Sigma-Delta Analog-to-Digital Converter

Sigma-delta Analog-to-Digital Converter (ADC) is used in many application including telecommunications for analog to digital conversion.

A Sigma-delta ADC has a modulator and a digital filter (also called as decimation filter) as shown in Fig. 1. A modulator converts the input analog signal into digital bit streams (1s and 0s).

Fig1. Sigma Delta ADC

The Sigma-delta ADC method is based on delta modulation where the difference between two successive samples (increase or decrease) is quantized.

Working of Sigma-delta ADC

The Sigma-delta ADC consists of two parts i.e. modulator & digital filter. The modulator circuit consists of an integrator, comparator and 1 bit DAC (Digital to Analog converter). The working of a modulator can be explained using a conversion example.

In Table, the Alphabets X, B, C, D, and W correspond to points in the signal path of the block diagram in Fig1. Suppose the input X is a DC input of 3/8 V. The resultant signal voltage at each point in the signal path for each signal sample is shown in the Table.

B = A − Wn-1 (previous value of W)

C = B + Cn-1 (previous value of C)

D = 0 (if C is negative)

= 1 (if C is o or positive)

W = +1 (if D is 1)

= −1 (if D is o)

Sample Number (n) X B C D W
0 3/8 0 0 0 0
1 3/8 3/8 3/8 1 +1
2 3/8 -5/8 -2/8 0 -1
3 3/8 11/8 9/8 1 +1
4 3/8 -5/8 4/8 1 +1
5 3/8 -5/8 -1/8 0 -1
6 3/8 11/8 10/8 1 +1
7 3/8 -5/8 5/8 1 +1
8 3/8 -5/8 0/8 0 -1
9 3/8 11/8 11/8 1 +1
10 3/8 -5/8 6/8 1 +1
11 3/8 -5/8 1/8 1 +1
12 3/8 -5/8 -4/8 0 -1
13 3/8 11/8 7/8 1 +1
14 3/8 -5/8 2/8 1 +1
15 3/8 -5/8 -3/8 0 -1
16 3/8 11/8 8/8 1 +1
17 3/8 -5/8 3/8 1 +1
18 3/8 -5/8 -2/8 0 -1

Note: A repetitive pattern develops every sixteen samples and that the average of the signal W over samples 1 to 16 is 3/8. It shows that the feedback loop forces the average of the feedback signal W to be equals to the input X.

Summary

  • Delta modulation is a 1-bit quantization method.
  • The output of a delta modulator (D) is a single-bit data stream where the relative number of 1s and 0s indicate the level or amplitude of the input signal.
  • The fig2. shows the Sigma-delta ADC conversion.
  • The number of 1s over a given number of clock cycles establishes the signal amplitude during that interval.
  • A maximum number of 1s corresponds to the maximum positive input voltage. 4096 1s occurs when the input is positive maximum.
  • A number of 1s equal to one-half the maximum corresponds to an input voltage of zero. 2048 1s occurs when the input is zero.
  • No 1s (all 0s) corresponds to the maximum negative input voltage. 0 1s occur when the input is negative maximum.
Fig2. Sigma-delta ADC conversion

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