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Digital to Analog Converter for High Fidelity Audio Applications Matt Smith Alfred Wanga CSE598A Project Summary • High Quality Audio Applications – Accurate Reproduction [16 bit] – Low Noise • Versatile – Support for Standard Sampling Rates – Specifications that allow use in Various Audio Applications R-2R Ladder Architecture * Buffer inserted on output for low output impedance D Flip Flop Schematic D Flip-Flop Simulation Results CLK DIN Q QNOT Pass Switch • NMOS pass transistor only • CMOS Transmission gate not needed because we don’t go near VDD • Sized to allow proper operation Voltage Reference Voltage Reference Results • Circuit modified from Homework #3 provides 2.5V and 1.1V voltage reference • Reference output stable down to ~3.3V supply voltage • 2.5V reference varies by 400mV over -40C to 85C (3.2 mV/˚C) • 1.1V reference varies by 150mV over -40C to 85C (1.2 mV/˚C) Voltage Reference Results • Power supply rejection ratio is 48dB • LSB accuracy corresponds with 7mV p-p supply noise Resistor • 200K resistor takes ~6800 um2 • We used values of 400K, 200K, 16K, and 8K • Resistors are a large part of area, but there is room to spare in the pad frame Output Buffer • Unity gain opamp • Open loop gain = 2560 Output Buffer The high end wasn’t a problem – we wouldn’t go that high But what to do about the low end? Output Filter This noise is partly an artifact of simulation, but a low-pass filter is a good idea anyway Complete Design Schematic Final Layout Bias Opamp 400K resistors 0 7 200K resistors 15 8 Transistor Switches D - Flip Flops Simulation Results Complete-circuit simulation with top 7 bits active and the rest tied high Simulation Results • Full-circuit simulation with all 16 bits operating. • LSB increments are 28uV • Noise peaks are 1/4LSB Design Assessment • • • • Maximum output voltage: 3.441V Minimum output voltage: 1.604V Voltage swing: 1.836V LSB voltage change: 28uV – Calculated and observed agree to +- 0.2uV • Just a little noisy … simulation problem or real? Future Work / Improvements • Temperature stability (reference) • Wide swing output • Decrease noise on output