Over-sampling, Noise Shaping, Analogue to Digital Converter (ONSADC)

For some years work has been carried out with BAE Systems on a Over-sampling, Noise Shaping, Analogue to Digital Converter (ONSADC).
This is a fast, noise-shaping, high bandwidth, multi-bit, Sigma-Delta converter and the final design stage would be suitable for a PhD
program.

A key parameter of a ONSADC is the Over-Sampling Ratio (OSR) which is the ratio of the input to the output sample rates. In normal ONSADCs
the OSR is set high, usually above at least 32, with 128 being typical.
In our work, where we need medium performance over a wide bandwidth,while using only a moderately high sampling frequency, we use OSRs down
to four.

As we also use a continuous-time, high-order, forward filter, the analysis of the resulting mixed continuous-time and sampled feedback
system becomes exceptionally complex. ONSADCs perform best when operated on the edge of instability, so the design process has to be
just right. When we add to this the fact that the ONSADC is to be realised as a IC, with all the usual associated component tolerances, we
see that this is a tricky problem indeed.

In an ONSADC one can continually trade off resolution (number of bits)against the output sample rate so, while an OSR of four gives
low-resolution at a high out-put sample rate, the same system can be "switched" to use a much higher OSR giving a very high-resolution at a
lower output sample rate. Under these latter circumstances the noise and linearity specifications become exceptionally important.

We have the design process moderately well tied down, and are now including the second and third-order effects (e.g. the effects of making
the system with non-ideal components). An old 0.8um BiCMOS process is being used because it offers exceptionally good component tolerances and
a good power supply range. Much of the circuitry has been designed as a good "first draft", but much work remains to be done.

The project will involve mostly analogue circuit design, though some supporting digital circuitry is also required.

In the longer term it is hoped that this mathematical and practical work will lay the foundations for optical ONSADC systems with very high
sample rates that will permit radar signals to be digitised at the antenna. In the medium term the aim would be to design a converter that
offers high-resolution at input bandwidths of around 100MHz.If you are interested in applying for such a projectr please contact me by email in the first instance.

Dr. Martin Reekie
Martin.Reekie@ee.ed.ac.uk