2 Comparing AC Performance Trade-offs and Understanding Input Network Metrics

Following are five active (amplifier) or passive (balun) analog input network metrics that can help keep the input network design focused and on track:

• The input network input impedance or voltage standing wave ratio (VSWR):a unit less parameter that shows how much power is reflected into the load over the bandwidth of interest. The input impedance of the network is the defined or specified value of the input stage of the analog input network (for example, the load); usually 50Ω.
• The application bandwidth: this is the beginning and ending frequencies used in the system, typically –3dB from a reference point.
• The passband flatness: this is usually defined as the amount of fluctuation or ripple tolerable within the specified bandwidth: 1dB or +0.5dB for example. These levels can be increased or decreased, or defined with a slope.
• AC performance: single-tone: the signal-to-noise ratio (SNR), spurious free dynamic range (SFDR) and two-tone: third-order intermodulation distortion (IMD3).
• The input drive level: is a function of the bandwidth, input impedance and VSWR specifications. This level sets the gain or amplitude required for a full-scale input signal at the converter. The level is highly dependent on the front-end components: the balun, amplifier and anti-aliasing filter and can be one of the most difficult parameters to achieve.

These metrics encapsulate the entire front-end interface design, not just the analog-to-digital converter (ADC). Considering these metrics first can help decide between an active or passive front end.

The following section shows five different front-end designs to compare these metric trade-offs, as shown in Figure 2-1.

Figure 2-1 Five Front-end Designs: Balun Only, an LNA, a Balun Plus an FDA, a Single-Ended FDA and the TRF1208