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Difference between DAC types-weighted resistor,R-2R ladder

This page covers difference between various DAC types including block diagram, equation etc. It covers weighted resistor DAC, R-2R inverting ladder DAC, R-2R non-inverting ladder DAC etc. It mentions advantages and disadvantages of them.

As we know DAC is the short form of D to A Converter. DAC converts digital information into analog information (either voltage or current). Switches, Resistors, Op-Amps are used to implement DAC.

Binary Weighted Resistor DAC

weighted resistor DAC

Figure-1 mentions block diagram of binary weighted resistor DAC. It utilizes summing Op-Amp circuit. Weighted resistors are used to distinguish each bit from MSB to LSB. Transistors are used to switch between Vref and ground (bit high or low).

Output voltage from binary weighted resistor DAC is expressed as follows:
Vo = { RF/(2N-1*R)}* {2N-1*VN-1 + 2N-2*VN-2 + ....... + 21*V1 + 20*V2 }

Advantages:
• It is Simple in Construction.
• It provides fast conversion.
Disadvantages:
• This type requires large range of resistors with necessary high precision for low resistors.
•  Requires low switch resistances in transistors.
• Can be expensive. Hence resolution is limited to 8-bit size.

R-2R non-inverting ladder DAC

R-2R ladder non-inverting DAC

Figure-2 mentions R-2R Ladder DAC of non-inverting type. It uses non-inverting type of Op-Amplifier.

DAC output non-inverting type equation1

Equation-1 above mentions output analog voltage for R-2R ladder type of DAC which is non-inverting type.

R-2R inverting ladder DAC

R-2R ladder inverting DAC

Figure-3 mentions block diagram of R-2R Ladder DAC of inverting type. It uses inverting type of Op-Amplifier.

Output Voltage for N -Bit DAC is expressed as follows:
DAC output inverting type equation2

Advantages:
• Only two resistor values are used in R-2R ladder type.
• It does not need as precision resistors as Binary weighted DACs.
• It is cheap and easy to manufacture.

Disadvantages:
• It has slower conversion rate.

For N bit DAC:
•  Number of different levels = 2N
•  Number of Steps = 2N - 1

Resolution or step size of DAC = Analog output/Number of steps = Va/( 2N - 1 )

% Resolution = (Step Size/Full scale output) x 100 %

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