 Pages) CIRRUS

10 / 24 page

CS5550

DS630F1

2.1 Theory of Operation

The analog signals at the analog inputs are subject

to the gains of the input PGAs. These signals are

then sampled by the delta-sigma modulators at a

rate of (MCLK/K) / 8.

2.1.1 High-Rate Digital Low-Pass Filters

The data is then low-pass filtered, to remove

high-frequency noise from the modulator output.

The high rate filters on both channels are imple-

mented as fixed Sinc3 filters.

2.1.2 Digital Compensation Filters

The data from both channels is then passed

through two 4th-order IIR “compensation” filters,

whose purpose is to correct (compensate) for the

magnitude roll-off of the low-pass filtering opera-

tion. These filters “re-flatten” the magnitude re-

sponse of the AIN1 and AIN2 channels over the

relevant frequency range, by correcting for the

magnitude roll-off effects that are induced by the

Sinc3 low-pass filter stages.

2.1.3 Gain and Offset Adjustment

After the filtering, the digital codes are subjected to

value adjustments, based on the values in the DC

Offset Registers (additive) and the Gain Registers

(multiplicative). These registers are used for cali-

bration of the device (see Section 3.4, Calibration).

After offset and gain, the data is available to the

user by reading the appropriate registers.

2.2 Performing Measurements

The CS5550 performs measurements at an output

word rate (sampling rate) of (MCLK/K) / 1024.

From these instantaneous samples, FILT1 and

FILT2 are computed, using the most recent N in-

stantaneous samples that were acquired. All of the

measurements/results are available as a percent-

age of full scale. The signed output format is a

two’s complement format, and the output data

words represent a normalized value between -1

and +1. The unsigned data in the CS5550 output

registers represent normalized values between 0

and 1. A register value of 1 represents the maxi-

mum possible value. Note that a value of 1.0 is

never actually obtained, the true maximum register

value is [(2^23 - 1) / (2^23)] = 0.999999880791.

After each A/D conversion, the CRDY bit will be as-

serted in the Status Register, and the INT pin will

also become active if the CRDY bit is unmasked (in

the Mask Register). The assertion of the CRDY bit

indicates that new instantaneous samples have

been collected.

The unsigned FILT1 and FILT2 calculations are up-

dated every N conversions (which is known as 1

“computation cycle”) where N is the value in the

Cycle Count Register. At the end of each computa-

tion cycle, the DRDY bit in the Mask Register will

be set, and the INT pin will become active if the

DRDY bit is unmasked.

DRDY is set only after each computation cycle has

completed, whereas the CRDY bit is asserted after

each individual A/D conversion. When these bits

are asserted, they must be cleared before they can

be asserted again. If the Cycle Count Register val-

ue (N) is set to 1, all output calculations are instan-

taneous,

and

DRDY

will

indicate

when

instantaneous calculations are finished, just like

the CRDY bit. For the FILT results to be valid, the

Cycle-Count Register must be set to a value great-

er than 10.

A computation cycle is derived from the master

clock and its frequency is (MCLK/K)/(1024*N). Un-

der default conditions with a 4.096 MHz clock at

XIN, instantaneous A/D conversions are per-

formed at a 4000 Hz rate, whereas FILT calcula-

tions are performed at a 1 Hz rate.

2.3 CS5550 Linearity Performance

Table 2 lists the range of input levels (as a percent-

age of full-scale registration in the FILT Registers)

over which the output linearity of the FILT Register

measurements are guaranteed to be within ±0.1%.

FILT1

FILT2

Range (% of FS)

0.2% - 100%

1% - 100%

Linearity

0.1% of

reading

0.1% of

reading

Table 2. Available range of ±0.1% output

linearity, with default settings in the

gain/offset registers.

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