SBOSAA5B April 2022 – September 2022 OPA2675
Achieving optimum performance with a high-frequency amplifier such as the OPA2675 requires careful attention to board layout parasitic and external component types.
a) 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, a window around the signal I/O pins should be opened in all of the ground and power planes around those pins. Otherwise, ground and power planes should be unbroken elsewhere on the board.
b) Minimize the distance (< 0.25 in, or 6.35 0 mm) from the power-supply pins to high-frequency 0.1 µF decoupling capacitors. The power-supply connections (on pins 7 and 14 for a VQFN package) should always be decoupled with low-ESR capacitors. The ground and power-plane layout should 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. An optional supply decoupling capacitor connected across the two power supplies (for bipolar operation) improves second-harmonic distortion performance.
c) Careful selection and placement of external components preserve the high-frequency performance of the OPA2675. The resistors used should have a very low reactance. Surface-mount resistors, metal film and carbon composition based axially-leaded resistors can provide good high-frequency performance. Keep the leads and PCB trace length as short as possible. 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. Where double-side component mounting is allowed, place the feedback resistor directly under the package on the other side of the board between the output and inverting input pins.
d) The frequency response is primarily determined by the feedback resistor value as described previously. Increasing the value reduces the bandwidth, whereas decreasing it gives a more peaked frequency response. The 402 Ω feedback resistor used in the Typical Characteristics at a gain of +4 V/V on ±6 V supplies is a good starting point for design. Note that a current-feedback op amp requires a feedback resistor even in the unity-gain follower configuration to control stability. A 511 Ω feedback resistor, rather than a direct short, is recommended for the unity-gain follower application.
e) 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, or 1.27 mm to 2.54 mm) should be used, preferably with ground and power planes opened up around them. Estimate the total capacitive load and set RISO from the plot of Figure 8-6. Low parasitic capacitive loads (< 5 pF) may not need an RISO because the OPA2675 is nominally compensated to operate with a 2 pF parasitic load.
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).
The high output voltage and current capability of the OPA2675 allows multiple destination devices to be handled as separate transmission lines, each with respective series and shunt terminations. 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.