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Digispark Atod input 0 - 5V ?

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dr pepper

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I started to use a digispark for a project, only because I have some.
I'm using adc 2 and 3 channels.
The channels need to measure a 0-5v signal, trying this gave me odd results, I found out that there is a 3.6v zener tried to ground on both these anlogue inputs lines via a very low ohm resistor.
It looks as though the zeners are something to do with the Usb connection to the host, as the high & low data lines for Usb are also on these pins.
Would I be able to replace the zeners with a 5v1 so I can get my analogue range, or will I blow the host's Usb port?
 

Nigel Goodwin

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Presumably the device runs at 3.3V, hence the zeners for protection - simply use a potential divider to reduce the voltage to 0-3.3V (or whatever you want). Personally I generally scale inputs so they don't need scaling in software, this gives sensible results with the LSD in single numbers - so for a 10 bit A2D I would scale to give either 10.23V FSD or 102.3V.

Stuffing 5V up a 3.3V input is going to break things!.
 

dr pepper

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No the attiny has a 78m05, it runs at 5v, the 'm05 is for ext power.
I'm thinking Usb is 3.3v hence the zeners.
And yes I adjusted the divider which did scale 50v down to 5v, it now divides 50v down to 3.5 and seems to work, only thing is the atod ref is still 5v so I'm losing a 1/3 of the resolution.
Another input is around 3v so I cannot change the chips atod reference from 5v.
 
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