R-2R DAC

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  R-2R DAC


This is Combinational Logic Circuit

The R-2R Digital-to-Analogue Converter, or DAC, is a data converter that converts a digital binary number into an analogue output signal proportionate to the digital number's value using two precision resistors.

R-2R DAC Circuit with Four Zero (LOW) Inputs




(RF/RIN)*VIN is the output voltage of an inverting operational amplifier. If we set RF equal to R, RF = R = 1, and R is connected to ground (0V), there is no VIN voltage value (VIN = 0), hence the output voltage is (1/1)*0 = 0 volts.
As a result, the output voltage of a 4-bit R-2R DAC with four grounded inputs (LOW) will be “zero” volts, and a 4-bit digital input of 0000 will yield an analogue output of 0 volts.

So, if we link input bit VA HIGH to +5 volts, what happens? What is the R-2R ladder network's equivalent resistive value, as well as the op-output amp's voltage?

R-2R DAC with Input VB




 




The output voltage, VOUT, is computed at 625mV, which is one-eighth (1/8th) the value of the +5V input (5/0.625 = 8) voltage, with input VB HIGH and logic level “1” and all other inputs grounded at logic level “0.”

We can also observe that when only input bit VA is applied, the output voltage is doubled, which is to be expected because the 2nd bit (input) has twice the weighting of the 1st bit.

Let's check what happens if we link input bit VC HIGH to +5 volts and observe what happens to the output voltage.

R-2R DAC with Input VC






The output voltage, VOUT, is computed at 1.25 volts, which is one-quarter (1/4) the value of the +5V input (5/1.25 = 4) voltage, with input VC HIGH and logic level “1” and the remaining input bits at logic level “0.” This voltage is double the output of input bit VB but also four times the value of bit VA, as can be seen.
Because input VC is the third bit, its weighting is double that of the second bit and four times that of the first bit.

Finally, let's look at what occurs when we connect input VD HIGH to +5 volts and observe what happens to the output voltage.




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