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Version 0.8 – January 2017

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4. Measurement Modes

The SDP3x-Digital is a highly versatile differential pressure sensor and is very flexible regarding the measurement speed. This

flexibility not only allows for optimizing the sensor’s performance to a specific application, but also for adapting the sensor to

different use cases. For example in one use case the sensor is detecting the smallest and quickest changes, whereas at

another time the sensor can measure in larger intervals while consuming only little energy.

4.1

Continuous Mode vs Triggered Mode

In continuous mode the sensor is measuring at the highest speed and writes the measurement values to the I2C results buffer,

where the I2C master can read out the value when it requires.

In triggered mode the sensor is default in an idle state and wakes up when the command is sent. It then powers up the heater

and does a measurement. During this time the sensor doesn’t acknowledge any I2C read header or stretches the clock. When

the measurement is finished the sensor returns to the idle state and makes the measurement result available to be read out.

For more details see chapter 5.3.3.

Continuous mode

Triggered mode

Description

Measures continuously

Measures once after command is sent

Measurement speed and

rate

Measurement result can be read out

continuously and at any time, but not faster

than 1ms (upgrade to 0.5ms expected in

March 2017).

Measurement result is available within +/-

45ms after command.

Clock stretching is available.

Measurement method

Sensor configuration is optimized for speed

and accuracy.

Sensor configuration is optimized for low

power consumption.

Recommended use

Best used where speed and accuracy are

most important.

Best used where energy consumption is

more important than speed.

4.2

Continuous Mode and ‘Average till Read’

In continuous measurement modes a new measurement result is available every 1ms (upgrade to 0.5ms expected in March

2017). A new value can be read out every 1 ms and the IRQn will go low when a new measurement result is available.

If the ‘average till read’ option is chosen, the sensor averages all values (xi) prior to the read out. This has the benefit that the

user can read out the sensor at its own desired speed, without losing information and thus prevents aliasing. During the first

25 ms of averaging the averaged value is obtained as the arithmetic mean.

������̅ = ∑

������������

������

������

������=1

for ������ < 25 ms

When the reading speed is even slower than 25 ms, the sensor will continue to average, but with another algorithm. In this

algorithm exponential smoothing is used, with a smoothing factor

������ = 0.05.

������������ = ������ ∙ ������������ + (1 − ������) ∙ ������������−1,

������0 = ������,

̅

for ������ > 25 ms

Where S0 is the arithmetic value after the first 25 ms.

Please refer to relevant literature for more information about exponential smoothing.

4.3

Temperature Compensation Modes and Absolute Pressure Dependency

The SDP3x is temperature compensated both for differential pressure and for mass flow compensated differential pressure. In

use cases where the SDP3x is used to measure mass flow it is advised to use mass flow temperature compensation. In this

case no absolute pressure compensation is required.

For more information about temperature and absolute pressure compensation for differential pressure sensors, for example

volume flow measurements in bypass, refer to the selection guide in the differential pressure download center on our website.