Sigma Delta data converters

As the semiconductor world moves to smaller and smaller geometries the challenge to mix analog functionality with large digital blocks looms large. A significant need is to implement data converters ( Analog to digital and digital to analog ) on large digital signal processors. I note that the engineering community is moving towards the use of oversampled data converters( sometimes known as delta - sigma or sigma - delta converters). Low order OS converters ( second order) have been available for quite sometime in standalone configurations.



As I started using these techniques myself I was surprised to note how counter intuitive the concept of the OS converter is. Understanding successive approximation and flash converters is fairly intuitive. However, OS converters are not.



Another interesting fact about OS converters is the merging of analog and digital techniques. The analog part of the converter is small in size( but perhaps not small in impact) while as far as size is concerned the digital parts are large and complex. In order to design an OS converter one needs a deeper understanding of signal processing, than for other types of data converters. First of all the OS converter contains significant digital content and is quite complicated. For the A to D we have the decimation filter ( digital) following the modulator. For the D to A we have an interpolating filter ( digital) following the modulator.


For the purely analog designer, this may cause some issues in implementation. In addition to these strange architectural constructions, there is also a whole new aspect of the noise spreading
constructs.

I recently I took a look, purely out of curiosity, at how much information on these types of devices was available in the learned journals ( I took a look at only one: IEEE Journal of Solid State Circuits) as well as the web. The following are the numbers I found:

JSSCC: From 1990 to 2009 the total number of easily accessible papers were: 18। These were the most readable and packed with information and understandable with not too much theory.

The Web yielded the following harvest।

106,000 hits for sigma - delta A to D converters, 92,000 for sigma - delta D to A converters
104,000 for digital decimation filters, 387,000 for digital interpolation filters। The surprise was 6,320,000 hits on sigma - delta modulators and finally 225,000 hits for noise shaping networks। The interest is intensse!

How does one determine what one needs as far as the parameters of a sigma delta A/D are for a particular application?

Basic parameters are sampling rate, order of the modulator, resolution in bits.

These parameters depend on the technology chosen for implementation, which determines supply voltages ( 1.8V to 350V!!), sampling frequencies ( 20khz to 1 ghz or more?), and the complexity of the analog modulator design.

My next post describes the critical parameters and their impact.