Home>Characterization Test Center>Test How-To

Test high-speed ADCs for analog-input phase imbalance

Martin Rowe, Senior Technical Editor- April 19, 2011

Output harmonics from an ADC can produce errors in the device's digital output. Many ADCs have differential inputs, and you must apply signals with identical amplitudes that are 180º out of phase. If you don’t, the ADC's output will contain unwanted harmonics that produce measurement errors.

Adding cable length can compensate for mismatches, producing two opposing sine wave on the oscilloscope. Courtesy of Analog Devices.

Thus, you need identical signal paths from your analog signal sources to the ADC under test. In addition, you need two analog signal sources that produce clean sine waves that are 180º out of phase and the signal sources must be phase locked as well.

Test setups can affect test signals. You can learn how to minimize unwanted harmonics before you test an ADC for production or for use in a design. Rob Reeder, a senior applications engineer at Analog Devices, shows you the problems that can result from cable and PCB trace mismatches and how the mismatches vary with frequency. He also explains how you can use a test system consisting of two RF signal generators and an oscilloscope to measure the effects of differential imbalance on an ADC's input. You can download his paper, “Test high-speed ADCs for analog-input phase imbalance,” to learn more.