Digital to Analog Converter for High Fidelity Audio Applications

Matt SmithAlfred 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

• 400K resistor takes ~13,700 um2

• We used values of 400.010K, 200.005K, 16.146K, and 8.073K

• Resistors are a large part of area, but there is room to spare in the pad frame

• Using large resistors decreased power and tx_gate size

Output Buffer

• Unity gain opamp• Open loop gain = 2560

Output BufferThe high end wasn’t a problem – we wouldn’t go that high

But what to do about the low end?

Output Filter

Noise peaks are up to 12LSB

* Noise also decreased significantly by the addition of a 10uF capacitor from Vref to ground

Complete Design Schematic

0 7

815

Source-follower and

output buffer

200K resistors

D - Flip Flops

Bias

Final Layout

400K resistors

Transistor Switches

Heavy-duty unity-gain buffer

Simulation Results - Overall

Schematic simulation of the entire circuit

Simulation Results – Zoom-In

• Full-circuit simulation with all 16 bits operating.

• LSB increments are 28uV

• Noise on the vast majority of transitions is < 0.05 LSB

Simulation Results - Noise

MSB transition point – momentary noise: 3.56mV (128LSB)

• Noise and non-linearity occur at significant bit transitions

• Trade-off between noise and non-linearity

• Tx_gates too small gives large non-linearity

• Tx_gates too large give large noise spikes

• Placing a huge capacitor (10uF) on the Vbias power line to the array helps transient noise

• Noise and non-linearity are worst for MSB and decrease by powers of 2 for less-significant bits

Simulation Results - Speed

Fall time: 6.5us Rise time: 80ns

• Absolute maximum frequency where the circuit can achieve full amplitude is 1/(6.58us) = 152kHz

• Little distortion of full-amplitude square wave at the limit of human hearing (20kHz)

• No distortion of full-amplitude sine wave at 20kHz

Design Assessment

• Maximum output voltage: 3.441V

• Minimum output voltage: 1.604V

• Voltage swing: 1.836V

• LSB voltage change: 28uV

• Maximum Differential Non-linearity: 1LSB

• Integral Non-linearity: Approximately 16LSB

• Power consumption: 23.5mW

Future Work / Improvements

• Temperature stability (reference)

• Wide swing output

• Better low-pass filter would allow larger tx_gate and less integral non-linearity