|It may be Greek to you, but sigma delta converters are not really hard to understand.|
|Q. Can you please explain, simply, as to a Bear of Little Brain1, how sigma-delta converters work?|
If a voltage is measured many times, the average of the measurements will be more accurate than most individual measurements. This is "over-sampling." (Dither3 may be necessary to randomize the errors in the individual measurements.)
There is a definite theoretical minimum limit to the possible noise of an analog-todigital converter (ADC). When an ADC samples a signal at a frequency of fs the digital output contains the signal and this "quantization noise" is usually spread evenly from dc to
fs / 2. By sampling at a higher rate of Kfs, the noise is spread over the wider band from dc to Kfs / 2. If we then remove all the noise above fs / 2 with a digital filter the signal-to-noise ratio (SNR) of the digital output is improvedeffectively improving the ADC resolution.
Normally the SNR increases with the square root of K, so very high sampling rates are necessary for useful increases in SNR. But a Σ-Δ modulator does not produce uniformly distributed quantization noise. Although the total noise is unaltered in a Σ-Δ system, most of it is at high frequencies (HF). This is known as noise shaping and permits much lower values of K.
If the digital output from the Σ-Δ modulator is filtered to remove HF, leaving the frequencies from dc to fs / 2 (where the wanted signals are) then the SNR and resolution of the digital output are improved. A Σ-Δ ADC simply consists of a Σ-Δ modulator and a digital low-pass filter, both of which are easily made with modern highdensity digital technology. The principle of Σ-Δ ADCs has been known for more than 40 years, but the ability to build one on a chip is relatively recent.
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For More Information:
How a Sigma-Delta Converter Works (pdf, 126,435 bytes)
Basics of Sigma-Delta: Tutorial MT-022 (pdf, 289 kB)
Advanced Sigma-Delta Concepts: Tutorial MT-023 (pdf, 936 kB)
Useful Analog Devices Links: