Linear Technology AN132 Application Note Fidelity Testing for A+D Converters

About this manual

The document details the design of a Wien bridge oscillator with an LED-driven CdS photocell gain control to achieve extremely low distortion levels suitable for testing 18-bit A/D converters. The text explains how this specific circuit eliminates J-FET conductivity modulation errors and maintains stable output amplitude through automatic gain control (AGC).

The note also covers the verification of the oscillator's performance using specialized analyzers like the Audio Precision 2722, noting limitations of standard instruments at low distortion levels. Finally, it presents test results for an LTC2379 A/D converter driven by an LT6350 amplifier, displaying spectral error components and tabulated readings.

What's inside

• Introduction
• Oscillator Circuitry
• Verifying Oscillator Distortion
• AD Testing
• Tools for A>D Fidelity Testing

Frequently asked questions

What is the minimum measurable distortion of the HP-339A analyzer mentioned?

The HP-339A specifies a minimum measurable distortion of 18ppm.

What is the typical Total Harmonic Distortion + Noise limit for the Audio Precision 2722?

The Audio Precision 2722 has a specified limit of 2.5ppm THD+N, with 1.5ppm being typical.

What distortion level did the test system achieve with the LTC2379 and LT6350?

The combination produced a 2nd harmonic distortion of -111dB, which is about 2.8ppm.