emcinductor
In the paper "Synthetic Sensors: Towards General-Purpose Sensing" by Gierad Laput, et. al, they use a 100 mH inductor as an EMI (electromagnetic interference) sensor. They have not provided much details on the board (shown below)
I was wondering what a circuit would look like which the inductor can be an ADC input to a microcontroller. Thanks for any ideas.
I took a rather thin USB extension cord, ran it 4 times through a ferrite core (one that comes in two halves in a piece of plastic and can be closed with a click) and ever since I haven't had any errors on my PC.
You want to filter two kinds of noise: low-frequency noise, like some remaining 100 Hz ripple, and the 66 MHz from the digital part of the controller. For the low frequency an inductor is impractically large, and a resistor is more appropriate, so that you get an RC low-pass filter. Unfortunately the datasheet doesn't say how much current VDDANA needs, but that won't be much, let's guesstimate that at 100 µA. Then a 100 Ω series resistor will only drop 10 mV, which is acceptable. Combined with a 47 µF capacitor this gives a low-pass filter with a cutoff frequency of 33 Hz. The attenuation at 100 Hz will not yet be very high, but the power supply should already have filtered most of the ripple anyway.
For the HF noise there are special EMI filter in the form of ferrite beads. They have a very low DC resistance, usually less than 1 Ω, and a peak impedance which is often specified at 100 MHz. The Murata BLM21AG102SH1, for instance, has an impedance of 1000 Ω at 100 MHz and and a DC resistance of 0.28 Ω:
The graph shows 850 Ω at 70 MHz. Due to the low DC resistance it's no problem to place it in series with the resistor we already had for the low frequencies.
So the BLM21 ferrite in series with a 100 Ω resistor and a 47 µF capacitor should filter both low-frequency and high-frequency noise. It's wise to place a 100 nF capacitor parallel to the electrolytic, close to pins 17 and 18 (GNDANA and VDDANA).
Best Answer
You can see one design from the Elektroscluch [0]. It seems to be as simple as connecting an inductor to ground and then feeding the "positive" end into an amplifier (and in your case an ADC). I don't see why you need two inductors here. This circuit uses one.
Given the sampling rate of 500kHz (if you copy the paper), you might want to put in a low pass filter around 200-250kHz anyway. On the PCB in the supersensor papre it looks like there may be an RC filter just off to the right, which maybe feeds into an op-amp buffer (unity gain amp) U8. That's a guess though, since there's no schematic.
Their conference paper does cite a couple of references that may be useful, but they seem to be more related to power line monitoring, not inductive/contactless. For example:
https://ubicomplab.cs.washington.edu/publications/electrisense/ https://ubicomplab.cs.washington.edu/publications/lightwave/
Both these papers show that lots of interesting emission in the low 100s of kHz which matches with the sampling rate in the paper.
[0] https://github.com/LOM-instruments/Elektrosluch-3-plus/blob/master/hardware/elektrosluch_schematic.pdf
Another example they cite is the Syntonistor project from CMU - that seems closer, although a lot of the conditioning circuitry here doesn't seem to be on the Synthetic sensor PCB (but who knows, we only have a photo of the top side). Also it's designed to detect 60Hz, but you could tap off the signal just after the inductor.
https://www.andrew.cmu.edu/user/agr/projects/syntonistor/