Similar to all high-speed devices, best system performance is achieved with close attention to board layout. The THS4551DGKEVM user guide (SLOU447) shows a good example of high-frequency layout techniques as a reference. This EVM includes numerous extra elements and features for characterization purposes that may not apply to some applications. General high-speed signal path layout suggestions include:
- Continuous ground planes are preferred for signal routing with matched impedance traces for longer runs; however, both ground and power planes must be opened up around the capacitive sensitive input and output device pins. When the signal goes to a resistor, parasitic capacitance becomes more of a band-limiting issue and less of a stability issue.
- Good high-frequency decoupling capacitors (0.1 µF) are required to a ground plane at the device power pins. Additional higher-value capacitors (2.2 µF) are also required but can be placed further from the device power pins and shared among devices. For best high-frequency decoupling, consider X2Y supply decoupling capacitors that offer a much higher self-resonance frequency over standard capacitors.
- Differential signal routing over any appreciable distance must use microstrip layout techniques with matched impedance traces.
- Higher-speed FDAs such as the THS4551 include a duplicate of the output pins on the input feedback side of the larger 16-pin VQFN (RGT) package. This feature is intended to allow the external feedback resistors to be connected with virtually no trace length on the input side of the package. This internal feedback trace also provides a second feedback path for connecting a feedback capacitor on the input pin sides for band-limited or multiple feedback filter designs. This internal trace shows an approximate 3.3-Ω series resistance that must be considered in any design using that path. The TINA-TI™ model does not include that element (to be generally applicable to all package styles) and must be added externally if the RGT package is used. Use this layout approach without extra trace length on the critical feedback path. The smaller 10-pin WQFN package lines up the outputs and the required inputs on the same side of the package where the feedback RF resistors must be placed immediately adjacent to the package with minimal trace length.
- The input summing junctions are very sensitive to parasitic capacitance. Any RG elements must connect into the summing junction with minimal trace length to the device pin side of the resistor. The other side of the RG elements can have more trace length if needed to the source or to GND.