Similar to all high-speed devices, best system
performance is achieved with a close attention to board layout. The THS4541 evaluation module (EVM) shows a good example of high
frequency layout techniques as a reference. This EVM includes numerous extra
elements and features for characterization purposes. General high-speed, signal-path
layout suggestions include:
- Continuous ground planes are preferred for signal routing with matched impedance traces for longer runs; however, open up both ground and power planes around the capacitive sensitive input and output device pins. After the signal is sent into a resistor, parasitic capacitance becomes more of a bandlimiting issue and less of a stability issue.
- Use good, high-frequency decoupling capacitors
(0.1 µF) on the ground plane at the device power pins. Higher value capacitors
(2.2 µF) are 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.
- When using differential signal routing over any appreciable distance, use microstrip layout techniques with matched impedance traces.
- Higher-speed FDAs, such as the THS4541, include a duplicate of the output pins on the input feedback side of the larger 16-pin VQFN (RGT) package. This duplication is intended to allow the external feedback resistors to be connected with virtually no trace length on the input side of the package. Use this layout approach with no extra trace length on this critical feedback path. The smaller 10-pin, WQFN (RUN) package lines up the outputs and the required inputs on the same side of the package where the feedback (Rf) resistors are placed immediately adjacent to the package with minimal trace length.
- The input summing junctions are very sensitive to parasitic capacitance. Connect any Rg elements 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 ground.