Estimating the signal band noise in delta-sigma modulators

Sigma delta modulators are popular devices used in a multiplicity of applications. One of the most prolific of these is the A/D converter. A delta – sigma A/D basically consists of a delta-sigma modulator ( typically first or second order), followed by a decimation filter. The modulator operates in such a way that it generates a high pass response for the noise in the system. This response is known as the NTF or noise transfer function of the modulator. In this way the modulator suppresses noise within the passband but allows the out of band noise components to have a high pass characteristic. A low pass system of decimation filters removes this latter noise also. It becomes imporatnt,in the practical sense, to estimate noise in the passband. An expression can be developed to do this for higher order modulators with fairly accurate results. This subject is dealt with in a recent brief paper released by Signal Processing Group Inc. It may be found in> “engineer’s corner”.

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