Sampling Back-EMF

I'm trying to interface an arduinoscope to sample then view on my computer the Back-EMF around a pulsed coil. The specificity of coil system is to generate under special resonance a ringing Back-EMF hence positive & negative voltage spikes.

What circuit do you recommend to connect my coil to say ADC0 having in mind I need over-voltage protection wether positive or negative spikes to not destroy arduino board.

P.S. I happen to have both duamilanove & mega, the latter can sample in differential mode.

Thank you in advance

You need two things. First of all a potential divider to create a mid level common referance point, that is 2.5V so two 10K resistors should be fine, connected to the arduino's +5 and ground. This point then becomes the common ground for your measuring circuit, that means that your arduino's power supply or the power supply of what you are trying to measure must be floating, do not connect the grounds together.

Then you need a potential from your input voltage to the common ground with the mid point of this going to the arduino's analogue input. This is to cut down the voltage on the input to a maximum of +/- 2.5V.

Sounds complex but draw it out. I would add some protection diodes as well just in case. http://www.thebox.myzen.co.uk/Tutorial/Protection.html

Many thx Grumpy_Mike.

Just to make sure I understand your design, it works only for differential ADC mode ?

Do you use Mega board ?

No it works for a single ended analogue input.

Do you use Mega board

I have one but this would apply to any board.

The important thing is that the supply or the system you are measuring must float with respect to each other so the 2.5V point in the arduino can be tied to the common (ground) line of your measuring system.

P.S. I happen to have both duamilanove & mega, the latter can sample in differential mode.

Are you sure of that statement? I wasn't aware that the mega (mega1280 chip) ADC could measure in differential mode, just has more single ended ADC input pins.

Also once a 2.5v reference voltage is supplied to a analog input pin via a voltage divider, wouldn't it be possible (and better) to capacitor couple the external signal you are wishing to analyze, as it's the transient information you are interested in measuring?

Lefty

Are you sure of that statement? I wasn't aware that the mega (mega1280 chip) ADC could measure in differential mode, just has more single ended ADC input pins.

yes, i'm sure of this statement because it is written in the datasheet of Atmega (see chapter 26 in particular from page 292) but I've never done it which is why i'm asking support here.

I do have one doubt about if both inputs are sampled at same time hence really reflecting the instantanous differential voltage.

The other doubt is about how to wire both input, the datasheet does not show any example... Should I just connect my voltage to both pin and not ground anything ?

Any insight would be appreciated.

Your right, the datasheet does describe using ADC in differential mode. However it is not something the Arduino core library supports I'm sure.

So you would have to write your own register level code to setup for that mode I guess. That's certainly something I couldn't do. Your right in that they don't go into a lot of detail on wiring or using differential mode. Maybe someone else around here can comment of this ADC mode of operation and how one might write code to use it. Would most likely require a nice library to utilize usefully.

Lefty