Achieving optimum performance with a high-frequency amplifier such as the THS6222 requires careful attention to board layout parasitic and external component types. The THS6222RHFEVM can be used as a reference when designing the circuit board. Recommendations that optimize performance include:

1. Minimize parasitic capacitance to any ac ground for all signal I/O pins. Parasitic capacitance, particularly on the output and inverting input pins, can cause instability; on the noninverting input, this capacitance can react with the source impedance to cause unintentional band limiting. To reduce unwanted capacitance, a window around the signal I/O pins must be opened in all ground and power planes around these pins. Otherwise, ground and power planes must be unbroken elsewhere on the board.
2. Minimize the distance (less than 0.25 in, or 6.35 mm) from the power-supply pins to high-frequency 0.1 µF decoupling capacitors. At the device pins, the ground and power plane layout must not 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. An optional supply decoupling capacitor across the two power supplies (for bipolar operation) improves second-harmonic distortion performance. Larger (2.2 µF to 6.8 µF) decoupling capacitors, effective at lower frequencies, must also be used on the main supply pins. These capacitors can be placed somewhat farther from the device and can be shared among several devices in the same area of the PCB.
3. Careful selection and placement of external components preserves the high-frequency performance of the THS6222. Resistors must be of a 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 leads and PCB trace length as short as possible. Never use wire-wound type resistors in a high-frequency application. Although 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. Where double-side component mounting is required, place the feedback resistor directly under the package on the other side of the board between the output and inverting input pins. The frequency response is primarily determined by the feedback resistor value, as described in Section 7.2.1. Increasing the value reduces the bandwidth, whereas decreasing the value leads to a more peaked frequency response. The 1.24 kΩ feedback resistor used in Section 5.8 is a good starting point for a gain of 10 V/V design.

4. Connections to other wideband devices on the board can 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, 0.050-in to 0.100-in, or 1.27-mm to 2.54-mm) must be used, preferably with ground and power planes opened up around them.
5. Socketing a high-speed part such as the THS6222 is not recommended. The additional lead length and pin-to-pin capacitance introduced by the socket can create an extremely troublesome parasitic network, and can make achieving a smooth, stable frequency response almost impossible. Best results are obtained by soldering the THS6222 directly onto the board.
6. Use the V_S– plane to conduct the heat out of the package. The package attaches the die directly to an exposed thermal pad on the bottom, and must be soldered to the board. This pad must be connected electrically to the same voltage plane as the most negative supply voltage (V_S–) applied to the THS6222. Place as many vias as possible on the thermal pad connection and connect the vias to a heat spreading plane that is at the same potential as V_S– on the bottom side of the PCB.