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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 converter. The signal’s difference, after passing through the integrating circuit,

reaches the input of the comparator, where it is compared to the reference voltage (the com-

parator works as a 1-bit quantizator). The input signal from the comparator controls 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

Figure 2 shows the general scheme of the microcontroller 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 capacitor and simulta-

neously on the input of the comparator begins to rise. As soon as the reference voltage is

reached the output status is altered and the voltage decreases. After falling below the refer-

ence 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 pro-

gram-operated feedback loop and digital filter.

DIGITAL

FILTER

1 BIT DAC

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