Crystal Semiconductor is a manufacturer of integrated circuits used in Professional and Consumer Audio, Multimedia, Video, and Telephone systems. They have been producing Digital Audio Converters since 1984. Ron Knapp has over fifteen years experience as a design engineer specializing in high resolution IC and hybrid converters. He has worked for Analog Devices, Maxim, and Power Integrations. Ron has been with Crystal since 1990. He holds a B.S. in Systems Engineering from Boston University and an M.S.E.E. from Worcester Polytechnic Institute in Massachusetts.

Ron started his talk by discussing the importance of converter linearity and error. He then went on to discuss early approaches to digital to analog (D/A) conversion using resistor switching networks and R2R ladders. The problem with these is the difficulty in attaining matched resistive values. Without the benefits of laser trimming, resolutions of only up to 12 bits were possible. Laser trimmed networks could attain resolutions of up to 16 bits. In 1984, Crystal Semiconductor introduced a DAC using CMOS and a capacitive charge redistribution approach. The benefit of this approach was its inexpensive calibration using parallel networks of capacitors on each bit. This allowed a means of mass producing converters and automatically calibrating them for improved linearity. Crystal at the same time introduced an Analog to Digital converter using a successive approximation approach and the same DAC.

After an introduction to basic sampling theory and the causes and effects of aliasing, Ron pointed out the main disadvantages of the earlier approcaches to conversion: Since the input signal to an ADC (and the output from a DAC) had to be heavily attenuated at frequencies above one-half the sample rate (the Nyquist frequency), very high order filters were required to be used with these devices. These filters had serious phase and signal distortion problems, not to mention that they were often noisy or prohibitivly expensive.

The solution to the filtering problem was oversampling. By increasing the sample rate to a multiple of the audio band, then digitally filtering the output, the requirements placed upon the analog filters could be greatly relaxed. A single-bit oversampling system called Delta Sigma Modulation was devised. The Modulator consists of a Differential Amplifier followed by an integrator, then a comparator and a 1 bit DAC fed back to the Differential Amplifier. The output of this modulator was a 1 bit stream that went into a digital decimation filter at a rate of 64 times the output sample rate. Using this system, the analog filtering requirements were nothing more than a single pole well above the audio band.

Of course, with a single bit system, quantization noise would be extremely high. However, using a technique called noise shaping, the IC is able to shift the quantization noise well above the audio band where it is easily filtered. The result is a resonably priced line of converter products that today have resolutions approaching 20 bits. As well as products that achieve a high level of integration allowing several converters on a single chip, or I.C.s that perform both conversion and digital signal processing.

Ron discussed some aspects of design considerations when trying to design these parts into audio and computer systems. He also touched on the topics of jitter and idle channel tones.

Ron recommended two books: Principles of Digital Audio by Ken Pohlman, and a compilation of IEEE papers called Oversampling Delta-Sigma Converters edited by James Candy (et al). He also gave to all present a 34 page Crystal Semiconductor A/D & D/A Converter Applications Seminar Workbook. and offered to send data books to anyone who wanted one.