AN4207 - STMicroelectronics

AN4207 - STMicroelectronics
AN4207
Application note
Getting started with STM32F37x/38x SDADC (Sigma-Delta ADC)
Introduction
The STM32F37x series of microcontrollers combines a 32-bit ARM Cortex-M4 core with a
DSP and FPU instructions running at 72 MHz with advanced analog peripherals.
This series introduces the combination of a Cortex-M4 core with a precise 16-bit SigmaDelta ADC.
This document outlines the main features of the SDADC and shows how the SDADC can be
used in various application cases. By way of example, four application cases are presented
in this document:
1.
Temperature measurement using PT100
2.
Pressure measurement using MPX2102A
3.
Wave recorder
4.
ElectroCardioGram (ECG) acquisition
To help you get started quickly, the four application cases are implemented in C language
and are available as part of the STM32F37x DSP and standard peripherals library package
stm32f37x_dsp_stdperiph_lib and the STM32373C-EVAL demonstration firmware package
stm32373c-eval_fw
Please note that this document is not intended to replace the Sigma-Delta Analog to Digital
Converter (SDADC) section in the STM32F37xx/STM32F38xx reference manual RM0313.
All values given in this document are guidance only. Please refer to the related datasheet to
get guaranteed and up-to-date values.
Table 1.
Applicable products
Type
Microcontrollers
December 2012
Part numbers
STM32F37X, STM32F38x
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www.st.com
Contents
AN4207
Contents
1
Basics of sigma-delta converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2
Overview of 16-bit SDADC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1
Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.1
Clock selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.2
Input modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.3
SDADC voltage references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1.4
Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.5
Matching impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.1.6
Low power modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3
Sigma-delta (SD) vs. successive approximation
register (SAR) analog-to-digital converters . . . . . . . . . . . . . . . . . . . . . 17
4
Application examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5
2/28
4.1
Temperature measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2
Pressure measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.3
Wave recorder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.4
ElectroCardioGram (ECG) acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.5
Power meter application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Doc ID 023945 Rev 1
AN4207
List of tables
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Applicable products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Voltage step sizes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Typical SDADC input channel input impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
SDADC vs. SAR ADC feature comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Temperature sensor voltage range. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Pressure sensor voltage range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
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List of figures
AN4207
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
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Block diagram of a sigma-delta analog to digital converter . . . . . . . . . . . . . . . . . . . . . . . . . 5
Input signal range in differential mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Typical connection of differential sensor to differential channels . . . . . . . . . . . . . . . . . . . . . 8
Input signal range in single ended offset mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Input signal range in single ended zero reference mode . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Offset error in SDADC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Gain error in SDADC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Gain calibration using the accurate reference voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
SDADC software calibration sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Equivalent input circuit for input channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
ADC architecture vs. resolution and sampling rate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Block diagram of voice recorder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Flowchart of wave recorder application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Block diagram of the electrocardiogram (ECG) acquisition application . . . . . . . . . . . . . . . 24
Flowchart of ECG acquisition application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Three phase power meter application using STM32F37x/38x devices . . . . . . . . . . . . . . . 26
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AN4207
1
Basics of sigma-delta converters
Basics of sigma-delta converters
Sigma-delta converters, also known as oversampling converters, consist of two basic
circuits: a modulator and a digital filter (Figure 1). In the modulator, the input signal is added
to the negative feedback signal from the digital to analog converter (DAC). The signal’s
difference, after passing through the integrating circuit, reaches the input of the comparator
(1-bit ADC), where it is compared to the reference voltage (the comparator works as a 1-bit
quantizer). The input signal from the comparator (1-bit ADC) controls the 1-bit converter and
reaches the input of the digital filter, which decreases flowability and transforms the 1-bit
stream into 16-bit words. The used filter topology that ensures the low-pass stage is Sinc³.
Figure 1.
