I've been messing around with a LMT86 temperature sensor
This sensor is deemed high output impedance as the documented loads it drives is 50uA.
ADC noise on STM32 isn't 'new'. Just that for a first time I'm observing such volatility up close. The problem is mere millivolts differences means a different temperature reading from the sensor, and accordingly there is already a sort of 'amplification' from the sensor itself.
Threads found from a Google search are like such.
https://embdev.net/topic/stm32f0-adc-input-noise-investigations
https://itectec.com/electrical/electronic-stm32-adc-noise-2/
https://www.eevblog.com/forum/microcontrollers/stm32-adc-noise/
It is explored in a thread discussing the temperature sensor here:
https://forum.allaboutcircuits.com/threads/lmt86-2-2v-temperature-sensor.181850/
Among the discussions is that given this is a high impedance source, an Op-Amp is needed to buffer inputs to the ADC.
So next I strapped up an LMV358 Op-Amp as a buffer and here are the observations. This is with the Op-Amp
common parameters for charts below: adc clock 84 mhz / 4 ~ 21 mhz, 84 clocks sample time
2000 samples, 10 samples/s, measuring room temperature about 30.5-31 deg C
mean 30.882000
std 6.782125
min -213.360000
median 30.960000
max 57.800000
No, temperature did not accidentally drop to near absolute zero. It is room temperature all the while during measurement, a multimeter did not change a single millivolt while variances are recorded by the ADC.
Next, I set up a filter to remove all values outside +/- 2 deg C from the median, 555 outlier values removed.
mean 30.973156
std 0.733296
min 29.030000
median 30.890000
max 32.950000
Next remove that Op Amp and connect the sensor LMT86 directly to the ADC (on stm32f401)
2000 samples, 10 samples/s, measuring room temperature about 30.5-31 deg C
mean 30.930100
std 4.062343
min 3.670000
median 30.810000
max 59.260000
Next, I set up a filter to remove all values outside +/- 2 deg C from the median, 500 outlier values removed.
mean 30.817787
std 0.698166
min 28.810000
median 30.810000
max 32.800000
It is a dilemma kind of. The Op-Amp probably reduce the impedance (significantly?), but that the errors/variance are about the same !
What is more concerning is the large outlier values as those are observed in the unfiltered input. This didn't seem to go away when the OpAmp is placed as a buffer between the sensor to stm32f401.
I've done a little different experiment, I turned the stm32f401 into a little 'oscilloscope', here is what is seen at PA0 ADC1, the temperature sensor LMT86 is connected there.
This is at 1 k samples per sec. scaled at 0.1 xdiv (so 10 is actually 1 second).
STM32F401 is running at 72 MHz, ADC clock = 72 / 2 = 36 MHz.
Sample time is 3 clocks for these charts.
1.77-1-8V is the voltages from LMT86. What is more interesting are the spikes, they seemed rather irregular and quite large, the large ones seemed to be around 0.3 v.
It seemed there is no observable periodic A/C signals, as A/C signals are around 50/60 Hz, they should be rather visible if they are present.
Another view at 10 kHz. xdiv is 0.01 so 10 on x-axis is 0.1 of a sec.
The above 'scope' charts seem to suggest why, apparently, that 'median' filtering seemed to work. The sensor voltages is concentrated in a narrow band.
I'd continue to work the tests to see if those (noise/spikes) could be identified to particular sources. Prior to this, as I used rather low sample rates, I'd guess it is a reason sometimes I do not encounter the spikes and the readings fall within range. But given the spikes, and a rather long sample time of 84 clocks, if a spike occur, it may change the sampled voltages in the ADC.
Note that the 220 uF capacitor is connected at 3.3v rail for these 'oscilloscope' charts.
It seemed this may be affected by impedance issues, 3 clocks at 36mhz is a pretty short sample time. And the LMT86 has a 50 uA max output.
It seemed as a wild possibility, that these could be radio waves. There is a wire/cord connecting the temperature sensor (probe) of about 70 cm. That works out to be around 420 MHz for full wavelength. Well, but for now it is a wild guess.
https://forum.allaboutcircuits.com/threads/lmt86-2-2v-temperature-sensor.181850/post-1667079
edit:
I placed that LMV358 in the path as a buffer, full negative feedback, so gain = 1. This should work like a low pass filter with cut-off at about 1 MHz (in spec). Then I do that ADC 'scope' again.
500 k samples per sec, I chanced upon this. It doesn't always look like this, it so happen to be one of a random capture. Note this is the output from the Op Amp with the sensor + cable as input.
I could nearly imagine, if I put a diode and a capacitor in the former, I'd probably hear some sounds. It seemed there is a possibility it is after all radio waves.
The 220 uF bypass cap is still connected at 3.3v to GND.
Thanks @Georgy Moshkin, the 'scope' graphs is a little app I created on stm32f401 that capture ADC samples from PA0 ADC1. It records 1200 samples in a buffer at a time using DMA and send them (the buffer) across after the capture. Capture is initiated with a single char command received from the host, During capture nothing is transmitted. This is configured to run at 72 MHz / 2 = 36 MHz ADC clock speeds. Sample time is 3 clocks per sec, so adding another 12 clocks for the conversion time, it is 15 clocks which gives a max sample rate of 2.4 MHz sample rates. This is the fastest achievable on stm32f401 ADC.
