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Forum Index : Microcontroller and PC projects : Getting the best out of Pico ADC
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| Bleep Guru  Joined: 09/01/2022 Location: United KingdomPosts: 509 |
In another thread, Volhout mentioned the ADC and how with a bit of averaging you can get quite good results. I was wondering what would be the best method to get the best out of the Pico ADC? I'm currently measuring some small voltages, so I'm amplifying them, I'm then doing the following. Dim Float Iin(11),Volt(11),Iout(11) Dim Float filtIi(11),filtIo(11),filtV(11) Dim Integer y ADC open 166666, 3 'Stabalise jitter in the ADCs 'Take 12 readings, for the Iin, Iout and Volts find the median value 'Do this 12 times and then do the same again on those 12 values For y=0 To 11 ADC Start Iin(),Volt(),Iout() filtIi(y)=Math(median Iin()) filtIo(y)=Math(median Iout()) filtV(y)=Math(median Volt()) Next ' Filter out high and low readings again. raw(0)=Math(median filtIi()) raw(1)=Math(median filtIo()) raw(2)=Math(median filtV()) Using the built in median, I beleive should order the values then take the middle 2 and average them, because there is a even number of samples. This gives me reasonably good results and is fast, but I wondered if there was a better way? speed is not critical I'm only taking a sample per second, but I don't want the smoothing to swallow up too much time. I originally was taking a group of samples, finding the largest and smallest, removing them and then averaging the rest, all in BASIC, so did take a fair time, but this median method seems to give me much more stable results. Thanks for any input. Regards, Kevin. |
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stanleyella Guru Joined: 25/06/2022 Location: United KingdomPosts: 2129 |
When I first tried adc it was recommended to use an array and average it but I just use the samples or take one sample and case test it. I get good results. |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6100 |
Internally, the pico takes 10 readings, discards the two highest and the two lowest. It then averages the remainder. The only real advantage by doing it yourself is, the readings are spread over a longer time-frame. In 'some' cases, that is significant. Jim VK7JH MMedit MMBasic Help |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4246 |
The error in the pico adc is a non-monotonic fault al msb transition and msb-1 transition. When there is sufficient noise in the signal you can average to hide it. Without noise, you can't. Regardless how many samples. If you are measuring RMS ac signals, or fft the data the adc error is not dominant. If you want to measure accurate dc voltages, you can substantially increase the adc performance by adding noise (i.e. play sound, choose noise, and in hardware attenuate the signal to 10 lsb or so. Add that to the input voltage, and average (100x or so).. You may be able to achieve 5 digit resolution. Professional multimeters average over 100ms. This automatically removes 50hz and 60hz noise. Volhout Edited 2023-07-14 07:16 by Volhout PicomiteVGA PETSCII ROBOTS |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
Another way to reduce the effect of the ADC fault starts with a low pass filter on the input (as low as your application can accept). It should not be possible for the difference between adjacent samples to exceed the slew rate of the filter, so if it does it must be an error. Discard that sample and replace it with an an extrapolation of the preceding samples. This does not work for an input that changes too slowly, so is best for AC rather than battery volts for example. Edit Taking inspiration from Volhout. I guess adding a tone to the input at the filter corner frequency then averaging it out later might improve it. Edited 2023-07-14 09:27 by phil99 |
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9308 |
I believe I read in the Pico manual, that the ADC will perform much better if you disable the on-board switch-mode regulator, and power the board from an external linear regulator. I don't think I was dreaming about that..... Smoke makes things work. When the smoke gets out, it stops! |
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| phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
With Volhout's method of error masking the extra supply noise might actually be useful! |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4246 |
