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Design wideband output networks for high speed DAC

22 Apr 2014  | Jarrett Liner

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Nowadays, the demand for new IC components and technology continues to grow at phenomenal rates. The commercial and defence industry are leaders in this charge. Most new specifications dispatched to the semiconductor industry today revolve around reduced size, weight and power or SWaP. Here in the semiconductor industry we meet those requirements by ever improving technology and clever designs. However, performance is still a key demand as well, especially for Digital to Analogue Converters (DAC) technology in the GSPS space. To keep this pace, the analogue output matching network is often overlooked as a key element.

In order to provide more clarity, high frequency is considered to be over 1GHz, high speed is considered to be over 1GSPS; most importantly the end user may incorporate an amplifier after the DAC therefore useable signals are less reliant on signal level, and more on noise and fidelity. In this paper, the matching component and their interconnectivity will be discussed. Particular attention will be given to the key specifications to consider when selecting a transformer or balun along with connection configuration techniques. Finally, ideas and optimisation techniques will be provided to show how to achieve a wideband smooth impedance transformation for DACs operating in the GHz region.

Setting the stage
DACs have wide range of uses; some of the most obvious uses include complex waveform generation at high frequency for commercial and military communications, wireless infrastructure, Automatic Test Equipment (ATE), and RADAR and military jamming electronics. Once the system architect has found the right DAC, the output matching network must be considered to preserve the signal constructed. The component selection and topology become even more important as the GSPS DACs applications require operation in the super-Nyquist, where the desired spectral information is in the 2nd, 3rd or 4th Nyquist zone.

Figure 1: DAC sinx/x output frequency response vs. mix mode.

Intellectual prerequisites
First let us consider the role of the DAC and its position in the signal chain. A DAC functions much like a signal generator. It can provide single tone to complex waveforms at a range of centre frequencies (Fc). Historically the Fc max is in the first Nyquist zone, or half the sample frequency. Newer DAC designs have internal clock doublers to effectively double the first Nyquist zone; we can refer to this action as 'mixed-mode' operation. The natural output frequency response curve of a DAC using mixed-mode takes on the shape of a sinX/e^(X^2) curve (figure 1).

System architects can consult the product datasheet to understand component performance. Often performance parameters such as power level and Spurious Free Dynamic Range (SFDR) will be listed at various frequencies. The clever system designer can extend the use of the same DAC into the super-Nyquist zones mentioned earlier. It's noteworthy to mention the expected output level will be significantly lower at higher frequencies (zones), for which many signal chains might include the additional gain block or driver amplifier after the DAC to compensate for this loss.

Component considerations
The best performing GSPS DAC is only as good as the end user designs and measures it. To shine the best light on a good DAC, only the best components should be selected to support the performance. Key circuit decisions have to be made at the beginning. Does the datasheet performance of the DAC provide enough output power? Will an active device be needed? Does the signal chain need to transfer from the DACs differential output to a single ended environment? Will there be a transformer or balun? What is the proper impedance ratio for a balun? For this paper the use of a balun or transformer will be the focus here.

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