Measuring & Modeling Wideband Baluns for Application to ADC Input Stages

Wideband transformers used in ADC and RF applications
Within the huge range of wideband transformers, there are selections on passband frequency, turns ratios, and pin configurations. Figure 1 shows an example configuration range from a typical industry sample kit (Ref. 1).

The two most commonly used in ADC input interfaces are No. 1 and No. 3 above. The second one is very similar to No. 1, but the selection range seems much larger with the secondary centertap. Configurations No. 1 and No. 2 are often called “flux-coupled baluns,” which is not particularly descriptive, as all configurations of Figure 1 have to be flux coupled to operate.

Configuration No. 3 is also very common and typically aimed at higher-frequency applications. Configuration No. 3 is often referred to as a “transmission line transformer” but is also called a common mode choke. It can also deliver a “Balun, Balanced to Unbalanced” operation -- or more commonly unbalanced to balanced in single-ended input to ADC differential input interfaces common to ADC evaluation boards (Ref. 2).

The focus here will be on baluns of configuration No. 1 (or equivalently, No. 2). While this shows a secondary centertap, it is perfectly acceptable to not use that connection, and that will be the assumption here. Floating the centertap in the configurations shown later will remove the gain and phase balance concerns typical of that configuration.

Transformer vendors use many different approaches to describing their operations. All, however, will report insertion loss or bandwidth specifications assuming some source impedance. In RF applications, those specifications are normally assuming the secondary is terminated in an impedance that is n²*Rs. This doubly terminated assumption gives some point of comparison between devices, but, in fact, transformers will give some level of operation and passband response with a huge range of source and load impedances. Any Spice modeling approach should track that physical reality but probably start with the doubly terminated assumption for measurement purposes.

Network analyzer balun measurements and coupled inductor model element solution
Typical lab network analyzers can easily make an S21 transmission measurement. For best results, the device under test (DUT) input and output impedance should be 50Ω. Figure 2 shows the measurement setup for a wideband 1:2 ohms (or 1:1.414 turns) ratio transformer (Ref. 3).

After calibration, the network analyzer simply drives into the blocking caps and then into the primary of the transformer. If the secondary load impedance is set to n²*Rs, it will appear on the input side of the transformer as an Rs termination, eliminating any line reflection effects. However, the test schematic of Figure 2 should also translate to a source impedance on the output side of the board matched to the network analyzer measurement impedance -- most commonly 50Ω.

The exact solution for those impedance mapping elements (R1 & R2) gives 70.7Ω where those shown in Figure 2 are close 1 percent values. R1 and R2 are solved to give an impedance looking out of the balun secondary equal to n²*Rs (100Ω here) while also presenting a 50Ω source looking back into R2. Those exact solutions are shown here where “n” is the turns' ratio.

 

Using these exact values will give an anticipated measured midband insertion loss for this test circuit as:

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