how do Vref work w/ DAC and ADC

Discussion in 'General Electronics Chat' started by new2circuits, Apr 22, 2009.

  1. new2circuits

    Thread Starter Member

    Apr 22, 2009
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    So far I've figured out ( I think) that the # steps on the output of and ADC is based on the full scale voltage divided by the #bits (resolution) of the ADC. For example, a 4 bit adc with a 5 V full scale input will have steps in 5/16 (or 0.3125V) increments.

    Question is: What does an external Voltage Reference do for the ADC ?
    And does it play in the calculation above at all ?

    Hope this is clear.
    Any insight is appreciated.
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    The voltage reference provides an accurate and stable point for the ADC and DAC for their conversions.

    A voltage reference can be picked with different levels of voltage accuracy and temperature stability. Your main power supply will fluctuate with different current draws and temperatures and can change based on current conditions. The reference will not be so susceptible to these changes. Power supplies are usually designed with +/- 5% types of accuracies whereas references can be +/- 1% or better. So, references offer better single board stability and also more board to board stability when many are made.

    If the converter in your example used an external reference, Vref, the full scale input would be from 0 to Vref and the steps would be in Vref/16 increments.

    Other converters may use -Vref to +Vref as the full scale or something else. Look to the specific data sheet to find what the full scale is for a part you are using.
     
  3. dude521

    Active Member

    Nov 1, 2008
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    From what I understand, the DAC works by having a bunch of resistors in series, specifically it will have 2^bits (2^4 = 16 in this case) resistors. When you feed a binary number into the DAC, it will output the voltage at that particular resistor. The Vref- will be at the bottom of the resistor string while the Vref+ will be at the top. Some DACs use different terminologies, but looking at the datasheet should make it clear how the Vref affects the output voltage.

    It should not affect the steps very much. So long at there is 5V between Vref- and Vref+, the step size will always be 5/16.
     
  4. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    There is more than one way to physically make a DAC, most of which I could not explain.

    The fact that you put a Vref into the circuit makes it sort of irrelevant what the power supply voltage to the DAC is. The step size is defined by the Vref voltage, which can be just about anything. The supply voltage and reference voltage can be very different, ie. the supply can be 5 volts while the reference is only 1 volt. Your steps would be defined off of the 1 volt reference, 1/16 in your example not 5/16.

    Remember, as with ADC, DACs can operate from 0 - +Vref, -Vref - +Vref, or whatever is defined in the datasheet. The range of Vref and supply voltages will also be defined in the data sheet. Some converters, both ADC and DAC, will contain an internal reference.
     
  5. new2circuits

    Thread Starter Member

    Apr 22, 2009
    21
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    Thanks allot for the clarification. Much appreciated.
     
  6. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
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    Happy to help!
     
  7. new2circuits

    Thread Starter Member

    Apr 22, 2009
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    One last question if you don't mind on this front:

    What exactly is the relationship between Vin and Vref for an ADC (or DAC for that matter) ?

    Can Vin be less than, greater than, or equeal to Vref (i.etruly independent of each other) ?

    I'm having difficulty really understanding the relationship.

    Thanks for your help.
     
  8. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
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    Vin is the analog signal that you are trying to digitize.
    Vref, along with the number of bits of accuracy, is the voltage that defines how large the individual voltage steps are between unique digital inputs.

    ADC can work with the Vin between GND and Vref, or -Vref and +Vref or however the manufacturer decides to make their part. Look at the data sheet for the part. It will be defined there.

    I hope this helps.
     
  9. new2circuits

    Thread Starter Member

    Apr 22, 2009
    21
    0
    It does help. Thanks again.
     
  10. new2circuits

    Thread Starter Member

    Apr 22, 2009
    21
    0
    Thanks for your help. Much appreciated.
     
  11. tahirsengine

    New Member

    Jun 20, 2012
    2
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    Thank you... that made many things clear for me. Very straight and to the point answer. Thanks alot.
     
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