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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....
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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.
- Fall '08