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Document number: BST-BME280-DS001-23 Revision_1.23_012022

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When the IIR filter is disabled, the temperature resolution is 16 + (osrs_t – 1) bit, e.g. 18 bit

when osrs_t is set to ‘3’.

3.4.4

IIR filter

The humidity value inside the sensor does not fluctuate rapidly and does not require low pass filtering.

However, the environmental pressure is subject to many short-term changes, caused e.g. by

slamming of a door or window, or wind blowing into the sensor. To suppress these disturbances in the

output data without causing additional interface traffic and processor work load, the BME280 features

an internal IIR filter. It effectively reduces the bandwidth of the temperature and pressure output

signals10 and increases the resolution of the pressure and temperature output data to 20 bit. The

output of a next measurement step is filtered using the following formula:

data_filtered=data_filtered_old⋅(filter_coefficient−1)+data_ADC

filter_coefficient

Data_filtered_oldisthedatacomingfromthecurrentfiltermemory,anddata_ADCisthedatacoming

from current ADC acquisition.

Data_filteredisthenewvalueoffiltermemoryandthevaluethatwillbe

sent to the output registers.

The IIR filter can be configured to different filter coefficients, which slows down the response to the

sensor inputs. Note that the response time with enabled IIR filter depends on the number of samples

generated, which means that the data output rate must be known to calculate the actual response

time. For register configuration, please refer to Table 28. A sample response time calculation is shown

in chapter 9.4.

Table 6: filter settings

Filter coefficient

Samples to reach ≥75 %

of step response

Filter off

In order to find a suitable setting for filter, please consult chapter 3.5.

When writing to the register filter, the filter is reset. The next ADC values will the pass through the filter

unchanged and become the initial memory values for the filter. If temperature or pressure

measurements are skipped, the corresponding filter memory will be kept unchanged even though the

output registers are set to 0x80000. When the previously skipped measurement is re-enabled, the

output will be filtered using the filter memory from the last time when the measurement was not

skipped. If this is not desired, please write to the filter register in order to re-initialize the filter.

Since the BME280 does not sample continuously, filtering can suffer from signals with a frequency higher than the sampling

rate of the sensor. E.g. environmental fluctuations caused by windows being opened and closed might have a frequency <5 Hz.

Consequently, a sampling rate of ODR = 10 Hz is sufficient to obey the Nyquist theorem.

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