how to pick Vref for A/D converter?

Can someone explain how do we pick Vref for, say, an 8-bit A/D converter? Is there an optimum one can choose, some equations etc? thanks.

It is always good to have a round number of mV per bit, so you can choose the following:
Vref=4.096V, reading is 4.096/256 = 16mV/bit
Vref=3.84V, reading is 3.84/256 = 15mV/bit
Vref=2.56V reading is 2.56/256 = 10mV/bit

For Vref =5V the reading is 5.00/256 = 19.53mV/bit and it is much more difficult to process ..
However, check with data sheets on a particular A/D converter what are the limitations on Vref max and Vref min ..

Higher value of Vref (within the A/D converter Vref rating, of course) results in less susceptibility to analog noise on the input. In general, the Vref selected should equal or exceed the expected dynamic range of the analog input.

Using extenal reference is good for ADC. Like many ADC in Analog Device.

Yibin.

First of all, you make sure the reference is within the ADC specifications. Also, make sure it can provide the current the A/D requires. Temperature stability has to be such that the performance of the system can be met. This also depends on the number of bits of the A/D.

Next, compare the reference with the maximum input voltage you expect. If it can always be lower than the reference, choose the reference just a little higher than the maximum input voltage. If the input voltage has to be divided down anyway, use as high a reference as possible. This will give you the best resolution and noise immunity.
For example, if the maximum voltage is 2V, then use a 2.048 or 2.5V reference, but if you need to divide down the input voltage, use a 4.096V or 5V reference.

Then, depending on what you use the circuit for, you can select a reference that is a power of two in mV, such as 2.048V or 4.096V, which will produce round numbers in mV/ count, making result conversion for absolute voltage measurements very easy.
If, on the other hand, you only need to do comparative measurements, then other voltages, such as 5V are fine.
For example, if you need to measure voltages for a voltmeter construction, use a 2.048 or 4.096V reference.
If you need to use the A/D in a system where you just compare measurements, without regards to absolute values, use a 2.5 or 5V reference.

Sometimes the choice can be dictated by the availability of other components, such as matched resistor networks to use as dividers. If you need to measure a 10V voltage, you could use a 2:1 divider, made up of identical tracking resistors in a resistor network. The obvious choice is a 5V reference, since 4.096 would not allow 10V measurements with this simple, stable divider and the resistor network does not come in a version that includes two tracking unequal resistors (you would need say a 10K and a 14.4K resistor in the same package.

How to design the Vref?
Which constrcution we select on common?

following things have to be consider when selecting Vref.
(1) what is the range of your analog signal ( i.e 0-5V or -3 to 3V etc) and whats the resolution ( say 10mv/bit or 20mv/bit etc..).
(2) what voltage ADC will generate internally for Vref
(3) is this Ok for the resolution you want
(4) Else you can give external Vref accordingt o your resolution. resolution can be find using Vref/2^(no. of bits)

If u use external Vref, u will need an external circuit for reference adjustment.
the larger the Vref the more immune to the external noise.
It is better to choose Vref such that the dynamic range of the input approximately
equals the dynamic range of accepted input of the ADC. This improve performance much

Use Internal Reference circuit to avoid any flactuations on vref due to bonding wire inductances