R-2R Ladder DAC

DAC circuit, theory and output of DAC

1. Lab Component “ R-2R LADDER DAC” Presented by, Chandu L Y (18BBTEC011) Department of Electronics and Communication Engineering 1 LINEAR INTEGRATED CIRCUITS 4BEC402

2. R-2R LADDER DAC AIM: To build and study DAC using R-2R ladder network. CIRCUIT DIAGRAM:

3. Explanation: A Digital to Analog Convertor is used when the binary output from a digital system is to be converted into its equivalent analog voltage or current. This type of converter has only two values of resistors R and 2R. The conversion speed reduces in this type due to parasitic capacitance. It is the simplest type o DAC where the switch between ground and inverting input of the op-amp is controlled by the input bit. The Gain is defined by equation: A v = -R/2R = -1/2 The output voltage of R-2R Ladder DAC is given by the equation: V0 = V R *(R F /R)[b1/2 1  + b2/2 2  + b3/2 3  + b4/2 4 ]

4. Working: This uses Kirchhoff’s current law, which states that the sum of currents entering a node must be equal to the sum of currents leaving a node. In the ladder, at each node, the current is split in half. By switching the currents into each node, the total current flowing is binary weighted. Using the principle of superposition, when you add more current into a resistance the total voltage appearing is the sum of the voltages caused by all the individual currents i.e. as each bit is activated, so the voltage increases at the output. This results in the conversion of the input digital stream in analog value.

5. Simulation of 4-bit R-2R Ladder DAC:

6. OUTPUT combinations for designed circuit: Design: R=Rf= 50Ω 2R = 100Ω Op-amp 741= +15V and -15V V(reference) = 16V Digital Input (4-bit)= HLLH (1001)

7. Advantages of R-2R ladder ADC: Easier to build accurately as only two precious metal film resistors are required. Number of bits can be expanded by adding many sections of same R/2R value Due to the small resistance spread only 2: the R-2R ladder can be fabricated monolattical with high accuracy and stability.

8. Applications: Audio Amplifier Video Encoder Display Electronics Data Acquisition Systems Motor Control Software Radio, etc..