Home>Manufacturing Test Center>Test How-To

Build a circuit to test ADCs

D Sargent- June 1, 2012

Link to Vojtěch Janásek's article, "Low-distortion oscillator tests measurement circuits."

Today’s 20-bit to 24-bit ADCs need low-distortion signals for testing how well the ADCs digitize analog signals. Distortion in the source signal will add to any distortion that the ADC produces. Thus, low-noise signals are critical.

When he couldn’t find an oscillator with sufficiently low distortion, Vojtěch Janásek, an engineer at Janascard in the Czech Republic, built his own. The oscillator produces distortion that’s more than –140 dB below the oscillator’s fundamental output signal. In addition, Janásek designed a notch filter that removes the oscillator’s fundamental frequency. That lets him view the distortion produced by the ADC.

The oscillator uses an inverted Wien-bridge topology with amplitude stabilization through an LED-driven CdS (cadmium-sulfide) photocell isolator. Using SPICE simulations before building the circuit, Janásek showed how the oscillator’s voltage noise-spectral density is highest at its resonant frequency, then falls at higher frequencies.

To verify the performance of his oscillator and filter, Janásek connected the final test signal to a data-acquisition system and frequency-analysis software. This particular module has a 14-bit ADC with a 400-ksamples/s sample rate. The module averages eight samples to reduce sampling speed to 50 ksamples/s and takes 128 ksamples to perform spectral analysis. Janásek’s measurement showed that the circuit’s THD (total harmonic distortion) is –145 dB.

Janásek’s article “Low-distortion oscillator tests measurement circuits," includes schematics for both the oscillator and filter circuits, plus plots of their performance. T&MW