DIGITAL TO ANALOG CONVERTOR

1. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi DIGITAL TO ANALOG CONVERTOR A quick presentation to introduce our peers on the techniques, specifications and applications of the DAC

2. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Table Of Contents Introduction Types Of DAC Specifications Applications Conclusion

3. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi INTRODUCTION The DAC fundamentally converts finite-precision numbers (usually fixed-point binary numbers) into a physical quantity, usually an electrical voltage. Normally the output voltage is a linear function of the input number. Usually these numbers are updated at uniform sampling intervals and can be thought of as numbers obtained from a sampling process

4. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi

5. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Output is a sequence of piecewise constant values or rectangular pulses, means that there is an inherent effect of the zero-order hold on the effective frequency response of the DAC resulting in a mild roll-off of gain at the higher frequencies

6. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi

7. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Types of DACs Many types of DACs available. Usually switches, resistors, and op-amps used to implement conversion Three Types: Resistor String Binary Weighted Resistor R-2R Ladder PWM (Pulse Width Modulation)

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10. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi SELECTION SWITCHES

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12. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Advantages: simple fast for < 8 bits Disadvantages: high element count for higher resolutions, reason: number of resistors: number of switches: slow for > 10 bits

13. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Binary Weighted Resistor DAC Utilizes a summing op-amp circuit Weighted resistors are used to distinguish each bit from the most significant to the least significant Transistors are used to switch between Vref and ground (bit high or low)

14. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Summing OP-Amps Inverting summer circuit used in Binary Weighted Resistor DAC. V(out) is 180° out of phase from V(in)

15. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Binary Weighted Resistor Assume Ideal Op-Amp No Current into OP-Amp Virtual ground at inverting input

16. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Calculation

17. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Cont’d

18. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Advantages and Disadvantages Advantages Easy principle/construction Fast conversion Disadvantages Requirement of several different precise input resistor values: Requires large range of resistors (2048:1 for 12-bit DAC) with necessary high precision for low resistors one unique value per binary input bit. (High bit DACs) Larger resistors ~ more error. Precise large resistors – expensive.

19. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi R-2R ladder DAC Example Convert 0001 to analog

20. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi R-2R DAC Example (cont.)

21. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Conversion Equation

22. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Advantages Only two resistor values Does not need as precision resistors as Binary weighted DACs Cheap and Easy to manufacture Disadvantages Slower conversion rate

23. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Pulse Width Modulation

24. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi SPECIFICATIONS Resolution Speed Linearity Settling Time Reference Voltage Errors

25. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi RESOLUTION Defined by the change in output voltage for a change of the LSB. Related to the size of the binary representation of the voltage. (8-bit) Higher resolution results in smaller steps between voltage values

26. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi SPEED Defined by the rate at which the register value is updated. Also called the conversion rate or sampling rate Speed is limited by the clock speed of the microcontroller and the settling time of the DAC

27. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi LINEARITY Represents the relationship between digital values and analog outputs Should be related by a single proportionality constant. (constant slope)

28. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi SETTLING TIME Time in which the DAC output settles at the desired value ± ½ VLSB. Ideally, an instantaneous change in analog volatage would occur when a new binary word enters into the DAC

29. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi REFERENCE VOLTAGE Used to determine how each digital input will be assigned to each voltage division Types: Non Multiplier DAC: Vref is fixed Multiplier DAC: Vref provided by external source

30. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi ERRORS Gain Offset Full scale Non-Monocity

31. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi GAIN ERROR Occurs when the slope of the actual output deviates from the ideal output

32. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi OFFSET ERROR Occurs when there is a constant offset between the actual output and the ideal output

33. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi FULL SCALE ERROR Occurs when the actual signal has both gain and offset errors

34. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Non-Monotonic Error Occurs when an increase in digital input results in a decrease in the analog output

35. 03/16/2007 Compiled by Rafael, Ravikanth and Hrishi Common Applications Electronic Cruise Control Computer Printers Sound Equipment (e.g. CD/MP3 Players) Project applications Motor speed controller Solenoid valves (pneumatics)