Block diagram of a sigma-delta analog to digital converter
Vin
1-bit ADC
1-bit stream
Digital filter
&
Decimator
16-bit data
1-bit DAC
MSv31135V1
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Overview of 16-bit SDADC
AN4207
2
Overview of 16-bit SDADC
2.1
Main features
STM32F37x/38x devices have three embedded Sigma-Delta Analog to Digital Converters
(SDADC). They can be synchronized together and each has the following main features:
2.1.1
●
Effective number of bits (ENOB) equal to 14 bits
●
5 differential input pairs, or 9 single-ended inputs, or a combination
●
High-performance data throughput:
–
16.6 ksps input sampling rate when multiplexing between different channels
–
50 ksps input sampling rate for single-channel operation
●
Programmable gain: x0.5, x1, x2, x4, x8, x16 and x32
●
Selectable reference voltage: VDDSD, 1.22 V, 1.8 V and VREF
Clock selection
The SDADC clock is supplied by SDADCCLK which divides the system clock (SYSCLK) by
a selectable prescaler: 2, 4, 6, 8, 10, 12, 14, 16, 20, 24, 28, 32, 36, 40, 44 and 48.
The typical operating frequency of SDADC is 6 MHz in fast mode and 1.5 MHz in slow
mode.
Example:
If SYSCLK is set to 72 MHz, the SDADC divider should be set to SYSCLK/Typical
Frequency:
Fast speed mode: prescaler = 72 MHz / 6 MHz = 12
Low speed mode: prescaler = 72 MHz / 1.5 MHz = 48
2.1.2
Input modes
The SDADC has three possible input modes, that can be combined.
●
Differential mode:
●
Single ended offset mode
●
Single ended zero reference mode
Differential mode
It is recommended to use differential mode when the sensors being used produce very
small signals that are very susceptible to noise. This is especially the case when using
thermocouple and bridge type sensors (pressure sensors).
In differential mode, the SDADC converts the difference between SDADCx_AINyP and
SDADCx_AINyM. The result can be either positive or negative depending on which input is
at higher voltage.
Note:
The SDADC can not measure negative voltages and the input voltage on each channel must
stay within the electrical limits of the device.
The input range is [-Vref/(2*gain), + Vref/(2*gain)] and the conversion value is in the range [32767, +32767]
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AN4207
Overview of 16-bit SDADC
Example:
For a 1.22V reference voltage and a gain set to 1, the input range is +/- 0.61V
The formula is:
Vin = SDADCx_AINyP - SDADCx_AINyM = ReadData * Vref/(2 x gain x 32767)
with ReadData is two’s complement read data from the SDADC data register
(SDADCx_JDATAR or SDADCx_RDATAR)
Figure 2.
Input signal range in differential mode
SDADCx_AINyP - SDADCx_AINyM
+ Vref/(2xgain)
- Vref/(2xgain)
MSv31128V1
Figure 3 shows how to connect a bridge type sensor to the 16-bit SDADC.
As shown below both positive (SDADCx_AINyP) and negative (SDADCx_AINyM) inputs are
connected to sensor outputs.
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Overview of 16-bit SDADC
Figure 3.
AN4207
Typical connection of differential sensorVref
to differential channels
SDADC_VREF+
Vref
SDADCx_INyP
SDADCx_INyM
STM32F37x
SDADC_VREF-
MSv31129V1
Single ended offset mode
In single ended offset mode, conversions are performed by connecting the negative input to
0 V internally, leaving the corresponding pin for the negative input (SDADCx_AINyM) free to
be used for other purposes. The signal to be measured is applied to the positive input
SDADCx_AINyP. This mode of operation is similar to differential mode, except that the
output data is only from 0 to +32767, and not from –32767 to +32767, therefore half the
dynamic range is lost, consequently the SNR is degraded.
The formula is:
Vin = SDADCx_AINyP = ReadData * Vref/(2 x gain x 32767)
with ReadData is two’s complement read data from SDADC data register
(SDADCx_JDATAR or SDADCx_RDATAR)
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AN4207
Overview of 16-bit SDADC
Figure 4.
Input signal range in single ended offset mode
SDADCx_AINyP
Vref/(2xgain)
MSv31130V1
Single ended zero reference mode
The signal is applied to the positive input SDADCx_AINyP, and the negative input is set to
the signal reference (normally 0 V). This mode injects an input common mode of half scale
to the ADC thus maintaining the dynamic range the same as in differential mode (-32767 to
+32767). In this mode, the injected common mode is dependent on gain variations.
