lab6_dac_notes - ECE 327 Electronic Devices and Circuits...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: ECE 327: Electronic Devices and Circuits Laboratory I Notes for Lab 6 (Digital-to-Analog Converter (DAC) Lab) 1. Strictly speaking, this laboratory is not about digital-to-analog conversion . • A DAC converts an abstract numerical quantity to a physical quantity (e.g., a voltage) • “Current-summing” DAC uses binary-weighted currents — switches select which currents to sum – In practice, single-pole double-throw (SPDT) switches are used (i.e., no “floating”) – Can generate currents with binary-weighted resistors * Turning on and off currents quickly causes switching transients (i.e., introduces noise) * Instead, switch constant current flow toward and away from sum junction * Binary-weighted resistors are difficult to implement (especially for high number of bits) – R –2 R ladder — used to make a more clever current-summing DAC ( Figure L6-1 ) * Need R , 2 R , and switch per bit · Much easier to match R and 2 R (helps with linearity — refer to INL and DNL ) * DAC input impedance is 2 R regardless of number of bits and switch states · Each new bit just splits off more current · Constant current means no switching transients – Multiplier DAC (mDAC) — like a “digital potentiometer” * Ratiometric : Output is a ratio of input (usually power supply) · Principle used in relaxation oscillators to make them insensitive to power supply changes · Frequently used with sensors for same purpose – Use zero-order hold (ZOH) to construct analog signal from digital time series * Result is pulse-amplitude modulated (PAM) version of digital signal * Low-pass filter to smooth edges if necessary (sometimes system is enough of a filter) · “Oversampling” can increase filter performance and decrease its complexity and cost · “Oversampling”: reconstructing a time series at much higher frequency by interpolation • Alternative: “Oversample” and use “1-bit DAC” (looks sexy on shiny metallic label) – “1-bit DAC”: Each point is represented as one of two analog states ( cheap to implement) – Filtering the quickly switching output produces the desired analog signal – Marketers make empty/nonsensical claims of quantization noise reduction (real benefit: cost ) – Pulse-density modulation (PDM, Σ- Δ , Δ- Σ , SDM) is frequently used * Simple low-pass filter (LPF) restores analog signal * LPF can be omitted if load has appropriate frequency response • Pulses are often used in digital communication – pulse-code modulation (PCM) represents binary c odes as serial high and low p ulses * e.g., compact disc digital audio (CDDA) — “pit”-“land” transitions encode “1” * a sequence of pulses (e.g., 7 subsequent pulses) represents one “code”a sequence of pulses (e....
View Full Document

This note was uploaded on 08/05/2011 for the course ECE 209 taught by Professor Staff during the Fall '08 term at Ohio State.

Page1 / 4

lab6_dac_notes - ECE 327 Electronic Devices and Circuits...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online