MT-022 Datasheet, PDF (2/12 Pages) Analog Devices – ADC Architectures III: Sigma-Delta ADC Basics

ANALOG
INPUT
+_∑
–
(A) DELTA MODULATION
SAMPLING CLOCK
∫
1-BIT
DAC
MT-022
DIGITAL
OUTPUT
ANALOG
INPUT
+_∑
(B) DIFFERENTIAL PCM ^–
SAMPLING CLOCK
N-BIT
FLASH
ADC
∫
N-BIT
DAC
DIGITAL
OUTPUT
Figure 1: Delta Modulation and Differential PCM
If the analog signal remains at a fixed dc level for a period of time, an alternating pattern of "0s"
and "1s" is obtained. It should be noted that differential PCM (see Figure 1B) uses exactly the
same concept except a multibit ADC is used rather than a single comparator to derive the
transmitted information.
Since there is no limit to the number of pulses of the same sign that may occur, delta modulation
systems are capable of tracking signals of any amplitude. In theory, there is no peak clipping.
However, the theoretical limitation of delta modulation is that the analog signal must not change
too rapidly. The problem of slope clipping is shown in Figure 2. Here, although each sampling
instant indicates a positive excursion, the analog signal is rising too quickly, and the quantizer is
unable to keep pace.
SLOPE OVERLOAD
Figure 2: Quantization Using Delta Modulation
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