2 Choosing the ADC and balun

Understanding the application requirements upfront is crucial when selecting the right type of ADC and ultimately determining the front-end development approach. Assume that there is a defined sampling rate, number of channels, digital output interface type and useful internal digital features that you could use or that are necessary for the application. I’ll use the ADC3669 converter throughout this narrowband front-end example.

To begin, you must first understand the analog input characteristics of your chosen ADC. If you scroll down to the analog input parameters section of any converter datasheet, you should see the parallel R||C specified in the specification tables. If not, check whether there is a simplified analog input model. As a final option, use the ADC’s S-parameters, which are typically listed on the product’s webpage. For example, the ADC3669 data sheet lists a model input with a resistor (R) = 100Ω and a capacitor (C) = approximately 1.85pF (aggregated) differential impedance terms. See Figure 1.

The next step is to choose the proper transformer or balun for the ADC, which include comparing these specifications between vendors: the return loss (RL), insertion loss, and phase and magnitude imbalance. If these parameters are not specified on the data sheet, ask the manufacturer, or measure them using a vector network analyzer or VNA.

Choosing between a standard flux-coupled transformer or balun will depend on the BW requirements. Standard transformers are often <1GHz, whereas a balun can achieve much higher BW. Reference [1] describes transformer and balun parametric and ADC requirements in detail.

For NB matching, the example calls for a reactive resistor-capacitor-inductor (RCL) match with the last component in shunt; see Figure 2 and References [2] and [3] for information about matching pads and topologies. Collecting and understanding the application requirements will enable you to select the front-end BW and balun. For the example, I selected the TCM2-33WX+ balun from Mini-Circuits with a 1:2 impedance ratio and 3GHz of BW after measuring and understanding this balun’s capabilities from previous examples used on the ADC3669 evaluation module (EVM). The TCM2-33WX+ provides a relatively low input drive in order to reach ADC’s full scale input range.