2 The Art of Choosing the Correct Balun Matching Network

Over the years, we have seen many attempts to simulate and get the balun match perfected. After weeks to months of simulation and trying to understand some level of PCB parasitic, the match does not quite work out when the PCB design is fabricated. The recommendation is to start the design process differently, use the following topology as shown in Figure 2-1. This can give you a great sandbox to play in using any components you wish to complete the match. Wonder if all of this effort and tradeoffs are actually worth it, the suggestion is to refer back to Figure 1-1. The following section describes each component to know the need or function within the input matching network to the ADC.

C1/C2: typically a 0.1uF, blocks DC from being fed into the balun or transformer. Some balun designs lead to ground and or DC can aggravate the balun’s function leading to poor performance. So, that is what they are for, put them in.

R1: allows for back termination near the outputs of the balun after the DC blocking capacitors, not always needed, if your trace lengths are long enough you can need this component. Assuming no perfect match across the band of interest is something that cannot be achieved, you can need to back terminate to handle any standing waves that can accumulate as the imperfect match rolls back and forth across your frequency range.

R2/R3/R4: This allows for various matching techniques to be employed. These three components are the heart of the match and can take the form in several combinations to solve the balun or ADC matching conundrum. For widest band matches these three components generally are configured as a matching pad. This helps to dissolve the standing waves between the balun and ADC providing for a “stiff” 50ohm impedance that is generally needed by both devices. Though these are represented as resistors, these components can take the form of capacitors and or inductors as well.

C3: this capacitor, typically a 0.1uF, ties the center point of the R3s together and allows for an AC current path. This is also a good idea because when over-ranging the ADC’s input full scale, this allows for this AC current to go somewhere…you are welcome. Side note: this capacitor can also be located at R5 instead.

R5: allows back termination, on the opposite side near the ADC’s inputs, and is again not always necessary. This provides the same function as R1 but from the opposite perspective to help resolve standing waves that can accumulate. Typically, the need for R1 and or R5 are required when trace connections are 300mils in length or more.

R6: these are your kickback components. These are typically in the form of resistors but in some cases inductors or low-Q ferrite beads can help snub any residual charge kickback that comes back onto the analog input network from the internal sampling circuit in the ADC. These component placeholders are essential when using unbuffered ADCs.

Figure 2-1 Generalize Passive Network Component Placeholders

Again, be weary if you just plan to run two traces from the output of the balun to the inputs of the ADC. Even if you collect s-parameters, simulate, and prove the output out to your colleagues, this can prove to be costly unless you have previous experience with the balun and ADC combo.