CS5378 datasheet(57/88 Pages) CIRRUS

57 / 88 page

CS5378

DS639F2

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 register is written, the USEOR

bit in the FILTCFG register enables offset correc-

tion.

15.3 Offset Calibration

An offset calibration algorithm in the CS5378 can

automatically calculate an offset correction value.

When using the offset calibration algorithm, back-

ground noise data should be used as the input signal

for calculating the offset of the measurement chan-

nel.

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, an updated offset correction

value is automatically written to the OFFSET reg-

ister. When the offset calibration algorithm is fully

settled, the ORCAL bit should be cleared to main-

tain the final value in the OFFSET register.

CS5378 Datasheet(PDF) 57 Page - Cirrus Logic

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