LINEAR Application Note 93 Specifications User Guide

Application Note 93 February 2003

Instrumentation Applications for a Monolithic Oscillator A Clock for All Reasons Jim Williams

INTRODUCTION Clock Types Oscillators are fundamental circuit building blocks. A Commonly employed oscillators are resonant element substantial percentage of electronic apparatus utilize os- based or RC types.2 Figure 1 shows two of each. Quartz cillators, either as timekeeping references, clock sources, crystals and ceramic resonators offer high initial accuracy for excitation or other tasks. The most obvious oscillator and low drift (particularly quartz) but are essentially application is a clock source in digital systems.1 A second untunable over any significant range. Typical RC types area is instrumentation. Transducer circuitry, carrier based have lower initial accuracy and increased drift but are amplifiers, sine wave formation, filters, interval genera- easily tuned over broad ranges. A problem with conven- tors and data converters all utilize different forms of tional RC oscillators is that considerable design effort is oscillators. Although various techniques are common, a required to achieve good specifications. A new device, the simply applied, broadly tunable oscillator with good accu- LTC1799, is also an RC type but fills the need for a simply racy has not been available. applied, broadly tunable, accurate oscillator. Its accuracy

and drift specifications fit between resonator based types and typical RC oscillators. Additionally, its board footprint, a 5-pin SOT-23 package and a single resistor, is notably small. Note that no external timing capacitor is required.

TYPICAL TYPICAL FREQUENCY FREQUENCY TEMPERATURE POWER SUPPLY

CLOCK TYPE ACCURACY RANGE TUNABILITY COEFFICIENT REJECTION RATIO COMMENTS Quartz 0.005% 10k Hz to Poor 0.5ppm/°C 1ppm/V High Stability and Initial Accuracy at Expense of

200MHz Easily Achieved. Tunability. Essentially No Tunability. 1 • 10–9

See Comments Stability Achievable with Compensation Techniques

Ceramic 0.5% 250k Hz to Poor 30ppm/°C 20ppm/V Lower Performance and cost than Quartz. Resonator 60MHz Essentially Untunable LTC1799 1.5% 1k Hz to Good 40ppm/°C 500ppm/V Add 10 to 50ppm/°C Temperature Coefficient,

33MHz Plus Resistor Depending on Resistor Type. Extremely Small

Temperature Footprint— SOT-23 and 1 Resistor Coefficient

Typical RC 10% 1Hz to Good 200ppm/°C 2500ppm/V Requires Careful Design and Component Selection Based Clock 25MHz for Best Results

Figure 1. LTC1799 Compared to Other Oscillators. Quartz and Ceramic Based Types Offer Higher Frequency Accuracy and Lower Drift but Lack Tunability. RC Designs are Tunable but Accuracy, Temperature Coefficient and PSRR are Poor

, LTC and LT are registered trademarks of Linear Technology Corporation.

Note 2: This forum excludes such exotica as rubidium and cesium

Note 1: Strictly speaking, an oscillator (from the Latin verb, “oscillo,”

based atomic resonance devices, nor does it admit mundane but dated

to swing) produces sinusoids; a clock has rectangular or square wave

approaches such as tuning forks.

output. The terms have come to be used interchangably and this publication bends to that convention.

AN93-1