ADC reading higher voltage than Vdd?

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hyperion007
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ADC reading higher voltage than Vdd?

Postby hyperion007 » Sun Nov 03, 2013 11:15 am

Hi,

Just a quick question. If a PIC has 5.02V at Vdd, what value would the 12-bit ADC show if the voltage on the ADC pin is 5.08V?

The problem is that I don't have a constant supply of 5.08V to use as reference on the +Vref pin
Last edited by hyperion007 on Sun Nov 03, 2013 10:09 pm, edited 1 time in total.

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Re: ADC reading higher voltage than VCC?

Postby kersing » Sun Nov 03, 2013 11:59 am

You need to check the device data sheet to see if an input voltage above 5V (or Vdd) is actually allowed. If so, the value would be 4095 (there are 12 bits for values, so 2 ^ 12 values starting at 0, means 2 ^ 12 - 1 is the max)
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Re: ADC reading higher voltage than Vdd?

Postby hyperion007 » Fri Nov 22, 2013 3:21 pm

VDD + 0.3V
VSS - 0.3V
So I am well within the limits. And for the record, the ADC value does NOT roll over if the voltage read is slightly higher than VDD.
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medelec35
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Re: ADC reading higher voltage than Vdd?

Postby medelec35 » Fri Nov 22, 2013 5:20 pm

hyperion007 wrote:DC value does NOT roll over if the voltage read is slightly higher than VDD.

ADC never rolls over, it only reaches maximum value or 0.

It is anly when variables are used in calculations they can roll over or roll under< Could be a made up word :p

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Re: ADC reading higher voltage than Vdd?

Postby Brendan » Thu Dec 05, 2013 1:35 am

Just came across this thread while browsing.

For general future reference, the +0.3V above Vdd and -0.3V below Vss relate to absolute maximum limits governed by protective diode input structures, and has nothing to do with functional performance.

To guard against input spikes and ESD in general, MOS inputs commonly employ two reverse-biased diodes in non-RF applications. Consider the 'upper' diode with cathode to Vdd, the 'lower' diode with anode to ground, and the junction between the two connected to the input in question. To cut a long explanation short, the absolute maximum ratings above Vdd or below Vss relate only to the voltage where the natural forward-voltage characteristics of the protection diodes limits the forward current to within safe boundaries (at the quoted max/min voltage) and avoids diode damage.

At best the ADC range will apply to the available supply voltage, where the diode-based input limitations have no bearing whatsoever on designed input function.

Hope this helps :)


All the best,

Brendan
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