What determines what value of vref to use for ADC and DAC?

[matrixofdynamism]

vref is the maximum output that DAC shall give when the biggest binary input word (0xFF for 8 bits) is given.
vref is the maximum voltage for which an ADC shall give the biggest binary output word (0xFF for 8 bits). I don't know what will happen if the input to the ADC is bigger than vref though.

Why do many ADCs have external vref pin but others have internal vref which is still smaller than the vcc? Shouldn't vref be simply set to vcc by default in all applications?

 

[Easyrider83]

If input voltage > Vref+, adc will return maximum value = 2^n, where n - is a resolution, like 8, 10 or 12 bits
External reference are more stable and precicly. For example, 4.096V reference voltage are popular. With 12 bits ADC it gives 1 bit/mV ratio that makes calclulations simpler.

 

[KlausST]

Hi,

vref is the maximum output that DAC

This is not true for all DACs, but it is true for many DACs. Dont rely on your speculation. It is necessary to read the datasheet on this.
To be exact the max output often is VRef * 255 / 256 for an 8 bit DAC. So one LSB less than VRef.

vref is the maximum voltage for which an ADC shall give the biggest binary output word (0xFF for 8 bits)

Exactely the same as above. Read datasheet.
Even the max decodable voltage is 1 LSB less than VRef, too. Datasheet will tell you.

I don't know what will happen if the input to the ADC is bigger than vref though.

In all cases i know it gives out (2^n - 1). But here again: read datasheet. Some ADCs have additional status bits for overrange.

Why do many ADCs have external vref pin but others have internal vref which is still smaller than the vcc?

If VRef is wrong then the ADC result is wrong also.
VCC is not meant to be precise nor meant to be stable with: ageing, load, temperature.
VCC simply is meant as power supply.

Imagine the pressure in your house´s incoming water pipe. You need the pressure to supply water to yur house. But you won´t use the water pressure as reference when you fill your car wheels with air.
--> Therfore in 99% i use a reliable and stable voltage reference.
But the VRef is supplied by VCC. So the VRef regulation loop needs some headroom to regualte... and you need to take the lowest possible VCC into account. Even for a microsecond..

Shouldn't vref be simply set to vcc by default in all applications?

Answered above.

Hope that helps

Klaus