TN1180

Technical note

How to interpret pressure and temperature readings

in pressure sensors

By Tom Bocchino

Introduction

The purpose of this technical note is to provide a review of two's complement notation for ease of

design in pressure sensors and to guide the user in the interpretation of pressure and temperature

values in the device registers. MEMS pressure sensors measure absolute pressure (P) and

temperature (t) and store the values in two's complement registers which can be read via the

I

The pressure sensor stores pressure as a 24-bit word. Both values are stored as two's complement

integers as illustrated below.

Figure 1: Pressure sensor analog-to-digital data flow

May 2014

DocID026298 Rev 1

1/9

www.st.com

Low Noise

AnalogFront

End

(AFE)

Analogto

Digital

Conversion

(ADC)

Digital Control

Signal Processing

(DSP)

&

Register Bank

P

t

I

SPI

AnalogDomain

Digital Domain

PressureOut_XL (LSB)

28h

PressureOut_L (MIDDLE)

29h

PressureOut_H (MSB)

2Ah

Byte Description

Addr(h)

TemperatureOut_L (LSB)

2Bh

TemperatureOut_H (MSB)

2Ch

24-bit Pressure count

Two’s complement value

16-bit Temperature count

Two’s complement value

Sensors

and Bias