1/ No, noise will be proportional, meaning that when measuring a low voltage it would be 1 lsb, and high voltage 100 lsb. 2/ Pico PSU noise changes (peak peak, and spectrum) with load. Very inpredictable 3/ Have to look it up in the datasheets, but most likely the pico PSU switches at a frequency outside the frequency band of the ADC (500kHz fastest -> Nyquist is 250kHz). Dpending input filter (or none) this may not work. Resolution is what you gain by averaging. Accuracy requires a reference. The Pico reference is 3.3V. Generated from the power supply. So the measurement accuracy can never be better that the PSU is. The output voltage of the PSU changes with temperature, load (ARM working harder), video content (VGA pico), etc.. For best performance you use an LM4040 at the Vref pin. That will make you indendent of the PSU (ripple and drift). It all depends on what you want to achieve. Colleagues of mine have succeeded to create a 3-3/4 digit (0. - 3999) accurate multimeter from an 8 bit flash ADC. This flash ADC used internally 2 x 4bit ADC's and a 4 bit DAC (2 ADC stages) so it was littered with linearity errors and non-monotonity. It required pulling out all the tricks there are... literally measuring each of the 256 steps of each ADC in production... and applying noise, and averaging and....(patent-ed)... Success, Volhout... Edited 2023-07-14 17:17 by Volhout PicomiteVGA PETSCII ROBOTS |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4246 |
The input filter will smooth out the signal, removing disturbances. It will not work to solve the ADC error, since it is produced AFTER the filter. The averaging can remove it when (and only when) you have control over the input noise, that should be 5-10x larger than the error to be corrected. (100x is also possible, but requires significantly more averaging to restore the resolution). That will work fine, as long as you average over an exect number of cycles, though a triange wave is needed. Look at the distribution. Volhout Edited 2023-07-14 17:26 by Volhout PicomiteVGA PETSCII ROBOTS |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6798 |
As this is a silicon fault there is no reliable workaround. It's far, far easier to use a different chip for your ADC inputs - use those to connect the chip. :) Now we have a nice DAC to play with, I wonder if it would it be possible to have a "standard" external ADC chip supported by MMBasic? Actually, we already have one - the MX170 28-pin... Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4246 |
Without tricks the MX170 is only 10 bits. Not much better than the pico ADC that is a "degraded" 12 bit ADC. If you need a nice accurate ADC, use a TI ADS8326. Not very expensive, single channel 16 bit (1.5 lsb linearity) 100kSps SPI ADC. You can drive it with a 24bit (or more) SPI block running a MCLK of 2.4MHz (or lower, depending the speed you need it to work at). With it's 0.65mm pitch it is however a bit tricky to solder. Best is to put it on a 8 pin DIL adapter PCB that you can buy for near nothing. Volhout P.S. but the earlier mentioned theory (accurate noise free Vref, noise free power supply etc..) apply always, and become more important when resolution increases.. Edited 2023-07-14 18:26 by Volhout PicomiteVGA PETSCII ROBOTS |
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| phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
Once again my explanations are incomprehensible to anyone but me. The purpose of the filter is not to solve the ADC problem but to make it stand out so the defective samples can be removed. Any changes from sample to sample that are larger than the filter slew rate will allow must come from within the Pico. They can then be removed. This only works while the input is changing at a sufficient rate. A static input could be at a voltage that gives a constant error which would go undetected. Adding some ripple at a suitable frequency will ensure it is always changing. |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4246 |
Hi Phil, Thanks for the explanation. Let me try to explain the ADC error, then you may reconsider. picture this: The ADC is 12 bit, Vref = 3.3V Each LSB is 3300/4096 = 0.805mV when I apply 1.64919 V the reading of the ADC is decimal 2047 when I add 0.805mV (1.649995V) the reading should be 2048, but in fact it is 2051 then I add another 0.805mV, then the reading should be 2049, and in fact it is 2049. This is (roughly) what is happening at the MSB change. The magnitude is different per chip. I guess if you want to filter this kind of behaviour out, then you should design your input filter to allow maximum 1 lsb change per reading of the ADC (3 lsb should be flagged as impossible). At maximum speed (500kHz) you would (2us) allow a maximum input change of 4096x2 = 8ms. At the same time you would have to evaluate all individual samples. It can be done. When slowing down a bit. If you want a settling time of 1 second (is 5 tau) then you could work with 200ms first order filter (R-C) and sample the ADC at 20kHz. MMBasic could be able to keep up with that pace. But what if you connect at stable voltage of 1.649995V. No lsb change. Just a stable 2051 output, where it whould have been 2048. ??? Volhout Edited 2023-07-14 19:05 by Volhout PicomiteVGA PETSCII ROBOTS |
