AN4207

Application examples

Doc ID 023945 Rev 1

21/28

4.2

Pressure measurement

This application is intended to show how to use the 16-bit sigma-delta analog-to-digital

converter to perform pressure measurement using the absolute pressure sensor

MPX2102A mounted on the STM32373C-EVAL evaluation board.

The application source code is available in the

Project\STM32F37x_StdPeriph_Examples\SDADC

folder in the STM32F37x DSP and standard peripherals library package

stm32f37x_dsp_stdperiph_lib

.

On the STM32373C-EVAL board, The MPX2102A sensor is connected to SDADC1 channel

8P (PE8) and channel 8N (PE9). The MPX2102A sensitivity when powered by 3.3 V is 3.3 V

* 40 mV / 10 V = 13.2 mV/1000 mB = 13.2 microV/mB. To increase the sensitivity an

external 45.1 gain is applied, using the TVS632 operational amplifier installed on the

STM32373C-EVAL. The same operational amplifier is used to shift-down the input voltage

by 3.3 V/10 = 0.33 V

Note:

Refer to STM32373C-EVAL user manual for more details about how the MPX2102A is

connected to PE8 and PE9

The SDADC channel 8 is configured in differential mode. The external reference VREF (set

to 3.3 V on STM32373C-EVAL) is used as reference for SDADC. The conversion is

triggered by the TIM19 timer with interrupt enabled on End of Injected Conversion.

The input voltage is calculated using the formula below:

inputvoltage = (InjectedConvData * (SDADC_VREF / (SDADC_RESOL * SDADC_GAIN))) +

OFFSET_VOLTAGE;

Where:

InjectedConvData: The digital value read from SDADC data register

SDADC_VREF: is the sigma delta converter voltage reference: set externally to 3.3V

SDADC_RESOL: is the sigma delta converter resolution: 2e16-1

SDADC_GAIN: is the internal SDADC gain. In this example it is set to 4

OFFSET_VOLTAGE: The offset voltage added by the operational amplifier TVS632

(approximately 3.3 V/10 = 0.33 V)

And the pressure is calculated using the formula below:

PressuremB = ((1000000 * inputvoltage) / (MPX2102_SENSITIVITY * EXTERNGAIN));

Table 6.

Pressure sensor voltage range

Pressure (HPa)

Differential voltage on

sensor outputs

Differential voltage on

SDADC inputs

800

10.56 mV

146.256 mV

1000

13.2 mV

265.32 mV

1200

15.84 mV

384.384 mV