The formula is:
Vin = SDADCx_AINyP = (ReadData + 32767) * Vref/(gain x 65535)
with ReadData is two’s complement read data from SDADC data register
(SDADCx_JDATAR or SDADCx_RDATAR)
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Overview of 16-bit SDADC
Figure 5.
AN4207
Input signal range in single ended zero reference mode
SDADCx_AINyP
Vref/gain
MSv31131V1
Note:
When channel p (with p is even) is used in differential mode, the channel p+1 is
automatically used as minus input (SDADCx_AINyM) and therefore channel p+1 can not be
used in single ended mode (zero reference mode or offset mode).
When channel 4 is configured in differential mode channel 5 is automatically used as minus
input and therefore channel 5 can not be used either in single ended offset mode or in single
ended zero reference mode.
2.1.3
SDADC voltage references
The SDADC reference voltage is selectable among four sources:
1.
VREFINT1: A 1.2V embedded reference voltage
2.
VREFINT2: A 1.8V embedded reference voltage
3.
VDDSD: The SDADC analog supply voltage. It ranges from 2.2 V to 3.6 V
4.
VREFSD+: The external SDADC reference voltage. It ranges from 1.1 V to VDDSD
The table below shows the voltage weights per bit (step size) using three possible
references.
Table 2.
10/28
Voltage step sizes
VREFSD+
µV/bit
VREINT1: 1.2
18.46
VREINT2: 1.8
27.69
3.3V
50.35
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AN4207
2.1.4
Overview of 16-bit SDADC
Calibration
In order to get the best performance from the SDADC two parameters should be calibrated.
These parameters are offset and gain.
Offset calibration
The offset error is a constant value that is added to the ideal conversion value.Figure 6
illustrates the offset error.
The SDADC embedded in STM32F37x/38x devices provides automatic offset calibration
without adding external components. Its principle can be summarized in the following steps:
Note:
1.
Short internally both channel inputs (positive and negative)
2.
Perform conversion and store result in internal register (configuration register)
3.
Subtract automatically the calibration value from conversion value during standard
conversion.
It is recommended to run calibration at least once after SDADC configuration
Figure 6.
Offset error in SDADC
Digital value
Positive offset
1
Analog input
2
Negative offset
Null offset
MSv31132V1
Gain calibration
Another error source is the gain error. As illustrated in Figure 7 the gain error is the deviation
of the SDADC’s transfer function from the ideal straight line. It is due to the built-in
programmable amplifier. According to the device datasheet the typical analog gain error is
-2.7% which means there can be 884 counts (at maximum voltage) due to gain error. So
reducing the gain error is mandatory when performing accurate measurements.
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Overview of 16-bit SDADC
Figure 7.
AN4207
Gain error in SDADC
Digital value
Positive gain error
1
Null gain error
3
Analog input
Negative gain error
2
MSv31133V1
Two types of gain are implemented in the SDADC:
Analog gain: x1/2, x1, x2, x4, x8
Digital gain: x16 and x32.
Only the analog gains are considered in gain calibration.
Gain calibration requires an external accurate reference and it is performed by applying the
accurate reference voltage (AccRef) at the SDADC input and getting the SDADC output.
The gain is computed as following: AccRef/(Output x Vref/65535)
The computed gain can be stored in non-volatile memory (Flash memory) and used during
acquisition phase
Note:
12/28
Before running gain calibration it is mandatory to run offset calibration.
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AN4207
Overview of 16-bit SDADC
Figure 8.
Gain calibration using the accurate reference voltage
Vref
SDADC_VREF+
Sensor
1
Accurate
Reference
2
1
Acquisition phase
2
Gain calibration phase
SDADCx_INyP
I/O control
STM32F37x
SDADC_VREF-
MSv31134V1
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Overview of 16-bit SDADC
AN4207
Software procedure for offset and gain calibration
The flowchart below shows a typical SDADC application using both offset and gain
calibration.
Figure 9.
SDADC software calibration sequence
Input voltage acquisition
Configure SDADC (gain,
conversion mode, channel…)
Run offset calibration
Convert the accurate reference
Calculate the exact gain value
Store the exact gain value in
non-volatile memory or backup registers
Acquire the SDADC input
(sensor outpout)
Calculate the input voltage
using the exact gain value
Stop conversion?
