Vref, Vin , Supply voltage,ADC

Dodgydave

Joined Jun 22, 2012
9,657
The Vref is the limit of the a/d converter, so if you have a 10bit a/d with 1.024Vref, then each bit is worth 1mV , so if Vref is now 5V each bit is worth 4.88mV.

The modern pic micros now have an in built Vref from 1.024 to 4.096V.
 

ebeowulf17

Joined Aug 12, 2014
3,274
The Vref is the limit of the a/d converter, so if you have a 10bit a/d with 1.024Vref, then each bit is worth 1mV , so if Vref is now 5V each bit is worth 4.88mV.

The modern pic micros now have an in built Vref from 1.024 to 4.096V.
The question I'm less clear on is what the maximum safe input voltage is (in terms of damaging components, not in terms of getting accurate, meaningful readings.) I've seen devices which were safe up to their supply voltage, while others say they can be damaged by any input higher than Vref.

Perhaps I've misread or misunderstood specs. Either way, it's a frustrating and confusing gray area for me.
 

Dodgydave

Joined Jun 22, 2012
9,657
The question I'm less clear on is what the maximum safe input voltage is (in terms of damaging components, not in terms of getting accurate, meaningful readings.) I've seen devices which were safe up to their supply voltage, while others say they can be damaged by any input higher than Vref.

Perhaps I've misread or misunderstood specs. Either way, it's a frustrating and confusing gray area for me.
You can't input a Voltage higher than the Vref, even if the chip has an External Vref pin, no point in setting a Vref of 10V when using a 5V supply it will damage the chip.
 

ebeowulf17

Joined Aug 12, 2014
3,274
You can't input a Voltage higher than the Vref, even if the chip has an External Vref pin, no point in setting a Vref of 10V when using a 5V supply it will damage the chip.
I was thinking of the other way, not Vref higher than supply, but Vref lower than supply.

For example, the ADS1115 claims input limits referenced to supply voltage and ground:
http://www.ti.com/lit/ds/symlink/ads1115.pdf
upload_2018-2-5_8-3-54.png
This datasheet seems to indicate that even though the Vref is an internally generated 4.096V, if VDD is 5.0 then any analog input less than 5V (or even 5.3 considering the spec is VDD +0.3) should be safe, even if it's significantly over the Vref.

Meanwhile, on the other end of things there's the ESP8266 microcontroller, which is designed for a 3.3V supply voltage, but whose ADC uses a 1V Vref. I've read in numerous places that any input voltage over the 1V Vref can damage the chip.

I'm sure there are other examples, but it's these two in particular that got me confused and curious.
 
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