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:
• Av = -R/2R = -1/2
• The output voltage of R-2R Ladder DAC is given by the equation:
• V0 = VR*(RF/R)[b1/21 + b2/22 + b3/23 + b4/24]

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..