ON 20050801@2:57:04 PM at page:
http://www.piclist.com/techref/io/sensor/sigmdelt.htm#38565.622962963
[mikerj-clara-] Says
Microchip's application note AN700 gives some more detail on the implementation of a Sigma Delta A/D using a PIC http://ww1.microchip.com/downloads/en/AppNotes/00700a.pdf This is useful as it shows how to add another resistor to modify the input range to include negative and positive voltages.
A better document (IMO) is the Texas Instruments app note for their MSP range of micros. http://www-s.ti.com/sc/psheets/slaa104/slaa104.pdf
As explained in the TI document, the time constant is important for accuracy. The ripple voltage should be not more than 1 LSB per ADC cycle. The calculations to derive a suitable capacitance value are given within the app note.
One problem that these converters present is speed. Especial for the higher resolutions, conversion times can become significant, so it is important to optimise the ADC code to operate in as few cycles as possible. This also allows a lower value capacitor. The speed issue is not helped when you consider that the pre-charge time must be added to the conversion time, and near the ends of the ADC's range, the pre-charge can take a very significant amount of time. One possibility is to use a MOSFET switch to quickly charge the cap to the (buffered) reference voltage. However, this does increase complexity. Another option is to design the voltage input range to exceed the actual requirements by some degree, so that the converter is never operating at either extreme. Of course, the trade-off is lost resolution.
I have achieved ~12 bits of resolution from the comparators built into some of the newer parts such as the 16F627A and 16F88. The two comparators in these parts, plus the input multiplexor allows a 4 channel ADC to be built, each channel may have a different range. Providing the resistor values are kept reasonably high, a single PIC pin can drive the feedback/balancing arm on multiple converters with little or no loss of accuracy.
ON 20050814@10:27:19 PM at page:
http://www.sxlist.com/io/sensor/meat.htm#38578.5461689815
James Newton[JMN-EFP-786] removed post 38578.5461689815
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ON 20050815 at page:
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