End
MSv31136V1
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AN4207
2.1.5
Overview of 16-bit SDADC
Matching impedance
The impedance of the analog signal source, or series resistance (RAIN), between the source
and the MCU pin may lead to a voltage drop across it because of the current flowing into the
pin.
In SDADC, the channel input impedance depends on:
1.
SDADC clock
2.
Analog gain (0.5 - 8)
The figure below shows the equivalent input circuit for input channel where Rin is the input
impedance of SDADC analog input and can be calculated using the formula below.
1 R in = ----------------------2  f clk  C
Figure 10. Equivalent input circuit for input channel
SDADCx_AINxP
chclk
RAIN
C = 0.543 pF + 0.152 pF * gain
chclkz
VAIN
chclkz
SDADCx_AINxM
chclk
C = 0.543 pF + 0.152 pF * gain
RAIN: The impedance of the analog signal source
VAIN: The signal source
MSv31137V1
Table 3.
Typical SDADC input channel input impedance
Frequency
Gain
RIN
1.5 MHz
0.5
540 K
6 MHz
0.5
135 K
6 MHz
8
47 K
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Overview of 16-bit SDADC
2.1.6
AN4207
Low power modes
The 16-bit SDADC combines both high resolution and low power consumption making it
suitable for battery-powered products. If the SDADC is left powered-up continuously, it
consumes 1.2 mA maximum (typically 800 µA). The SDADC has three main modes for
reducing power consumption:
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1.
Slow mode: In this mode, the SDADC consumes 600 µA maximum but the sampling
rate is limited to 12.5 Ksps maximum
2.
Standby mode: In this mode, the SDADC consumes a maximum of about 200 µA but a
stabilization time of 300 SDADC clock cycles (50 µs @ 6 MHz) is required each time it
exits from Standby mode.
3.
Power down mode: In this mode, the SDADC consumes a maximum of about 2.5 µA
but a stabilization time of 600 SDADC clock cycles (100 µs @ 6 MHz) is required each
time it exits from Standby mode.
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AN4207 Sigma-delta (SD) vs. successive approximation register (SAR) analog-to-digital convert-
Sigma-delta (SD) vs. successive approximation
register (SAR) analog-to-digital converters
Analog to digital converters come in different architectures to be able to address the needs
of different applications. The main types available in the market are:
Successive approximation register (SAR) ADC: Successive-approximation-register
(SAR) analog-to-digital converters (ADCs) are frequently used in embedded systems, with
sample rates of less than 5 megasamples per second (Msps). Their resolution ranges from
8 to 16 bits. This type of ADC is used in industrial control applications.
Sigma-delta ADC (SDADC): Sigma-Delta analog-to-digital converters (ADCs) are used in
lower speed applications requiring high resolution. The resolution may attain 24 bits by
oversampling but the sampling rate is limited to only a few ksps.
Flash ADC: Flash analog-to-digital converters, are the fastest type of analog to a digital
converter. They are suitable for applications requiring a very high sampling rate. However,
flash converters have low resolution (12-bits). This type of ADC is used in oscilloscopes.
The Figure 11 gives an overview of the different ADC architectures, comparing their
resolution and sampling rate.
Figure 11. ADC architecture vs. resolution and sampling rate
24
Sigma Delta
Resolution (bits)
3
16
12
SAR
Flash
8
100 k
5M
5G
Sample rate (sample/s)
MSv31138V1
The STM32F37x/38x devices have two types of embedded ADCs: 12-bit SAR ADC and 16bit SDADC.
The Table 4 summarizes the differences between the two types of ADC.
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Sigma-delta (SD) vs. successive approximation register (SAR) analog-to-digital converters
Table 4.
SDADC vs. SAR ADC feature comparison
Feature
SDADC
SAR ADC
Max sampling rate
50 Ksps(1)
1 Msps
Resolution
16 bits
12 bits
Input mode
Single-ended differential
Single-ended
Embedded gains
0.5x to 32x
No
Number of channels
5 differential input pairs or 9
single-ended inputs
16 single-ended inputs
Number of instances
3 with synchro capability
1
Automatic Offset calibration
Yes
Yes
Analog watchdog
No
Yes
Trigger sources for regular
conversion
– Software
– Software
– Start of conversion of another – Embedded timers
SDADC
– External events
Trigger sources for injected
conversion
–
–
–
–
Software
– Software
Embedded timers
– Embedded timers
External events
Start of conversion of another – External events
SDADC
[VREF-, VREF+]
Input range
Reference voltage
1.22 V
1.8 V
VDDSD
VREFSD+
VREF+
Input impedance(2)
47 K to 540 K
125 K to 2500 K
1. The sampling rate is 50 ksps when selecting a single channel and 16.6 ksps when multiplexing between
different channels.
