ST AN1827 Application note
AN1827
APPLICATION NOTE
IMPLEMENTATION OF SIGMA-DELTA ADC
WITH ST7FLITE05/09
INTRODUCTION
The purpose of this document is to describe how to implement a 10-bit Sigma-Delta A/D converter using a simple external circuit and a Sigma-Delta conversion program.
The ST7FLITE05(09) has an on-chip ADC with 8-bit resolution and an input range of 0-V
The external Sigma-Delta ADC described in this application is designed for relatively slow al-
ternating signals (0,2 - 5Hz) in the range 0 - 10mV(p-p). The focus of the project is to provide
good relative accuracy, repeatable parameters and simplicity, resulting therefore a low cost of
the device (in its simplest form, apart from the microcontroller, only three RC elements are
necessary).
CC
.
Rev. 1.0
AN1827/0304 1/11
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IMPLEMENTATION OF SIGMA-DELTA ADC WITH ST7FLITE05/09
1 SIGMA-DELTA CONVERTER THEORY
The basic Sigma-Delta converter (Figure 1) is built of 2 basic circuits: a modulator and a digital
filter. In the modulator the input signal is summarised with the signal of negative feedback from
the D/A co nverter. The signal’s difference , after passi ng throug h the integr ating circu it,
reaches the input of the comparator, where it is compared to the r eference voltage (the comparator works as a 1-bit quantizator). The input signal from the comparator co ntrols the 1-bit
converter and reaches the input of the digital filter, which decreases flowability and transforms
the 1-bit stream into 10-bit words.
Figure 1. Sigma-Delta Conversion
+
∫
-
V
REF
+
-
DIGITAL
FILTER
1 BIT DAC
Figure 2 shows the general scheme of the m icrocon troller implementation. Within the system
the function of an integrator is fulfilled by the Ci capacitor, and the range of converted voltages
depends on the resistance ratio R1 and R2. The principle of operation is as follows: from the
moment of setting the high status on the output, the voltage on the Ci capa citor and simultaneously on the input of the comparator begins to rise. As soon as the reference voltage is
reached the output status is altered a nd th e vo ltage decr eases. After f alling below the r eference voltage the output status is altered to the high one and the cycle repeats. Provision of the
state of balance and the correct conversion of signal into its digital form is realised by a program-operated feedback loop and digital filter.
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2
IMPLEMENTATION OF SIGMA-DELTA ADC WITH ST7FLITE05/09
Figure 2. Sigma-Delta Imp lementation with Microcontroller
SERIAL OUTPUT
PORT
I/O
R
2
FIRMWARE
R
V
IN
1
C
i
V
REF
+
-
COMPARATOR
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IMPLEMENTATION OF SIGMA-DELTA ADC WITH ST7FLITE05/09
2 REALISAT ION WITH THE USE OF THE MICROCON TROLLER ST7FLI TE05(09)
The practical realisation of the Sigma-Delta converter has been used with our passive infrared
detector. A sensor of passive infrared c haracterise s with a low output vo ltage (belo w 10mV)
as well as with a usable range of frequ encies from 0,2Hz to 7Hz. Therefore the Sigma-D elta
converter is a perfect solution. It provides an a ccurate enough conversion of a signal in to its
digital form with use of a minimal number of external elements. That means simplicity of construction, relatively low production costs and good using properties.
Figure 3 presents the block diagram of the detector.
Figure 3. ST7FLITE05 interfacing with PIR Sensor
LD2980
TP
SERIAL OUTPUT
CONFIGURATION
POWER
OUTPUT
REL
ELEMENT PIR
VCC
V
CC
PB 0
PB 1
PB 4
ST7FLITE05
ANALOG
INPUT
ANALOG
INPUT
x8
OUTPUT
ADC
V
CC
PA 0
PA 1
PA 4
PA 5
PA 6
PA 7
A signal from the sensor passes through the capacitor to the input of the converter. The A/D
converter has been used as a comparator in the microcontroller as well as the source of the
reference voltage. By enabling the internal x8 amplifier, the amplitude of an output signal has
been decreased 20 times to provide correct operation of the amplifier. This allows a decrease
in the res ist o rs ’ value span in the sa me ratio and to increase the operational ac curacy of the
A/D converter working as a comparator. Additionally the temperature compensation of sensitivity has been introduced.
Figure 4 shows the work algorithm of the Sigma-Delta converter.
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