CMPE 146 DAC
Differential Nonlinearity (DNL)
1. Maximum deviation of an actual analog output step, between adjacent input codes, from the ideal step value of +1 LSB 2. An ideal DAC response would have analog output values exactly one code (LSB) apart, i.e. DNL = 0
Integral Nonlinearity (INL)
1. Maximum deviation, at any point in the transfer function, of the output voltage level from its ideal value 2. Two methods of calculation -End point a. Deviation from the straight line through the zero and full-scale points -Best straight line a. Use curve-fitting method on the transfer function b. In general, it is 50% of the end point method
Digital-to-Analog Conversion
1. Most of the signals in real world is analog or continuous 2. Digital computers processes or store signals in digital or discreet form 3. A conversion process is needed to convert a signal's digital form to an analog form that can be applied to the real world -For example, a computer converts the signal stored in a music file to sound 4. Digital-to-analog converter (DAC) -Converts digital word to an analog voltage that is a proportion of a reference voltage
R/2R Ladder DAC
1. N-bit DAC consists of -2N resistors of only two values in the ratio 1:2 -N switches 2. Output current Iout is the sum of individual currents through the higher-value resistors -Current ratios through the resistors: 1:2:4:8:...:2^(N-1) 3. Advantages -Fewer resistors to trim Scalable a. Easy to achieve higher number of bits -Network impedance is always R a. Regardless of N b. Constant reference current 4. Disadvantage -More components -More complicated circuitry
Voltage Divider DAC
1. N-bit DAC consists of -2^N resistors in series -2^N switches 2. Input code to the decoder turns on one of the switches 3. Advantages -Fast -Monotonic 4. Disadvantage -Large number of resistors and switches for high resolution
Binary-Weighted DAC
1. N-bit DAC consists of -N binary-weighted resistors -N switches 2. Output current Iout is the sum of individual currents through the resistors -Current ratios through resistors: 1:2:4:8:...:2^(N-1) 3. Advantage -Few components 4. Disadvantages -Need to tightly control resistor ratios to maintain monotonicity -Hard to achieve high resolution due to variations of resistors
Support Circuits
1. Voltage reference -Provides DAC a precise reference voltage to produce the correct output voltages 2. Filter -DAC produces a constant voltage level(step function) periodically -Filter smooths out the ragged edges of the piece-wise output signal -Essentially a low-pass filter to produce a smooth analog output signal a. Interpolates the signal between two successive output voltage levels b. Removes the high-frequency components 3. Quite often, the filter's output is connected to the input of a driver circuit the interfaces with the off-board component, e.g. motor, speaker
Ideal DAC
An example of a 3-bit DAC 1. The smallest resolvable output voltage is 1 LSB which is equal to fs/2^3 or fs/8 2. The output voltage has 8 discrete values representing a discrete linear transfer function
Nonlinearity
Is a DAC property 1. Error between the desired analog output and the actual output 2. Typically expressed in terms of number of LSB's 3. Two types of nonlinearity: Differential and integral
Monotonicity
Is a DAC property 1. In a monotonic DAC, the analog output always increases or remains constant as the digital input increases -The analog output never decreases during the input sequence 2. If the analog output decreases at any point during the input sequence, a DAC is said to be non-monotonic 3. Important property in many control applications where the direction of movement of an object is determined by the DAC -If the DAC behaves in a non-monotonic fashion, the object will be moved in the wrong direction 4. Example of a 3-bit non-monotonic DAC 5. Note that if DNL is greater than (more negative) -1 LSB, the DAC is non-monotonic
Settling Time
Is a DAC property 1. Interval between the output voltage starts to change and it sttles within += 1VLSB of the expected value 2. Determines the maximum output update rate
Resolution
Is a DAC property 1. Resolution of a DAC is the number of bits representing the digital values 2. Amount of variance in output voltage for every change of the LSBin the digital input -A measure of how closely we can approximate that desired output signal -Higher resolution provides smaller voltage division, this finer detail on output 3. Resolutuon: VLSB=Vref/2^N, where N=Number of bits of digital input to DAC
Update Rate
Is a DAC property 1. The maximum speed at which the DAC circuitry can operate and still produce correct output -Typically expressed in number of samples per second or simply just Hz 2. Depends on -Clock speed of input signal -Settling time of converter
