SLAAET1 March   2025 ADC08DJ3200 , ADC08DJ5200RF , ADC09DJ1300 , ADC09DJ800 , ADC09QJ1300 , ADC09QJ800 , ADC09SJ1300 , ADC09SJ800 , ADC12DJ1600 , ADC12DJ2700 , ADC12DJ3200 , ADC12DJ4000RF , ADC12DJ5200RF , ADC12DJ800 , ADC12QJ1600 , ADC12QJ800 , ADC12SJ1600 , ADC12SJ800 , ADC32RF52 , ADC32RF54 , ADC32RF55 , ADC32RF72 , ADC34RF52 , ADC34RF55 , ADC34RF72 , ADC3548 , ADC3549 , ADC3568 , ADC3569 , ADC3648 , ADC3649 , ADC3664 , ADC3668 , ADC3669 , ADC3683

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Active (Amplifier) vs. Passive (Transformer/Balun) Tradeoffs
  6. 3Amplifiers vs. Baluns: The Advantages
  7. 4Amplifiers vs. Baluns: The Disadvantages
  8. 5Understanding the Importance of Phase Imbalance
  9. 6Phase Balance with Amplifiers and Baluns
  10. 7Summary
  11. 8References
  12.   A Appendix A

5 Understanding the Importance of Phase Imbalance

If the application frequency plan includes even order distortion—the second (HD2), fourth (HD4), sixth harmonics (HD6), and so on—then you must also look at phase imbalance when designing the analog front-end interface. Both amplifiers and baluns have a finite amount of phase imbalance between the output signals, which typically gets worse (deviates) across higher and higher frequencies.

Phase imbalance is the term used to quantify the amount of phase imbalance between two differential signals. Since the ADC’s analog inputs are typically a differential interface, the two inputs need to be equal in amplitude and 180 degrees out of phase. For example, if Ain+ = –2 degrees and Ain– = 185 degrees, that produces a 7 degree shift, which translates into the frequency domain or fast Fourier transform (FFT) plot as a worse even-order distortion; that is, the second harmonic gets worse.

Unfortunately, there is really no real way to quantify how much phase imbalance your signal chain can take before starting to degrade system performance. This is because every component with a differential input or output interface, active or passive, can have some inherent finite amount of phase mismatch at some frequency. There is really no way to designed for balance an IC internally, a balun’s windings, or even multiple cables to the absolute designed for phase.

When performing balanced or differential test measurements in the lab, where you plan to use cables or adapters in the test setup, these extras need to phase-matched as well.

If there is still doubt, and you like a bit of math, please see Appendix A for a full phase imbalance derivation using an ADC model. Here, the ADC model uses a third-order transfer function and a pair of sinusoid signals to prove how phase imbalance gives rise to even-order distortion as shown in Figure 5-1.

Differential Input Signaling Mathematical ModelFigure 5-1 Differential Input Signaling Mathematical Model

Where each input signal is represented as:

Equation 5. x 1 ( t )   =   k 1 × sin ω t x 2 ( t )   =   k 2 × sin ω t - 180 ° + θ   =   - k 2 × sin ω t + θ

The ADC is a modeled as a third-order expression:

Equation 6. h x t   =   a 0 + a 1 x t + a 2 x 2 t + a 3 x 3 t

The output is the convolved expression of the two:

Equation 7. y t = 2 a 1 k × sin ω t + 2 a 3 k 3 × sin 3 ω t