What is Vref in ADC, what is purpose of vref in ADC

Thread Starter

mishra87

Joined Jan 17, 2016
906
Hello All,

I have few quarries left to be answered related ADC.
Point to point questions need to be answered point to point.

1. Can ADC measure the signal without using Vref and what is the role of Vref in ADC.
2. How to select Vref value for our application. Lets say we have to measure 0-100mV time varying signal in MCU, what could be appropriate Vref and why.
3. Detailed explanation of Vref in ADC would be appreciated.

Thanks in Advance !!!
 

danadak

Joined Mar 10, 2018
4,057
1) Depends on how part is configured, that is if Vref is stipulated as an external
reference and there is also an internal reference available for use. Generally
speaking it refers to the maximum value the A/D digital result can handle.
So if A/D is 8 bit, and Vref is 1V, then when input is 1V output digital value of
A/D is 256 (ignoring sign bits for the moment). If input is 1/2 V then A/D
digital result is 128.....

Resolution of A/D is Vref/2^n, n number of bits of A/D. So as Vref increases
resolution goes down, and vice versa.

2) Vref is basically the full scale / maximum value the A/D can convert. So a larger
Vref, generally speaking, means larger dynamic range, lower resolution. Smaller
Vref, generally speaking, smaller dynamic range, higher resolution. You trade off
dynamic range for resolution. There are also other factors that affect this, # bits
A/D converts to, input buffer G if it has one....

3) http://www.ti.com/lit/an/sbaa004a/sbaa004a.pdf

https://www.analog.com/en/education/education-library/analog-digital-conversion-1972.html Chapter 2

What A/D are you using ? Is it in a processor ? What is your range you want
converted and resolution target goals ?


Regards, Dana.
 
Last edited:

LesJones

Joined Jan 8, 2017
3,104
Reading the section of the data sheet on the MCU that you are using that deals with the ADC should give you that information. You would choose a reference voltage that was within the range that the particular ADC could deal with and a value that you could generate. I would chose the highest value within these constraints so that the value of one bit was not so small that the comparison in the ADC was not between very low voltages. So in practice I would choose a value of the MCU supply voltage or a bit less to make each bit a nice decimal voltage value. Some MCUs have a built in reference voltage. For example the PIC12F1840 reference voltage can be set to 4.096, 2.048 or 1.024 volts. So as this MCU has a 10 bit ADC with the 4.096 volts reference one bit would be 4 mV. In your example of 100 mV full scale I would amplify it using an op amp to scale it to a suitable value.

Les.
 

Dodgydave

Joined Jun 22, 2012
9,921
Vref sets the Maximum input signal to measure, so if your micro wants to measure a signal of say 1V, you set the vref to 1V, and the micro will divide the 1V into 8 or 10 bits or what ever your micro can divide by,.

for a 10 bit result is 1024 which is more accurate.
 
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