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Signal chain basics: Understand spurious signals in high-speed ADCs

Posted: 19 Dec 2011 ?? ?Print Version ?Bookmark and Share

Keywords:Data converters? ADC? fast fourier transform? spurious signals?

High-speed data converters are increasingly enabling new signal-chain architectures as their sample rates, resolutions and bandwidths increase. Data converters, which span the digital and analog domains, are more complex to debug than an analog-=only signal chain. This article discusses approaches for tracking down spurious signals in high-speed analog-to-digital converter (ADC) systems.

The figure shows a typical high-speed ADC signal chain. There are several potential sources of spurious signals in the system:

1) Signal chain before ADC input
2) The ADC itself
3) Sample clock
4) Power supplies

Figure: High-speed ADC signal chain.

The first step in debugging the data converter is to measure the performance so that it can be compared to the expected performance. High-speed ADCs usually are characterized with a continuous wave (CW) input. While this may not reflect the actual system requirements, it is easier to track down spurious signals and compare to expected performance.

To analyze the ADC output, you need to capture the digital output signal and examine the frequency domain by performing a fast fourier transform (FFT). If the input frequency is not coherent (an exact integer number of cycles in the length of the captured data), then a window function should be applied to the output data before FFT.

The input signal should be filtered before being input to the system, as signal generators typically have very poor distortion leading to large harmonics. The peak-to-peak amplitude at the ADC can be measured in the ADC output data, and the input adjusted to the desired input level.

There are several different types of spurious signals that can be generated in a system with ADCs. Due to the sampling nature of the ADC, spurs generated at frequencies above the 1st Nyquist zone (0 C fSAMPLE/2) will alias back into the Nyquist zone at the ADC output.

While the frequency of the spurs at the output can be easily calculated, another method is to observe the change in output frequency with input frequency. There are also several types of spur behaviors.

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