Re: protect ADC inputs
leto said:
Yes, I noticed that, about zener.
Now, Im reading the datasheet of M74HC4051 multiplexer and it says:
"All inputs are equipped with protection circuits againt static discharge and transient excess voltage".
would I have to be calm? because there is no details about voltage limits, etc.
Voltage limits are fully specified; you just have to know how to interprete the spec.
Find the section for Absolute Maximum Limits (or something like that)
There you will find the maximum current allowed into the input, sometimes called the clamp diode current. I think its about 20 mA. Since the inputs already have clamping diodes (for esd and transient protection as you read), you only need to add a series resistor. If you add a small resistor such as 200 Ohms as some have suggested, then your protection can be calculated as Vmax = 5 + 0.02*200 = 9 volts and Vmin = -0.02*200 = -4 volts. So you will be safe as long as no input exceeds 9 volts or -4 volts. If that's not enough protection, you need to increase the resistor until it protects you against the largest voltage expected.
Next look at the power rating which is probably about 500 mW. You will have to make some assumptions here. In the worst case, all 8 inputs might jump to the maximum voltage and you will be adding 8*0.02*0.7 = 112 mW dissipation in the clamping diodes which you will add to the normal dissipation of the chip in your circuit to get the total power. Next, you need to derate the 500 mW depending on the maximum temperature you expect. Finally, you must make sure that the total maximum power dissipated is less than the derated power. Remember this is the worst case so if you can stay within these limits you are very safe and can be calm. If not you either need to increase the value of the limiting resistors, make some less demanding assumptions, etc.
Also, you will need to calculate the offset due to the series resistor. If you're using a 1K and the leakage is 10 uA you will get a +/- 10mV error. If this is within your error budget (and I assume you have an error budget of course), then you can be calm. This would be a 2 bits of error for an 10-bit ADC meaning you might as well be using an 8-bit ADC.
This is probably more info than you wanted, but someone might get something from it.