2. For SDADC, the input impedance depends on the selected gain and the selected operating frequency
(1.5 MHz or 6 MHz).
For SAR ADC, the input impedance depends on used sampling frequency (0.05 - 1 MHz) and sampling
capacitor (8pF).
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4
Application examples
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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Application examples
Table 5.
AN4207
Temperature sensor voltage range
VDD = 3.3V
Rref = 1.8 K
Temperature
Resistance ()
Voltage (mV)
0°
100
91.667
20°
107.7
98.725
50 °
119.2
109.2667
The temperature measurement is performed in two steps:
1.
Temperature sensor calibration: this phase is performed with JP18 fitted in 2-3(REF)
position. A 100 Ohm resistor is connected to PE7 which is connected to VREF through
the reference resistor. The SDADC converter measures the analog voltage applied on
PE7 and then computes the correction coefficient. This calibrates the gain. The PT100
is not connected in this phase.
2.
Temperature measurement: this phase is performed with JP18 fitted in 1-2 (PT100)
position. The PT100 sensor is connected to PE7 which is connected to VREF through
the reference resistor. The SDADC converter measures the analog voltage applied on
PE7 and then computes the temperature which is given by the following formula:
TemperaturePT100 = (((((CoeffCorrection * (AvrgRegularConvData/SDADC_GAIN) *
REFERENCE_RESISTOR * 2) / SDADCRESOL) - RESISTANCE_ZERODEGRE) /
RESISTANCE_COEFFICIENT);
where:
AvrgRegularConvData is the average value of 256 samples
SDADC_GAIN is the internal SDADC gain. In this example it is set to 8
CoeffCorrection is the correction coefficient computed in phase 1.
REFERENCE_RESISTOR is the reference resistor 1K8 labeled R33 on STM32373CEVAL
SDADCRESOL is the sigma delta converter: 2e16-1
RESISTANCE_ZERODEGRE is the resistance of PT100 at 0 °C
RESISTANCE_COEFFICIENT is the coefficient of PT100 sensor
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4.2
Application examples
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
Table 6.
Pressure sensor voltage range
Pressure (HPa)
VDD = 3.3V
Note:
Differential voltage on Differential voltage on
sensor outputs
SDADC inputs
800
10.56 mV
146.256 mV
1000
13.2 mV
265.32 mV
1200
15.84 mV
384.384 mV
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));
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Application examples
4.3
AN4207
Wave recorder
This application shows how to use the 16-bit Sigma-Delta Analog-to-Digital converter to
record the human voice using the electric condenser microphone installed on the
STM32373C-EVAL evaluation board.
The application source code is available in the
Project\STM32373C-EVAL\src\waverecorder.c
file in the STM32373C-EVAL demonstration firmware package stm32373c-eval_fw
A microphone is connected to SDADC1 channel 6 (PB0) through an audio amplifier/filter.
The SDADC is configured in Single-Ended Zero Reference mode and voice recording is
triggered by TIM13 at a sampling rate of 8 KHz.
The wave recorder application uses a ping-pong buffer: one buffer is actively being written in
the MicroSD memory card while the second buffer is filled with the new samples.
Writing access to the MicroSD memeory card uses the open source file system FatFS.
Figure 12 shows the block diagram of the voice recorder application.
Figure 12. Block diagram of voice recorder
SPI
Cortex M4
CPU wth FPU/
MPU
72 MHz
STM32F37x
SRAM
Audio
AOP
16-bit
ΣΔ
SDADC
1st buffer
2nd buffer
TIM13
MSv31139V1
The flowchart of this application is given in Figure 13.
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Application examples
Figure 13. Flowchart of wave recorder application
Wave recorder
TIM13 IRQ
Create a wave file on
MicroSD
Write SDADC
sample in
selected buffer
Init SDADC
Init TIM13 used as
trigger
Stop recording ?
