Strange Frequency in Sensor Data

Question

I've built a fairly simple Temperature logging device on an Arduino Mega 2560 R3. I'm using TMP36's, 12 of them in total to log the data. Each sensors VS and GND pins are connected to common rails and each V_out pin is then connected directly to their own analog sensor pin on the Arduino.

The method for logging the temperatures is to record ten successive measurements on one sensor (pausing for 20ms between each measurement) then averaging those ten values, log that value to a data file, then move onto the next pin, repeating until all sensors have recorded a value. Then, wait until 10 seconds has passed and then repeat the process. (doesn't need to be faster since my experiment is hours to days long).

The first image (labeled Sensor 14) is the plot that i'm getting from one of the sensors attached to my heat source, which is about what I'm expecting (X-axis is seconds, Y axis is temperature in Celsius)

On my last test I only needed to needed to log a few locations and I left the rest of the sensors sitting on the desk essentially measuring the ambient temperature, which I'm 90% sure should be constant. However, I logged the data from the sensors anyways and plotted them for giggles and got the following plot (same axes as first)

This image has a pretty clear periodic temperature change, albeit a very small difference. The results I've gotten from this have no effect on my actual analysis, but the source for the periodic signal is what I'm curious about.

I'm no signals analyst, or electrical engineer, I'm wondering if seeing this type of periodic 'noise' indicative of some type of problem with my test rig or if this is fairly normal to see. (The sensor has a listed +- 2 degrees C accuracy with a 0.5 degree C linearity)

And as a second part to the question, would there be anything 'wrong' with removing this periodic signal from my temperature data? (running an fft on the noise signal, identifying the frequency and then using a bandstop filter to clear it out).

Many thanks for any advice!

(Edit 1)

I made a better plot in python and zoomed in on one section to show how the two signals look over a shorter period of time:

Answer 1

If you zoom in on the top graph you'll see about the same thing going on (look at the width of the line).

This is a problem with having the graphs auto-scale, you will eventually see something, either quantization noise as @Ignaciao suspects (most likely correct) or maybe some mains hum aliased into your sample bandwidth (which would look a bit different).

If your data acquisition system is using the internal ADC in the 2560, the resolution is only 10 bits. Simple averaging over many samples (which appears to be going on here) can appear to give more resolution but without proper dithering and proper filtering it won't work that well.

Keep in mind that without a preamplifier and using the ADC reference as the supply your ADC resolution is about 0.5°C (the accuracy and linearity are worse). Resolution is 4.9mV and TMP36 output is 10mV/°C.

You may note that it's bouncing back and forth by just about exactly 0.5°C, this is not a coincidence. The intermediate values are evidence of averaging, the fact it's not moving smoothly between states is because signal processing and dithering is inadequate.

Answer 2

A bit of speculation:

If you look at figure 5 in the data sheet for the TMP36 you can see that load regulation is broadly 0.006 degrees per micro amp drawn: -

If during the ADC process the current peak is (say) 1 mA (certainly not unheard of) then you will get an error of 6 degrees.

So, if your ADC is a multiplexed and unbuffered type that draws a spike of current during conversion then you'll get odd results. If you applied the maximum capacitance of 10 nF across the sensors output to ground you may well see this error fall dramatically.

Answer 3

Here's my guess for what you're seeing: The sensors only have a resolution 0.5c So they can put out the values 19.8, 20.3, and 20.8. Which can all be clearly seen in your graph. Now what about all those points on the slopes that are not one of the 3 values? I think those are occurring when the sensor changes during your 10 read average.

So the question remains. Is the room actually fluctuating back and forth by half a degree as the graph seems to indicate? It's hard to say. I think the only conclusion that you can come to based on this data is that you need more resolution if you care about 0.5c changes. I can say from experience that a fluctuation of 1-2 counts in the sensor reading is "normal"

Edit: I don't know who downvoted me. But here's more proof: The TMP36 has a scaling factor of of 10mV/degC. The arduino has 1024 counts of resolution, and assuming a vRef of 5v that gives 0.48degC per count. Almost exactly what I predicted. I said "sensor" in abstract, but what I should have said is that the system has a resolution of 0.5c.