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AN4207

Application examples

Doc ID 023945 Rev 1

19/28

Application examples

This section presents how the 16-bit SDADC embedded in STM32F37x/38x devices can be

used in application examples such as temperature measurement, pressure measurement,

three-phase power meter, voice recorder...

4.1

Temperature measurement

This purpose of this application is to show how to use the 16-bit resolution Sigma-Delta

Analog-to-Digital converter to perform accurate temperature measurement using the PT100

sensor connected to PE7 in the STM32373C-EVAL evaluation board.

The application source code is available in are available in the

Project\STM32F37x_StdPeriph_Examples\SDADC

folder in the STM32F37x DSP and standard peripherals library package

stm32f37x_dsp_stdperiph_lib

A current source circuit available on the STM32373C-EVAL board is used to provide a fixed

1 mA current (when VDD = 3.3 V) to the temperature sensor PT100 which is connected to

SDADC1 channel 3P (PE7) through a reference resistor 1K8 labeled R33.

The SDADC is configured in single-ended offset mode. The input range is from 0 V to

VREF/(2*gain).

In this application, the SDADC internal gain is set to 8 so the range is from 0 V to VREF/16.

The external reference VREF (set to 3.3V on STM32373C-EVAL) is used as reference for

SDADC using JP17 so the measurement ranges between 0 V and VREF/16 = 0.20625V. The

conversion is performed in continuous mode with interrupts enabled on End Of Regular

Conversion.

The temperature is computed using the formula below:

Rpt100 = 100 + 0.385 * T

 T = (Rpt100 - 100) / 0.385

Vpt100 = Rpt100 * Ipt100 = Rpt100 * VDD_ANA / 2 * Rref

 Rpt100 = Vpt100 * 1800 * 2 / VDD_ANA

 T = ((Vpt100 * 1800 * 2 / VDD_ANA) - 100) / 0.385

where:

Rpt100 is the resistance of the PT100 sensor

Vpt100 is the voltage measured on PT100 sensor

Ipt100 is ~ 1mA current crossing the PT100 sensor

VDD_ANA is the analog voltage

Rref is the reference resistor 1K8 labeled R33 on STM32373C-EVAL

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