TIM4 IRQ
Is selected
buffer full?
Switch selected
buffer
Write selected
buffer in
MicroSD
Clear TIM4
interrupt
Generate TIM4
interrupt
Stop recording: Disable
SDADC, Disable
TIM13…
Close the wave file
Clear TIM13
interrupt
Exit application
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Application examples
4.4
AN4207
ElectroCardioGram (ECG) acquisition
This application shows how to use the 16-bit Sigma-Delta Analog-to-Digital converter to
acquire a human ECG using two ECG electrodes TS1 and TS2 installed on the
STM32373C-EVAL evaluation board.
The application source code is available in the
Project\STM32373C-EVAL\src\applications.c
file in the STM32373C-EVAL demonstration firmware package stm32373c-eval_fw
Two ECG electrodes TS1 and TS2 are connected to SDADC1 channel 0 (PE12) through an
ECG amplifier/filter. The SDADC is configured in Single-Ended Zero Reference mode and
conversion is triggered by the TIM3 timer at a sampling rate of 480 Hz.
The ECG acquisition application uses a ping-pong buffer: one buffer is actively being filtered
(using a bandpass filter) and then displayed on the LCD while the second buffer is filled with
the new ECG samples.
Note:
ECG samples are filtered using the ARM DSP library
Figure 14 below shows the block diagram of the electrocardiogram acquisition application.
Figure 14. Block diagram of the electrocardiogram (ECG) acquisition application
SPI
Cortex M4
CPU wth FPU/
MPU
72 MHz
STM32F37x
SRAM
ECG
AOP
16-bit
ΣΔ
SDADC
DMA
1st buffer
2nd buffer
TIM13
480/sec
MSv31141V1
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Application examples
The flowchart of this application is given in the figure below.
Figure 15. Flowchart of ECG acquisition application
ECG acquisition
DMA half/
complete
transfer IRQ
Init SDADC, TIM3
used as trigger, DMA
Write SDADC
sample in
selected buffer
Init SDADC
Init TIM13 used as
trigger
Buffer pointer
reset?
Is Half Transfer
flag set ?
Set buffer
pointer at 2nd
half
Filter the SDADC
samples using a
BandPass FIR filter
Set buffer
pointer at 1st
half
Clear DMA half/
complete
transfer interrupt
Display filtered
samples on LCD
Reset buffer pointer
Stop acquisition?
Disable acquisition:
Disable SDADC,
Disable TIM3
Exit application
MSv31142V1
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Application examples
4.5
AN4207
Power meter application
The analog to digital converter is the most important part of a power meter application and
the SDADC embedded in STM32F37x/38x devices meets the requirements of this type of
application. Typically a class B power meter requires 1.5% current measurement accuracy
which means a 14-bit Effective Number Of Bits (ENOB) ADC.
For the voltage measurement there are no strict requirements and therefore the SAR ADC
can be used for voltage measurement synchronized with the SDADC that is used for
measuring the current.
Another constraining parameter of the Analog to Digital Converter is the sampling rate. For
power meter applications, a sampling rate of up to 12.8 ksps is sufficient for harmonic
spectrum analysis.
As shown in Figure 16, in the power meter application the phase measurement consists of
measuring voltages: Va, Vb and Vc in single-ended mode using the SAR ADC while
currents (Ia, Ib and Ic) are converted in differential mode using the SDADC. All are triggered
by the same timer (for example TIM19).
Figure 16. Three phase power meter application using STM32F37x/38x devices
Neutral
Phase c
Phase b
Phase a
Resistor divider
Resistor divider
Resistor divider
Va
ADC_IN0
Vb
ADC_IN1
Vc
ADC_IN2
STM32F37x
Current sensor 3
Ic
SDADC3_AIN0P
Ib
SDADC3_AIN0M
SDADC2_AIN0P
Ia
SDADC2_AIN0M
SDADC1_AIN0P
Current sensor 2
Current sensor 1
SDADC1_AIN0M
MSv31143V1
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5
Revision history
Revision history
Table 7.
Document revision history
Date
Revision
13-Dec-2012
1
Changes
Initial release.
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AN4207
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