1 The Art of Choosing the Correct Balun or Transformer

Before opening the CAD SW program to start on your PCB layout, we assume you have already decided on both the ADC you plan to use and the BW intended for your application. Now you need to decide, if the passive frontend is right for your design. Assuming again, you do not need to DC couple, that is, sample the DC frequency bin. Because a balun does not require an additional power supply, the benefits of using a balun include lower overall power consumption, and smaller board space requirements. Additionally, with no extra supply to contend with, a balun does not add noise to the overall RF signal chain that leads up to the ADC. This, in turn, means no degradation in SNR (signal-to-noise ratio) or NSD (noise spectral density) can occur.

Once established, a balun must be chosen, and the choices are numerous. When choosing a balun, the suggestion is to first look at your BW requirements. Choose a balun that has a bit more bandwidth than what is required so that the balun acts more like a window than a door. This is especially important at those higher frequency applications. See Figure 1-1. Shown are two different baluns used in the same application with the ADC3669, 16-bit, dual channel ADC. Even though both baluns are rated for the same BW, they can ultimately respond differently due to the combination of the ADC’s varying input impedance due to the ADC’s internal sample network, as well as the PCB trace parasitics. Notice that with no match applied with either balun, the BW falls quite rapidly, as noted in the two points described previously.

Figure 1-1 ADC3669 and Balun BW Comparison: Match (solid lines) vs. No Match (dashed lines)

A few other nuggets to think about during the balun down-selection process and before you go off and start simulating. Take a close look at the balun’s PCB footprint and layout recommendation in the data sheet, there is a reason why it is the way it is. The recommendation is to follow these recommendations exactly, unless you want to inadvertently make the balun respond differently. The balun was characterized using this footprint both for the data sheet collection and measuring the s-parameters, and can only perform up to spec under these circumstances. Which leads me to another thought, sometimes our balun vendors use the balun test board to measure the s-parameters, and do not de-embed the connectors nor the traces on the test board. So BW beware!

Lastly, start to understand the balun’s phase imbalance over your specific BW. The poorer the balun’s inherent phase imbalance the worse even order distortion (HD2 or second harmonic distortion) the ADC can manifest. If HD2 is important to your frequency planning application, it is recommended to pick a balun with good phase imbalance. There is really no good guide on this, as each ADC can also have the sensitivity to phase differences across its useable frequency range. Typically choosing a balun that has <5degrees of phase imbalance over your application operating BW can be a good start. This can add little to the aggregate even order distortion already existing in your RF signal chain lineup. For more information on balun phase imbalance and its impact to even order distortion, see the reference link. Figure 1-2 shows the difference between the same two matched baluns scenarios again, and impact it has on even order distortion using the ADC3669. Notice the HD3, odd order distortion, or third harmonic response is relatively the same across frequency and has no impacted differences.

Figure 1-2 ADC3669 HD2/HD3 Comparison between Two Baluns: High Cost vs. Lower Cost