The Digital to Analog converter is an electronic device which converts digital signal to analog signal. The digital signals are in the form of 0 and 1. This converter produces only one output for multiple binary inputs. The number of binary inputs can be represented as 2^n where n is the number of binary inputs. The Weighted Resistor and R-2R Ladder are two types of Digital to Analog Converter.
Weighted Resistor Digital to Analog Converter
This DAC uses binary weighted resistors to produce analog output equivalent to the binary input. The binary input is provided to the inverting adder circuit. Below figure represents the weighted resistor DAC. We can represent the binary number in the form of bits b0, b1, b2. The bit b0 is the least significant bit. The bit b1 is the most significant bit. The switches present will jump to ground when the input is equal to zero. When the input is 1 the switches will switch to position -VR which is the negative reference voltage.
The circuit consists of an Op-Amp which is an inverting op-amp as input is provided through the negative terminal. As we know from the virtual ground concept, the value of voltage at inverting and no-invertig end of the op-amp will be the same. Hence, the value of voltage at the input terminal node will be equal to 0V.
The value of current at each node can be determined by writing the nodal equations.
We have a 3-bit binary weighted resistor DAC. As there are only three input bits b0-b2. Then the number of possible outputs can be from 000 to 111 for a particular reference voltage VR.
Now for N-bit binary weighted resistor DAC the general form of output voltage can be
R-2R Ladder DAC
This R-2R ladder DAC consists of R-2R ladder structure as shown below. Then the DAC produces analog output which is equal to the binary input. The ladder network then produces output with an inverting adder circuit in it. The figure below explains the circuit of R-2R ladder DAC.
Similar to Weighted Resistor DAC this circuit also has b0 as the least significant bit and b2 as the most significant bit. When the input bits are equal to 0 the digital switches jump to ground. When the input bits are 1 the switches jump to negative reference value -VR.
We can also easily find out the output voltage equation for this DAC for individual binary inputs. But it is very difficult to get the generalized output voltage equation in this case.
The reason why we prefer R-2R Ladder DAC over Weighted Resistor are
- The design of this type of DAC is easy as only R and 2R resistors are present.
- We can also include more R-2R combinations depending upon the required number of bits.