Achieving optimum performance with a
high-frequency amplifier (like the OPAx863 devices) require careful attention to
board layout parasitics and external component types. The OPA2863DGK Evaluation
Module user's guide can be used as a reference when designing the
circuit board. Recommendations that optimize performance include the following:
- 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, it can
react with the source impedance to cause unintentional band-limiting. Open a
window around the signal I/O pins in all of the ground and power planes around
those pins to reduce unwanted capacitance. Otherwise, ground and power planes
must be unbroken elsewhere on the board.
- Minimize the distance (< 0.1 in) from the power-supply pins to high-frequency 0.01-µF decoupling capacitors. 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 also be used on the supply pins. These can be placed somewhat farther from the device and shared among several devices in the same area of the PC board.
- Careful selection and placement of external
components preserve the high frequency performance of the OPAx863
devices. Resistors must be a 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 output pin. Other network components, such as noninverting input
termination resistors, must 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. Keep resistor values as low as possible and consistent with load
driving considerations. Lowering the resistor values keep the resistor noise
terms low, and minimize the effect of its parasitic capacitance; lower resistor
values, however, increase the dynamic power consumption because RF
and RG become part of the amplifiers output load network.
- Socketing a high speed part like the OPAx863 devices are 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 OPAx863 devices onto the board.