Hi Md Ashraful Islam, 

you should use an 8bit DAC ( Digital to Analog Converter ). It has 8 inputs for the 8 bits to convert, one input for a reference voltage Vref and inputs for the power supply. The analog output voltage is a proportion of a reference voltage and the proportion based on the digital word applied.

i.e. Vout =  Vref * [ 2^(b0) +  2^(b1) + ... + 2^(b7)], where b0, b1, .. , b7 are the bits of the digital word applied.

Obviously, this is an explanation very simplify.

http://www.analog.com/library/analogdialogue/archives/43-09/EDCh%206%20Converter.pdf

The result is the ADC value * (voltage range/255).

So for a value of 127, voltage range of 5 V,  5/255*128= 2.49 V

The 255 comes from the maximum number (0 based) one can see in 8 bits. You may need to consider off set voltages too and add/subtract as needed.

For an 8-bit ADC with 0-5V input range, each bit results into 19.48mV or roughly 20mV.

Therefore, in your software you can simply multiply this number 20 (or 19.48 to be precise) to get the voltage reading.

Hope this satisfies your requirement.

Thanks and best luck!

Thanks a lot @ everyone 

There is confusion about ADC or DAC. In some sources, it is said there are 2n-1, in some sources it is said there are 2n discrete levels. Let’s finish this confusion.

ADC (Analog to Digital Converter)

For example; if we have 3-bits ADC and an analog sensor with the measuring range of 0 to 1 Volt. How will we convert this voltage to digital number? What will be the minimum voltage value that we can detect (means resolution)?

Answer:

23=8 and maximum number for 3-bits in binary system is (111)2 = 7. But there are 8 different numbers starting from 0 to 7. So there will be 8 different numbers for the voltage span of the sensor.

• The voltage span is 1-0=1 V
• 1V/8=0.125 V (The minimum voltage value we can detect with the given sensor.)

e.g. ADC’s output will be 0 (000)2 while the sensor’s voltage is 0 to 0.124 V. When it comes to 0.125 V, it will become 1 (001)2. It can be understood easily from the graph.

DAC (Digital to Analog Converter)

Same considerations are valid for DAC also. You must divide the voltage number to bit number.

When we look at the above example;

1V/8=0.125V

e.g. Digital number 5 (binary 101) will produce 0.625 V (5*0.125V).

When we come to 8 bit; there will be 28=256 (0...255) discrete levels for an analog signal. So you must divide the voltage value to 256.

For example; if the analog sensor’s voltage measuring range is 0-5 V. What will be the DAC’s output for the digital number of 33?

• 5/256= 0.01953125 V (resolution-1 bit digital variation’s voltage response)
• So, 33* 0.01953125= 0.64453125 V

You should take your numerized value ration ( value / 255 ) and multiply by your reference voltage ( usally Vcc, unless you have change it ) :

Example : for NumVoltage = 122 and Vref = 3.3V then :

Voltage = ( Vref x NumVoltage / 255 ) = 3.3 x 122 / 255 = 1.57V