Download presentation

Presentation is loading. Please wait.

1
**Digital to Analog Converters**

Tyler Smith Brent Nelson Jerry Jackson 10/14/04

2
**Topics Discussed What is a DAC? Choosing a DAC Resistor String DAC**

Weighted Resistor DAC R-2R DAC PWM DAC associated errors Applications Conclusion

3
What is a DAC A digital to analog converter (DAC) is a device that converts digital numbers (binary) into an analog voltage or current output.

4
Choosing a DAC There are six main parameters that should be considered when choosing a DAC for a particular project. Reference Voltage Resolution Linearity Speed Settling time Error

5
**Choosing a DAC Reference Voltage**

To a large extent the output properties of a DAC are determined by the reference voltage. Multiplier DAC – The reference voltage is constant and is set by the manufacturer. Non-Multiplier DAC – The reference voltage can be changed during operation.

6
**Choosing a DAC Resolution**

The resolution is the amount of voltage rise created by increasing the LSB of the input by 1. This voltage value is a function of the number of input bits and the reference voltage value. - Increasing the number of bits results in a finer resolution - Most DACs in the bit range

7
**Choosing a DAC Linearity**

The linearity is the relationship between the output voltage and the digital signal input.

8
Choosing a DAC Speed Usually specified as the conversion rate or sampling rate. It is the rate at which the input register is cycled through in the DAC. High speed DACs are defined as operating at greater than 1 millisecond per sample (1MHz). Some state of the art bit DAC can reach speeds of 1GHz The conversion of the digital input signal is limited by the clock speed of the input signal and the settling time of the DAC.

9
**Choosing a DAC Settling Time**

Ideally a DAC would instantaneously change its output value when the digital input would change. However, in a real DAC it takes time for the DAC to reach the actual expected output value.

10
Choosing a DAC Error There are multiple sources of error in computing the analog output.

11
Example of a DAC - AD7224 An example of a DAC would be the Analog Devices AD 7224 D/A Converter. The AD7224 is a precision 8-bit, voltage-output, digital-to-analog converter with an output amplifier. Specifications: DAC Type – R-2R Voltage Out Input – Dual 8 Bit Reference voltage – Non-Multiplier 2v – 12.5v Settling Time - 7μs Cost - Under $4.00

12
Example of a DAC - AD7224

13
**Types of DAC Circuits 1. Resistor String**

2. N-Bit Binary Weighted Resistor 3. R-2R Ladder 4. PWM DAC

14
**Resistor String DAC 3 Bit Resistor String DAC**

Components of a String DAC Resistor String Selection Switches Opamp

15
Resistor String DAC How many internal components would be needed to create an 8 bit resistor string DAC? Number of Resistors = Number of Switches = Impractical for a DAC with more than a couple bits input.

16
**Weighted Resistor DAC Basic Idea: Use a summing op-amp circuit**

Use transistors to switch between high and ground Use resistors scaled by two to divide voltage on each branch by a power of two - + R 2R 4R 2nR R/2 Vout

17
**Weighted Resistor Example**

Summing op-Amp: Vref = -2V Digital word = 1010 V1 = -2V V2 = 0V V3 = -2V V4 = 0V Rf = R/2 V1 R Rf 2R V2 4R V3 - Vout + V4 8R

18
**Weighted Resistor Summary**

Advantages Simple Fast Disadvantages Need large range of resistor values (2000:1 for 12-bit) with high precision in low resistor values Need very small switch resistances Summary Use in fast, low-precision converter

19
**R-2R DAC Basic Idea: Use only 2 resistor values**

Use equal resistances in parallel to halve the resistance Creates a series of voltage dividers cutting voltages in half Another summing op-amp

20
**R-2R Example Digital word = 001**

V0 has two 2R resistances in parallel connected to ground Equivalent of R between V0 and ground V1 now has a resistance R to V0 and R to ground V0 = V1/2 V1 has two 2R resistances to ground Equivalent of R between V1 and ground V2 now has a resistance R to V1 and R to ground V1 = V2/2 V2 = Vref V0 = V2/4 V0 = Vref/4 Vout = -V0/2 Vout = -Vref/8

21
**R-2R Summary Advantages Summary Only 2 resistor values**

Better than weighted resistor DAC

22
**Pulse Width Modulation**

Approximate analog signal by switching on/off at high frequency Integral of output voltage from PWM ideally is the same as integral of desired output voltage N-bit digital words updated at rate f DAC clock must run at rate 2n*f Example: Desired output = 7V, supply voltage = 10V Operate 10V at 70% duty cycle to approximate 7V In practice: use counter, comparator, clock, integrator

23
**PWM Summary Advantages Disadvantages Summary All digital Cheap**

High sampling rate required Sensitive to clock variations Summary Best when load is a (relatively) slowly responding system

24
Errors

25
**Errors Gain Error Offset Error Full Scale Error Linearity**

Non-Monotonic Output Error Settling Time and Overshoot Resolution

26
**Gain Error Slope deviation from ideal gain**

Low Gain Error: Step Amplitude is less than ideal High Gain Error: Step Amplitude is higher than ideal

27
Offset Error The voltage is offset from zero when all input bits are low

28
Full Scale Error Combination of gain error and offset error

29
Non-Linearity The linearity error is due to the fact that the resolution of the converter is not constant.

30
Non-linearity The largest difference between the actual and theoretical output as a percentage of full-scale output voltage

31
Non-linearity It is the difference of tension obtained during the passage in the next digital code. Should be 1 LSB in theory.

32
**Non-monotonic Output Error**

A form of non-linearity due to errors in individual bits of the input

33
**Settling Time and Overshoot**

Changes in input are not reflected immediately in the output Lag times result

34
**Resolution Errors Inherent errors associated with the resolution**

More Bits = Less Error and Greater Resolution Less Bits = More Error and Less Resolution

35
Applications

36
**Programmable gain OpAmps**

Voltage controlled Amplifier (digital input, Vref as control) Digitally operated attenuators (Vref as input, digital control)

37
**Programmable Filters Integrate DACs in filters**

Variable cutoff frequency commanded by a digital signal

38
DAC Applications Used at the end of a digital processing chain when analog signals are required Digital Audio CD Players, digital telephones, etc. Industrial Control Systems Motor speed, valves, etc. Waveform Function Generators Cruise Control

39
References Alciatore, “Introduction to Mechatronics and Measurement Systems,” McGraw-Hill, 2003 Horowitz and Hill, “The Art of Electronics,” Cambridge University Press, 2nd Ed Analog Devices AD 7224 DAC General Overview and Specifications D/A Converter Fundamentals and Definition Of Terms Data Converter Fundamentals

Similar presentations

© 2019 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google