At low frequencies, the terms balun and transformer are often used interchangeably because low-frequency baluns are almost always implemented using flux coupled transformers. For this reason it is often said that a balun is a type of transformer, but it is more accurate to say that a transformer can sometimes be used to implement a balun. Many other structures can also be used to implement balun functionality. Key specs for a balun are gain and phase balance, CMRR, insertion loss, isolation and delay flatness.

The single-ended to differential conversion is typically performed using passive magnetic baluns that are generally narrowband and increase in size with lower operating frequencies, wider range support also makes the baluns bulkier and costlier. Wideband passive baluns can be expensive with compromised phase and amplitude balance and insertion loss. Passive baluns offer the advantage of high linearity; however, their insertion loss directly hits the Output 3^rd Order Intercept Point (OIP3) of the preceding amplifier while impacting the overall noise figure. Poor phase and amplitude imbalance directly impacts the ADC's 2^nd order non-linearity. As with all RF and microwave circuits, each performance metric is only valid across some specified bandwidth. Increasing the bandwidth from octave, to decade, to multi-decade without sacrificing performance is a major challenge. In general baluns can be divided into two types. Those with magnetic coupling perform below 10 MHz, while those with only capacitive coupling have low-end performance limited to about 1 GHz, but can operate up to millimeter wave frequencies.

For wideband applications, baluns may not be able to mitigate 2^nd order harmonics that cause ripples throughout the desired pass band. At higher frequencies, the output impedance of the amplifier into a balun can change, and may result in a more pronounced imbalance across the entire frequency range. Conversely, baluns that have their frequency range extend to 10 MHz and lower will become significantly larger in size and consume more board space. Even though baluns can convert single-ended signals to differentials and provide the benefits of differential signaling, high-performance wideband passive baluns can be costly, large, and potentially unreliable, if mechanically constructed. Figure 2-1 shows a generic use-case of a balun being used to interface with an ADC.

For performance-demanding applications, a solution that combines the preceding amplifier and the passive balun becomes very attractive provided that the overall specifications of the active balun can beat those of the two cascaded parts. The ultimate test; however, is when cascaded with the ADC, the active balun must have negligible impact on the native ADC performance. It is preferred that the active balun does not reside on the same die as the ADC; otherwise, common-mode spur injection can limit ADC performance.

There are two general architectures used for active baluns – closed loop or open loop. For high-frequency applications, open loop structures are preferred for their stability advantages. While open-loop structures offer noise figure advantages, they tend to be AC coupled with a high-pass pole. A closed-loop implementation not only addresses this, but also yields an OIP3 or OIP2 that tracks the linearity profile of the ADC – high at low frequencies while gracefully degrading with higher frequencies. This is critical because across frequency, the active balun must always outperform the ADC by at least 6 dB to 8 dB to have minimal impact on the ADC.