10.1 Layout Guidelines
Achieving optimum performance with a high-frequency amplifier like the OPA818 requires careful attention to board layout parasitics and external component types. Recommendations that will optimize performance include.
- Minimize parasitic capacitance to any AC ground for all of the signal I/O pins. Parasitic capacitance on the output and inverting input pins can cause instability. On the noninverting input, parasitic capacitance can react with the source impedance to cause unintentional bandlimiting. Ground and power metal planes act as one of the plates of a capacitor while the signal trace metal acts as the other separated by PCB dielectric. To reduce this unwanted capacitance, a plane cutout around and underneath the signal I/O pins on all ground and power planes is recommended. Otherwise, ground and power planes should be unbroken elsewhere on the board. When configuring the amplifier as a TIA, if the required feedback capacitor is under 0.15 pF, consider using two series resistors, each of half the value of a single resistor in the feedback loop to minimize the parasitic capacitance from the resistor.
- Minimize the distance (less than 0.25") from the power-supply pins to high-frequency decoupling capacitors. Use high quality, 100-pF to 0.1-µF, C0G and NPO-type decoupling capacitors with voltage ratings at least three times greater than the amplifiers maximum power supplies to ensure that there is a low-impedance path to the amplifiers power-supply pins across the amplifiers gain bandwidth specification. At the device pins, do not allow the ground and power plane layout to be in close proximity to the signal I/O pins. Avoid narrow power and ground traces to minimize inductance between the pins and the decoupling capacitors. The power-supply connections must always be decoupled with these capacitors. Larger (2.2-µF to 6.8-µF) decoupling capacitors, effective at lower frequency, must be used on the supply pins. These are placed further from the device and are shared among several devices in the same area of the PC board.
- Careful selection and placement of external components will preserve the high frequency performance of the OPA818. Resistors should be of very low reactance type. Surface-mount resistors work best and allow a tighter overall layout. Metal film and carbon composition axially leaded resistors can also provide good high frequency performance. Again, keep their leads and PCB trace length as short as possible. Never use wirewound type resistors in a high frequency application. Because the output pin and inverting input pin are the most sensitive to parasitic capacitance, always position the feedback and series output resistor, if any, as close as possible to the inverting input and the output pin, respectively. Other network components, such as noninverting input termination resistors, should also be placed close to the package. Even with a low parasitic capacitance shunting the external resistors, excessively high resistor values can create significant time constants that can degrade performance. When OPA818 is configured as a conventional voltage amplifier, keep the resistor values as low as possible and consistent with the load driving considerations. Lower resistor values minimize the effect of parasitic capacitance and reduce resistor noise terms but because the feedback network (RF + RG for noninverting and RF for inverting configuration) acts as a load on the amplifier, lower resistor values increase the dynamic power consumption and the effective load on the output stage. Transimpedance applications (see Figure 13) can use feedback resistors as required by the application and as long as the feedback compensation capacitor is set considering all parasitic capacitance terms on the inverting node.
- Heat dissipation is important for a high voltage device like OPA818. For good thermal relief, the thermal pad should be connected to a heat spreading plane that is preferably on the same layer as OPA818 or connected by as many vias as possible if the plane is on a different layer. It is recommended to have at least one heat spreading plane on the same layer as the OPA818 that makes a direct connection to the thermal pad with wide metal for good thermal conduction when operating at high ambient temperatures. If more than one heat spreading planes are available, connecting them by a number of vias further improves the thermal conduction.
- Socketing a high speed part like the OPA818 is not recommended. The additional lead length and pin-to-pin capacitance introduced by the socket can create an extremely troublesome parasitic network which can make it almost impossible to achieve a smooth, stable frequency response. Best results are obtained by soldering the OPA818 onto the board.