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Topic: Atmega168 negative input protection (Read 999 times)previous topic - next topic

claudiu_mirescu

Nov 16, 2012, 07:46 am
Dear friends,

I would like to develop an electromyogram circuit using an Atmega168. For any of you who don't know, the electromyogram is the plot of the electrical activity of a muscle as it contracts. Depending on the amplifiers and filters used, it outputs voltage from -1V to +1V. I would like to read this voltage with an ADC pin of an Atmega168, considering I am interested only in the POSITIVE output of the electromyograph. I do not care about the negative waves, but:
1. Because the negative waves EXIST, is there any chance of destroying my Amega168? I know it does not read negative voltages, but in this case I do not need them. Will the Atmega ignore them or it will be fried by them?
2. If the answer is positive to the last question, how can I protect the Atmega from the negative voltage? I do not want it necessarily shifted above 0 (like when using a diode rectifier bridge), I simple don't need th signal.

Thank you very much.

twice

#1
Nov 16, 2012, 08:23 am
Build a Voltage divider, see:
http://www.psocdeveloper.com/forums/viewtopic.php?f=42&t=8573
Make Vcc=2V, do not add the capacitor. Ignore any value, you get from the ADC, between 0 and +1V, this is ignore any conversion < ADC resolution / 2.

However, improvements are needed because you lose resolution.

claudiu_mirescu

#2
Nov 16, 2012, 09:16 am
Thank you, it is a good idea. What about using an op-amp connecting the negative power supply to ground, would that output only the positive waves of the signal? Like in the first image on this link. http://mutable-instruments.net/forum/discussion/938/cv-inputs-how-to-protect-them-from-negative-or-too-high-voltages/p1

Would this lower the gain or influence the resolution of the signal?

majenko

#3
Nov 16, 2012, 01:17 pm
All you need is 2 diodes and a small resistor, and optionally a capacitor:

http://www.thebox.myzen.co.uk/Tutorial/Protection.html

Ideally you would use schottky diodes, as they have a lower voltage drop, and fast switching.

dhenry

#4
Nov 16, 2012, 01:20 pm
1. Yes.
2. Use a resistor of sufficient value. The pins are clamp diodes to limit voltages on the pins but you need to make sure that the current going through the clamp diodes doesn't exceed their limits. For -1v, I would use something like a 1k resistor on the pin.

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