09-28-2020, 01:18 PM
Why do I need an accurate A/D with good specs when I can just average the data from a hobby A/D and get to the same resolution?
Yes, you can get the appearance of high-resolution data from heavily averaged noisy, or lower quality data, but it doesn't carry much meaning. Why?
First, a self-evident point. The linearity will be limited by the linearity of the underlying data. Attempting to make a straight line out of a crooked one is a little like straightening a nail with a hammer. Be careful of your fingers, and don't be surprised if you lose the nail, entirely.
Second, any asymmetry will skew your results. Take a simple example, a one-bit A/D. Let's say you feed a symmetrical sign wave, from 0 to 5 volts, to a one-bit A/D, i.e. a comparator, that nominally switches at 2.5 volts. You then accumulate a long series of one-bit conversions. Ideally, half the results will be ones and half zeros. If you took a million points, the averaged conversion result would ideally be 500 thousand ones out of one million bits. If there is any offset in the comparator, that will offset the results. Then, if the comparator responds faster to positive or negative transitions, that will skew the results further. Also, if the input drifted a tad into one or the other stop, that would distort the waveform and skew the results. If there is a buffer for the input signal that slews at a different rate up and down, that will skew your results, etc.
Third, any stray feedback will interfere. That stray feedback could appear in the signal, causing, say, an output of one to capacitively couple back to increase the input magnitude. Remember, the stray effect only needs to be a few parts per million to show up in your results. Or, that stray feedback could act through the power supply, perhaps causing the switching reference voltage to drop slightly when the output was already a one. That stray feedback could be thermal. More switching at the comparator would cause self-heating, which could change the comparison point. Positive stray feedback will cause oscillation. Negative stray feedback will amplify. Neither will improve the stability or accuracy of your system. In practice, all these things happen, and more. I know from building Delta Sigma converters that use a one-bit D/A and a clocked comparator followed by a digital filter to extract a high resolution result. In order to get 20-bit data out of such a converter, you need 20-bit precision in the D/A. There is no free lunch.
So, if you think you can make a 20-bit A/D out of a 16-bit A/D by oversampling and averaging, think again.
Tom Lawson
Yes, you can get the appearance of high-resolution data from heavily averaged noisy, or lower quality data, but it doesn't carry much meaning. Why?
First, a self-evident point. The linearity will be limited by the linearity of the underlying data. Attempting to make a straight line out of a crooked one is a little like straightening a nail with a hammer. Be careful of your fingers, and don't be surprised if you lose the nail, entirely.
Second, any asymmetry will skew your results. Take a simple example, a one-bit A/D. Let's say you feed a symmetrical sign wave, from 0 to 5 volts, to a one-bit A/D, i.e. a comparator, that nominally switches at 2.5 volts. You then accumulate a long series of one-bit conversions. Ideally, half the results will be ones and half zeros. If you took a million points, the averaged conversion result would ideally be 500 thousand ones out of one million bits. If there is any offset in the comparator, that will offset the results. Then, if the comparator responds faster to positive or negative transitions, that will skew the results further. Also, if the input drifted a tad into one or the other stop, that would distort the waveform and skew the results. If there is a buffer for the input signal that slews at a different rate up and down, that will skew your results, etc.
Third, any stray feedback will interfere. That stray feedback could appear in the signal, causing, say, an output of one to capacitively couple back to increase the input magnitude. Remember, the stray effect only needs to be a few parts per million to show up in your results. Or, that stray feedback could act through the power supply, perhaps causing the switching reference voltage to drop slightly when the output was already a one. That stray feedback could be thermal. More switching at the comparator would cause self-heating, which could change the comparison point. Positive stray feedback will cause oscillation. Negative stray feedback will amplify. Neither will improve the stability or accuracy of your system. In practice, all these things happen, and more. I know from building Delta Sigma converters that use a one-bit D/A and a clocked comparator followed by a digital filter to extract a high resolution result. In order to get 20-bit data out of such a converter, you need 20-bit precision in the D/A. There is no free lunch.
So, if you think you can make a 20-bit A/D out of a 16-bit A/D by oversampling and averaging, think again.
Tom Lawson