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I said earlier that I would use a median filter on the raw A/D samples. A median filter takes the median of the last k input values and uses that as the output. It is very good at removing spikes, and it has a very low propagation delay for step inputs.
But after getting averaged results that are better than one LSB at the A/D I became worried that the filter would kill the resolution I have been getting. For example: if a sequence of raw data samples were 5,5,4,5,5,5,4,5 they would average 4.75. I would interpret this sequence to be saying that the input is a little less than 5 and noise is causing the LSB to toggle. The fraction of the samples that are 4 vs. the fraction that are 5 gives an indication of how much less than 5 the input is. Taking an average of the data samples gives me more information than I could get from any one sample. But if I run that sequence through a median filter I will get all 5's out of it. I will have thrown away some valuable information.
So I built two versions of the median filter. One is a normal version and the other is modified so that it will only reject spikes that are larger than +/- 1. It will pass the above sequence without changing it.
When I watch my averaged data I see some slow drift, probably thermal drift. The components are mounted on a breadboard that is exposed to air currents. Over a period of a minute or so I can see drift of about 0.1% or more. I thought this would be a good test for the median filters. I would expect that if noise is low and the input to the A/D is drifting, then the output of the median filter should tend to move in jumps. After putting in the normal median filter I see the jumps I expected on one of my two data channels. The other channel seems to change more smoothly. I would guess that this is because one channel has more noise than the other. The channel with less noise is more likely to move in jumps. The modified filter did not exhibit the jumps on either channel. I suspect that when I am reading the real input data there will be enough noise that it will not matter which version of the median filter I use, but for now I will use the modified one.
I think I am about done with the high speed interrupt routine. The final toll is: the interrupt takes 8 usec when it just drives the coil, 11 usec when it takes an A/D, and 24 usec when it takes an A/D, filters and accumulates. There are an equal number of the three kinds of interrupts, so the average is 14.3 usec, or 19% of the chip's throughput. Every 60th of a second there is an extra 7 usec on one of the interrupts to store the data and update clocks.
I am getting good quality data at a 60 Hz rate from a clean input signal. I guess the next step is to build a preamp and see how it looks when I read the signal from the receive winding.
Robert Hoolko
But after getting averaged results that are better than one LSB at the A/D I became worried that the filter would kill the resolution I have been getting. For example: if a sequence of raw data samples were 5,5,4,5,5,5,4,5 they would average 4.75. I would interpret this sequence to be saying that the input is a little less than 5 and noise is causing the LSB to toggle. The fraction of the samples that are 4 vs. the fraction that are 5 gives an indication of how much less than 5 the input is. Taking an average of the data samples gives me more information than I could get from any one sample. But if I run that sequence through a median filter I will get all 5's out of it. I will have thrown away some valuable information.
So I built two versions of the median filter. One is a normal version and the other is modified so that it will only reject spikes that are larger than +/- 1. It will pass the above sequence without changing it.
When I watch my averaged data I see some slow drift, probably thermal drift. The components are mounted on a breadboard that is exposed to air currents. Over a period of a minute or so I can see drift of about 0.1% or more. I thought this would be a good test for the median filters. I would expect that if noise is low and the input to the A/D is drifting, then the output of the median filter should tend to move in jumps. After putting in the normal median filter I see the jumps I expected on one of my two data channels. The other channel seems to change more smoothly. I would guess that this is because one channel has more noise than the other. The channel with less noise is more likely to move in jumps. The modified filter did not exhibit the jumps on either channel. I suspect that when I am reading the real input data there will be enough noise that it will not matter which version of the median filter I use, but for now I will use the modified one.
I think I am about done with the high speed interrupt routine. The final toll is: the interrupt takes 8 usec when it just drives the coil, 11 usec when it takes an A/D, and 24 usec when it takes an A/D, filters and accumulates. There are an equal number of the three kinds of interrupts, so the average is 14.3 usec, or 19% of the chip's throughput. Every 60th of a second there is an extra 7 usec on one of the interrupts to store the data and update clocks.
I am getting good quality data at a 60 Hz rate from a clean input signal. I guess the next step is to build a preamp and see how it looks when I read the signal from the receive winding.
Robert Hoolko