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Author Topic: Tension on shutted down gpio  (Read 2394 times)
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Hi all, I have 4 Arduino linked together, sometime one of this can be power off but on its gpio can be a voltage that come from other Arduino. This can damage the chip. So I must to isolate all the digital input with optocoupler, but i don't know what is the best solution for the analog input, can you help me?
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Why do yo have four Arduinos linked together?
How are they linked together?
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but i don't know what is the best solution for the analog input, can you help me?
Well try and find a way round having to do this but if you must then you need an opto isolator with two receivers. You used the second one as feedback to an op amp to compensate for the non linearity of the opto.

Alternatively there are linear opto isolators:-
http://uk.farnell.com/avago-technologies/hcpl-7510-000e/op-amp-isolation-ic/dp/1085055
It can be complex depending on how linear you want it to be.
http://www.ti.com.cn/general/cn/docs/lit/getliterature.tsp?literatureNumber=sbos129&fileType=pdf
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This can damage the chip.

It can, but that doesn't mean that it will.

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So I must to isolate all the digital input with optocoupler, but i don't know what is the best solution for the analog input,

You don't have to isolate. But if you do isolate, the best solution will depend on your application, your parts availability, your tolerance for complexity, your bank account, etc.

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can you help me?

Sure but not enough to go on.
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Like they said above, you my be able to get away with something simple like a 1k resistor protecing the pin insted of optocoupling
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Yes, I would give simple series resistors a shot. However I'm not sure 1K is the right value, you want to prevent current in excess of what the clamping protection diodes are rated for. I suspect 10K or more is more suitable, maybe more? The datasheet should give clamping diode ratings somewhere I would think?

Lefty

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Those diodes are rated 20ma. It would take over 200v to kill those diodes + 10k resistor.

Not to mention the low-pass filtered you got there.
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The protection diodes are rated at 0.5V max forward bias - that's rather less that 20mA I think - or have I missed the right part of the specs?

Actually specifying the voltage is a rather poor way of doing this as forward voltage varies significantly across the operating temperature range.
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Yeah they make good thermometers, easy to calibrate but do require a 2 point cal for <5% accuracy.

Bob

 
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as forward voltage varies significantly across the operating temperature range.

-2mv/c, pretty much.

How many degrees of temperature do you consider "significant"?
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The protection diodes are rated at 0.5V max forward bias - that's rather less that 20mA I think - or have I missed the right part of the specs?

Vf and Imax are two different specification for the internal clamping diodes. The first tells when it starts to conduct, the second how much current can conduct before damage.

Actually specifying the voltage is a rather poor way of doing this as forward voltage varies significantly across the operating temperature range.

Yea, the Vf spec is of just passing interest, we know that a 5 volt signal will fully turn on the positive clamping protection diode on a input digital pin on a 3.3 volt powered board, what we want to know is how much continous current the diode can withstand, that is the datasheet spec of interest.

Lefty

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So can I protect the analog input only with something such as 10K series resistor? If true is a very good info. In alternative i think to use analog switch but i need something that when is power off go in hight impedance. Do you know some ic like this?
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Many things can be protected with a large resistor - so long as no appreciable current flow is needed.  A digital input in normal operation takes < 1uA so even a 1M resistor would allow it to function (but much slower than normal - 10k is more reasonable a compromise).
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And is also ok for and analog input?
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For an analog input don't go any higher than a 10k resistor but it should work to protect it the same, if you go higher it will negatively affect how fast you can do an analog read, in the datasheet it says best with source impedance <10k ohms
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