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| Bleep Guru Joined: 09/01/2022 Location: United KingdomPosts: 509 |
This is becoming an interesting tread, if you are into electronics, which I am. :-) From the above, it looks like I'm probably getting about as good as I can possibly get with the ADC as it is, I'm measuring the voltage off of a 100A current shunt and am able to get 50mA resolution, ie I can increase the current by 50mA and consistently see a change in the output from the ADC, after my filtering as above, which would appear to be as good as I'll ever get. My question really was is there anything that software can do, massive oversampling, filtering, short term running average..... that can improve on the 9bits (claimed) accuracy that we are left with, without resorting to extra hardware? I know in theory Median isn't ideal, which was why I was asking, however the way it works, in that it orders the data values, takes the middle two values and average them (in my case, as I'm using an even number of data values), I then do all of this twice so 144 data samples per output result. Median seems to work quite well for me as I suspect I have a lot of noise (static inverters, long cable runs etc..) and throwing away most of the data seems to be a good thing and gives me a reasonably stable result, which other methods I'd tried did not. :-) Regards, Kevin. Edited 2023-07-14 21:52 by Bleep |
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| phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
Yes, that is why I suggested adding an AC component to the input, so you never have a stable voltage. I guess a triangle wave with a rate of change similar to the max. slew-rate of the input filter would make the errors visible. Once the out-of-limits samples are removed the AC component can be averaged out. |
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| stanleyella Guru Joined: 25/06/2022 Location: United KingdomPosts: 2129 |
I run my picos from 3.3V linear regs and 3.3V enable grounded. I have a 5 button a-d board , for arduino 5V but works fine as a game controller with pico. bp!=pin(31) 'if bp!<3.1 then 'a button has been pressed select case bp! case <.02 'left pressed case 1.5 to 1.8 'right pressed case 0.3 to 0.6 'up pressed case 0.9 to 1.1 'down pressed case 2.3 to 2.5 'fire button end select I have waveshare pico with 160x80 screen and a simple scope and just touching the a-d pin and the 50Hz noise clips the display, ie > 3.3V |
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| lizby Guru Joined: 17/05/2016 Location: United StatesPosts: 3150 |
Don't know if this is the appropriate thread, but I have an ADC question. I have a one-turn pot. If I multiply the value returned by 100 and set an integer to the result, I get values ranging from 1 to 233--so far so good. But from 1 to halfway around, the range is 1-8; and to a little over 3/4ths of the way, to 21. Then the remaining quarter-turn ramps quickly up to 233. With this program: setpin 31,ain dim integer i,j,k,l,m,n do: i=pin(31)*100: if j<>i then: j=i:?i;" ";:endif:pause 1000:loop I get this as I turn the pot (for instance): 10 15 17 21 37 66 234 233 15 14 12 8 5 1 5 6 10 11 21 13 6 1 6 8 9 8 10 11 10 12 11 13 20 33 126 235 233 232 19 18 21 20 27 30 29 47 44 42 41 65 64 63 62 113 111 110 108 107 106 104 103 228 230 229 233 231 233 8 7 30 25 1 7 8 7 8 17 22 21 20 21 31 52 51 30 20 19 228 231 6 31 41 16 15 31 What kind of pot do I have? What kind do I want if I'd like it to ramp up in a more regular fashion? PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed |
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| stanleyella Guru Joined: 25/06/2022 Location: United KingdomPosts: 2129 |
What's a one-turn pot? |
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| Bleep Guru Joined: 09/01/2022 Location: United KingdomPosts: 509 |
I assume this is over several turns of the pot? In which case I'd say you have a single turn logarithmic pot, so you are going off the end and back each turn. You probably want a linear pot and only go round once, well actually only about 3/4 of a turn max. Regards |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4246 |
Lizby, You are measuring an audio potmeter. These have a non-linear curve sometimes referred to as logaritmic potmeter. Normally there is a mechanical stop. But that seems missing. Volhout Edited 2023-07-15 02:14 by Volhout PicomiteVGA PETSCII ROBOTS |
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