11.1 Layout Guidelines
Achieving optimum performance with a high-frequency amplifier like the OPA2810 requires careful attention to board layout parasitics and external component types. The OPA2810EVM can be used as a reference when designing the circuit board. Recommendations that 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, it can react with the source impedance to cause unintentional band-limiting. To reduce unwanted capacitance, open a window around the signal I/O pins in all of the ground and power planes around those pins. Otherwise, ground and power planes must be unbroken elsewhere on the board.
- Minimize the distance (< 0.1") 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 OPA2810. 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. Good axial metal film or surface mount resistors have approximately 0.2 pF in shunt with the resistor. For resistor values > 10 kΩ, this parasitic capacitance can add a pole or zero close to the GBWP of 70 MHz and subsequently affects circuit operation. Keep resistor values as low as possible consistent with load driving considerations. Lowering the resistor values keep the resistor noise terms low, and minimize the effect of its parasitic capacitance, however lower resistor values increase the dynamic power consumption because RF and RG become part of the amplifiers output load network. Transimpedance applications (see the Transimpedance Amplifier section) can use whatever feedback resistor is required by the application as long as the feedback compensation capacitor is set considering all parasitic capacitance terms on the inverting node.
- Connections to other wideband devices on the board may be made with short direct traces or through onboard transmission lines. For short connections, consider the trace and the input to the next device as a lumped capacitive load. Relatively wide traces (50 mils to 100 mils) must be used, preferably with ground and power planes opened up around them. Estimate the total capacitive load and set RS for sufficient phase margin and stability. Low parasitic capacitive loads (< 35 pF) may not need an RS because the OPA2810 is nominally compensated to operate with a 35-pF parasitic load. Higher parasitic capacitive loads without an RS are allowed as the signal gain increases (increasing the unloaded phase margin) If a long trace is required, and the 6-dB signal loss intrinsic to a doubly-terminated transmission line is acceptable, implement a matched impedance transmission line using microstrip or stripline techniques (consult an ECL design handbook for microstrip and stripline layout techniques). A 50-Ω environment is normally not necessary onboard, and a higher impedance environment improves distortion. With a characteristic board trace impedance defined based on board material and trace dimensions, a matching series resistor into the trace from the output of the OPA2810 is used as well as a terminating shunt resistor at the input of the destination device. Remember also that the terminating impedance is the parallel combination of the shunt resistor and the input impedance of the destination device— this total effective impedance must be set to match the trace impedance. If the 6-dB attenuation of a doubly-terminated transmission line is unacceptable, a long trace can be series-terminated at the source end only. Treat the trace as a capacitive load in this case and set the series resistor value to obtain sufficient phase margin and stability. This does not preserve signal integrity as well as a doubly-terminated line. If the input impedance of the destination device is low, the signal attenuates because of the voltage divider formed by the series output into the terminating impedance.
- Take care to design the PCB layout for optimal thermal dissipation. For the extreme case of 125°C operating ambient, using the approximate maximum 177.2°C/W for the two packages, and an internal power of 24-V supply × 9-mA 125°C supply current (both amplifiers) gives a maximum internal power dissipation of 216 mW. This power gives a 38°C increase from ambient to junction temperature. Load power adds to this value and this dissipation must also be calculated to determine the worst-case safe operating point.
- Socketing a high speed part like the OPA2810 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 OPA2810 onto the board.