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I would use atmega328p to read the pulses transferred in human's body. So I wonder if that's good to directly expose the pins of the chip. Or there should be some type of components for protection and noise filtering.
e.g. maybe a 1n ceramic cap in series for filtering 50Hz house main noise? a 1K resistor in series for static protection?
Thank you!
There are two approaches that you can use that don't require much in the way of extra circuitry or processing. The easiest is to use a pair of electrodes complete a circuit to provide base current to a transistor or darlington pair, and connect the emitter to the Atmel pin. You could then configure the pin as an external interrupt (or maybe a comparator), or simply analog sample it, depending on what you want to do. A good description of your options is at http://www.electronicshub.org/touch-sensors/, the source of the images.
The second approach would be to build a capacitive sensor. You could use a megaohm range resistor, and just connect one end to the pin and the other to the air, hanging free. When the free end is touched, the noise floor will raise. You would need to sample pretty fast and process it, as well as doing some analog conditioning to make the signal suitable for single-ended acquisition, and you might need some gain. There are chips that do this for you that other people have pointed to that will do this job better than you and a microcontroller can achieve on your own.
Without acquisition circuitry, you won't read anything more significant than noise. This could be useful or not, depending on your application.
If you are trying to make a (very) rough touch detector, add some static discharge protection to an ADC port, identify peaks of variation and go for it.
But if you are trying to do somewhat serious signal processing and interpretation, you will definitely need amplifiers and filters.
As a rule of thumb, your total gain of the circuit should be enough so that the signal amplitude measured is slightly below the reference voltage of the ADC. Filters to be used include notch main filters and band-limiting anti-alias filters.
I would like to ask you to kindly expand on your explanation of the desired application.
If you walk about on a synthetic carpet, you can easily discharge a short 4kV pulse into a chip and kill it. So you need some sort of ESD protection. But what are you actually trying to sense?
There are several ways to measure human touch, but since you are mentioning pulses you need to specify from where on the body in order to get a good answer.
If it is touch you want to sense and you want a all digital solution you could look into using an extra chip AT42QT1070 (http://www.atmel.com/images/atmel-9596-at42-qtouch-bsw-at42qt1070_datasheet.pdf), but from the way you are writing your question i guess this is not what you want to do.
If you want to use the MCU i suggest you connect a mosfet that can react on the potential from your body. Since it's kinda hard to explain the connections i suggest you look at the circuit proposed here (http://www.aaroncake.net/circuits/Simple_Touch_Switch_Circuit.asp) and modify it (only care about Q1, R1 and R2). The drain could be connected to your input on the MCU.
biological electric signals are really small at the skin, you'll need an amplifier
EKG on a chip: ADS1298R http://www.eetimes.com/document.asp?doc_id=1259170
