CS5376A datasheet(60/107 Pages) CIRRUS

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CS5376A

value in the OFFSETx registers (0x25-0x28) from

the digital filter output data word.

Offset correction values are 24-bit two’s comple-

ment with a maximum positive value of 0x7FFFFF,

and a maximum negative value of 0x800000. If ap-

plying an offset correction causes the final result to

exceed a 24-bit two’s complement maximum, the

output data will saturate to that maximum value.

Output Data = Input Data - Offset Correction

Max Positive Output Value = 0x7FFFFF

Max Negative Output Value = 0x800000

Once the OFFSET registers are written, the USE-

OR bit in the FILTCFG register enables offset cor-

rection.

15.3 Offset Calibration

An offset calibration algorithm in the CS5376A

can automatically calculate offset correction val-

ues. When using the offset calibration algorithm,

background noise data should be used as the basis

for calculating the offset value of each measure-

ment channel.

The offset calibration algorithm is an exponential

averaging function that places increased weight on

more recent digital filter data. The exponential

weighting factor is set by the EXP bits in the

FILTCFG register, with larger exponent values

producing a smoother averaging function that re-

quires a longer settling time, and smaller values

producing a noisier averaging function that re-

quires a shorter settling time. Typical exponential

values range from 0x05 to 0x0F, depending on the

available settling time.

The characteristic equations of the offset calibra-

tion algorithm include an input value, X, an output

value, Y, a summation value, YSUM, a sample in-

dex, n, and an exponential value, EXP.

Y(n) = X(n) - [YSUM(n-1) >> EXP]

YSUM(n) = Y(n) + YSUM(n-1)

Offset Correction = YSUM >> EXP

Once the EXP bits are written, the ORCAL bit in

the FILTCFG register is set to enable offset calibra-

tion. When enabled, updated offset correction val-

ues are automatically written to the OFFSETx

registers. When the offset calibration algorithm is

fully settled, the ORCAL bit is cleared to maintain

the final values in the OFFSETx registers.

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