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

Posted: 29 May 2012 ?? ?Print Version ?Bookmark and Share

Keywords:analog-to-digital converter? digital-to-analog converter? nonlinear distortion?

In "Signal chain basics: Understand spurious signals in high-speed ADCs", we discussed approaches for tracking down spurious signals in high-speed analog-to-digital converter (ADC) systems. In this article, we examine similar techniques for high-speed digital-to-analog converter (DAC) systems.

There are two types of high-speed DACs: interpolating and non-interpolating (straight). Interpolating DACs, described in "Signal chain basics: Using the digital features of high-speed DACs", typically contain digital signal processing features such as interpolation, digital quadrature modulation, and internal phase-locked loops (PLLs). Straight DACs simply convert digital values to an analog output.

Figure 1 shows a DAC signal chain in a radio application. There are several potential sources of spurious signals from the DAC, each has distinct characteristics:
1) Digital interface
2) Input clock
3) PLL
4) Power supplies
5) Digital saturation
6) Improper output termination

Figure 1: Typical high-speed DAC signal chain.

DACs are characterized with both a continuous wave (CW) or modulated signal output (commonly a 3G or 4G communication signal like WCDMA or LTE). Datasheets contain AC specifications listed in a table as well as output spectral plots. Comparing against the spectral plots offers the most information for debugging system problems, as each problem type has different frequency characteristics.

To debug a high-speed DAC, first verify error-free data input. Input interface problems are common and cause bit errors, resulting in wideband noise. In a straight DAC this causes an elevated noise floor following a sin(x)/x frequency response (figure 2). In an interpolating DAC, the noise spectrum is shaped by the internal digital interpolation filters. This gives a distinctive output spectrum that matches the filter frequency response (figure 3).

Figure 2: Output spectrum for a straight DAC with interface errors.

Figure 3: Output spectrum for an interpolating DAC with interface errors.

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