Software

AN4159

20/27

Doc ID 023639 Rev 3

Case 2:

“The measured pressure altitude by the sensor is different than my portable GPS

Barometer”

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Absolute pressure sensor measures the air pressure at the sensing point. The base

rule of thumb is that the pressure drops by 1 mbar every 8.3 meters.

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The “sea level pressure average” is 1013.25 mbar (standard altitude measurement

done in most of digital barometer wrist watches)

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The “instant sea level pressure” is weather and location dependent and can vary by

more than 5 mbar (compare SFO and SIN airports on the web)

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Also beware that pressure altitude may be different than GPS altitude, which may also

be different than density altitude.

Case 3:

“The PCB design rule on page 4 says:

‘The area below the sensor (on the same side of the board) must be defined as

keepout area. It is strongly recommended not to place any structure in the top metal

layer underneath the sensor.’

Does this mean I can’t even place the ground plane immediately under the LPS331AP? Can

I place the ground plane in the PCB middle layer that is under the LPS331AP?”

●

You can place the ground plane in the PCB middle layer under the LPS331AP, but not

in the plane immediately under the LPS331AP.

Case 4:

“We are going to mount the LPS331AP and the LIS3DH on an accessory. The accessory is

powered by coin batteries and Vdd fluctuations are possible.

For this device/accessory, we plan to use lithium coin battery (CR2032), and directly supply

the voltage to the sensors.

However, depending on the load conditions, the voltage supplied by the coin battery greatly

fluctuates (as much as 0.4 V).

Assuming that the voltage supplied to the sensor is within the “normal operating voltage

conditions” (even after this 0.4 V drop),

Do you foresee any issues with the performance and/or characteristics of LIS3DH and

LPS331AP?”

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The max peak current from pressure sensor is 1 mA. A CR2032 is usually 220 mAH,

3.3 V with around 33

Ω intrinsic resistance

●

As the battery ages, the supply may drop down to 2.6 V.

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As long as the recommended decoupling capacitors are placed near the sensor, our

tests with batteries and switched resistive load shows that it should be fine even with

fast slew rates.

Case 5:

“Does LPS331AP have internal pull-up resistor on the I2C lines?”

●

No