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Juggling conversion choices
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28/11/2006
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Connecting between the analogue and digital worlds, data converters are used in virtually every end equipment segment. And the demands of their developers are similar to those of other components: do it faster, more accurately, using less power and for less cost. The challenge for data converter manufacturers is that these qualities tend to be mutually exclusive.
Data converters are best categorised by architecture. The highest performance versions fall into three main categories: successive approximation register (SAR); delta sigma; and pipeline – each representing a different compromise between speed, resolution, power consumption and cost.
Pipeline converters cater largely for applications where speed is of the essence. In a pipeline converter, data samples ripple through a pipeline of comparator stages. Each stage includes switch capacitor amplifiers for comparing the analogue signal to a first reference voltage, to a second reference voltage and to an average of the first and second reference voltages. Pipeline devices offer high speed, but resolution tends to be limited to 14bit.
In contrast, delta-sigma converters are best for high resolution applications, where slow, precise measurements are required or where the input signal is a continuous waveform. A typical application might be a blood glucose meter, where speed of measurement isn’t so much of an issue as accuracy. Bernd Rundel, Texas Instruments’ product line manager for Nyquist a/d converters, recounts: “In a delta-sigma converter, the converted signal does not correspond to a point in time. There is a simple sampling stage at the input at which many samples need to be taken and passed into a digital filter, prior to averaging them. To achieve a 20bit resolution, it would be necessary to make 1million conversions.”
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Author
Vanessa Knivett
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