SIGMA-DELTA ADC KEY CONCEPTS Oversampling Noise Shaping Digital Filtering Decimation Figure 3.3 An AdC is a circuit whose digital output is proportional to the ratio of its analog input to its analog reference. Often, but by no means always, the scaling factor between the analog reference and the analog signal is unity, so the digital signal represents the normalized ratio of the two Figure 3. 4 shows the transfer characteristic of an ideal 3-bit unipolar ADC. The input to an AdC is analog and is not quantized, but its output is quantized. The transfer characteristic therefore consists of eight horizontal steps(when considering the offset, gain and linearity of an AdC we consider the line joining the midpoints of these steps) TRANSFER CHARACTERISTIC OF AN IDEAL 3-BIT UNIPOLAR ADC 111 110 SAME OUTPUT CODE DIGITAL 101- OUTPUT 010 I QUANTIZATION ERR=±12LsB ←十 001 ANALOG INPUT Figure 3.4 Digital full scale(all"1"s)corresponds to 1 LSB below the analog full scale(the reference or some multiple thereof). This is because, as mentioned above the digital code represents the normalized ratio of the analog signal to the reference, and if this were unity, the digital code would be all O"s and"1"in the bit aboue the msb4 SIGMA-DELTA ADC KEY CONCEPTS Oversampling Noise Shaping Digital Filtering Decimation Figure 3.3 An ADC is a circuit whose digital output is proportional to the ratio of its analog input to its analog reference. Often, but by no means always, the scaling factor between the analog reference and the analog signal is unity, so the digital signal represents the normalized ratio of the two. Figure 3.4 shows the transfer characteristic of an ideal 3-bit unipolar ADC. The input to an ADC is analog and is not quantized, but its output is quantized. The transfer characteristic therefore consists of eight horizontal steps (when considering the offset, gain and linearity of an ADC we consider the line joining the midpoints of these steps). TRANSFER CHARACTERISTIC OF AN IDEAL 3-BIT UNIPOLAR ADC Figure 3.4 Digital full scale (all "1"s) corresponds to 1 LSB below the analog full scale (the reference or some multiple thereof). This is because, as mentioned above, the digital code represents the normalized ratio of the analog signal to the reference, and if this were unity, the digital code would be all "0"s and "1" in the bit above the MSB