Industrial applications often require a high resolution and performance from an analog-to-digital converter at a moderate speed (see Chap. 1), so that SAR ADCs (Chaps. 2 and 3) and delta-sigma ADCs are most commonly used. This chapter will therefore concentrate on the basics of delta-sigma converters and then on the discrete-time approach utilizing the switched-capacitor technique, which is most commonly used. The alternative continuous-time architecture will be discussed in Chap. 5.Delta-sigma converters were already introduced in Chap. 1. The analog signal A is coupled to a filter with feedback-loop as illustrated again in Fig. 4.1.The quantizer, which in Fig. 4.1 is a comparator, is placed within a regulationloop, resulting in the transfer function of Eq. (1.21):The quantization noise N is suppressed within the low frequency signal band and the remaining high frequency noise is suppressed by a digital low-pass filter. This way, the delta-sigma converters achieve the best signal-to-noise ratios [1]. ΔΣ ADCs can convert signals with low signal frequency with high resolution. The oversampling and the digital low-pass filtering will even suppress higher frequency noise of the signal. Such signals occur often in industrial applications that use sensors. Examples are pressure or temperature sensors, flow or energy meters. Additionally, the analog filter provides a linear transfer function. Consequently, the differential non-linearity is extremely good. However, non-linear parameters of the electrical circuitry will still cause an integral non-linearity.Further disadvantages are the low conversion rates caused by the required oversampling, the time delay caused by the digital filter and the fact that snapshots are not possible as the delta-sigma converter has to run continuously and synchronously with the clock.
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