As I need to move away from my desk for a while, here is a snapshot of the contraption I've setup for the experiment.
3.3v Power is currently drawn from the LDO on the STM32F401CCU board, that is supplying all the components seen in the experiment.
This includes the stm32f401 MCU itself (its VREF+ goes to VDD, VREF- goes to GND as seen in a schematic for the board). This same 3.3v LDO on the board powers the
- LMT86 temperature sensor (accordingly, this only draws micro Amps)
- LMV358 op amp
- STM32F401ccu
- SSD1306 LCD
On the left of the breadboard is a breadboard power supply that can be powered from a 5v USB mobile charger, it has 5v - 3.3v LDO on board. In this experiment, that external power supply is currently unused.
The 'transistor' in the bottom right in the foreground is the LMT86 temperature sensor. I connect it to the ADC over long wires (the cord) as this is intended to be used as a probe to calibrate thermistors with unknown parameters.
My guess is with the long wires, I've accidentally made this an antenna and that LMT86 is a diode kind of, and with amplification internally. That may have accidentally turned this setup into a radio, a simple diode detector. And what is seen at the STM32 ADC may be the AM demodulated signals.
If this is true, it probably explains the large volatility, variance seen in what is assumed to be a static voltage from the 'temperature sensor'.
Here is an update, I patched a LMT86 directly on the breadboard - no long wires / no cords / no op amps etc.
2000 samples, LMT86 direct to STM32F401 ADC PA0
LMT86 direct to STM32F401 ADC PA0 discarding 122 values outside 0.5 deg C range from median, this is barely around 5% of 2000 samples.
mean 31.195000
std 0.162021
min 30.740000
25% 31.110000
median 31.180000
75% 31.330000
max 31.620000
This result is astonishing, standard deviation is 0.162 deg C. 95% confidence interval is 1.959 sigma ~ 0.31736.
Now LMT86 measured on stm32f401 ADC pa0 is +/- 0.31736 deg C accurate at 95% confidence interval.
Then taking the full 0.5 deg C interval, that is 3.08 sigma, that is > 99.8% (accurate) confidence interval
https://en.wikipedia.org/wiki/Normal_distribution#Quantile_function
LMT86 is an accurate device, and it meets the specs published by TI, when a near noiseless measurement on stm32f401 ADC is possible.
The histogram is showing bars for temperatures well within 0.1 of a degree C. And I'm using only 12 bins for 2000 samples.
Working this back to the specs for LMT86, it is -10.9 mV average gain per deg C.
This translates to a +/-3.46 mV accuracy measured on STM32F401 ADC at 95% confidence interval.
And at 0.5 deg C at 3.08 sigma (> 99.8% confidence interval) it is +/- 5.45 mV at pretty much > 99.8% confidence interval measured on stm32f401 ADC.
Measurements are still stochastic, as I can observe varying numbers with each sample. But the samples and mean conforms well to a bell curve (normal distribution), after dropping the *outliers*.
Still can't rule out radio interference etc as noise sources which probably cause the variations and especially the outliers.
The data analysis looks like such
each line of these is the Anderson–Darling statistics
https://en.wikipedia.org/wiki/Anderson%E2%80%93Darling_test
https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.anderson.html
This analysis is done by rejecting values outside a tolerance starting with 2 deg C and reducing it.
The Anderson–Darling statistic reduces to the smallest and finally increase, showing a poorer fit as tolerance is narrowed.
Now the tightest fit to normal distribution is 0.5 deg C at 99.8% confidence interval
n = 2000, lmt86 direct - no cord
no filter
145.97371710597236 [0.575 0.655 0.785 0.916 1.09 ] [15. 10. 5. 2.5 1. ]
tol = 2 del 13
95.23121766618078 [0.575 0.655 0.785 0.916 1.09 ] [15. 10. 5. 2.5 1. ]
tol = 1.5 del 29
59.96725246650158 [0.575 0.655 0.785 0.916 1.09 ] [15. 10. 5. 2.5 1. ]
tol = 1 del 56
33.59035490934821 [0.575 0.655 0.785 0.916 1.09 ] [15. 10. 5. 2.5 1. ]
tol = 0.5 del 122
20.692436455313555 [0.575 0.655 0.785 0.916 1.09 ] [15. 10. 5. 2.5 1. ]
count 1878.000000
mean 31.195000
std 0.162021
min 30.740000
25% 31.110000
median 31.180000
75% 31.330000
max 31.620000
tol = 0.3 del 197
22.802556095985665 [0.575 0.655 0.785 0.916 1.09 ] [15. 10. 5. 2.5 1. ]This kind of also confirms the 'radio waves' interference hypothesis, that those outliers, and variances are due to interferences from radio waves.
But that'd need 1 more set of 'scope' charts, to verify it.
And one more round of checks on the 'scope' graphs. LMT86 patched directly on the breadboard - no long wires / no cords / no op amps etc.
10k samples per sec STM32F401 ADC PA0
There are still some visible artefacts, but that the amplitudes are much smaller.
This basically confirms the 'radio waves' interference hypothesis
