Achieving optimized performance with a
high-frequency amplifier like the OPA695 requires careful attention to board layout
parasitics and external component types. Recommendations to 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 band-limiting. To reduce
unwanted capacitance, a 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
(< 0.25") 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. Always decouple the power-supply connections with
these capacitors. An optional supply-decoupling capacitor across the two
power supplies (for bipolar operation) improves 2nd-harmonic distortion
performance. Larger (2.2 μF to 6.8 μF) decoupling capacitors, effective at a
lower frequency, must also be used on the main supply pins. These decoupling
capacitors can be placed somewhat farther from the device, and can be shared
among several devices in the same area of the PCB.
- Careful selection and
placement of external components preserves the high-frequency
performance of the OPA695. Use low-reactance-type resistors.
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. Keep the leads and PCB trace length as
short as possible. Never use wirewound-type resistors in a high frequency
application. The output pin and inverting input pin are the most sensitive
to parasitic capacitance; therefore, always position the feedback and series
output resistor, if any, as close as possible to the output pin. Place other
network components, such as noninverting input termination resistors, close
to the package. 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. The frequency response is
primarily determined by the feedback resistor value. Increasing the value
reduces the bandwidth, while decreasing the value gives a more peaked
frequency response. The 402-Ω feedback resistor (used in the typical
performance specifications at a gain of +8 on ±5-V supplies) is a good
starting point for design. Note that a 523-Ω feedback resistor, rather than
a direct short, is required for the unity gain follower application. A
current-feedback operational amplifier requires a feedback resistor, even in
the unity gain follower configuration, to control stability.
- 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) must be used, preferably with
ground and power planes opened up around them. Estimate the total capacitive
load and set the series isolation resistance from the isolation resistance
versus capacitive load characteristics. 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 usually not necessary on
board. In fact, a higher impedance environment improves distortion, as shown
in the distortion versus load plots. With a characteristic board trace
impedance defined (based on board material and trace dimensions), use a
matching series resistor into the trace from the output of the OPA695. Also
use terminating shunt resistor at the input of the destination device.
Remember that the terminating impedance is the parallel combination of the
shunt resistor and the input impedance of the destination device; set this
total effective impedance to match the trace impedance. The high output
voltage and current capability of the OPA695 allows multiple destination
devices to be handled as separate transmission lines, each with 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. Treat the trace as a capacitive load in this case, and
set the series isolation resistance from the isolation resistance versus
capacitive load characteristics. This setting does not preserve signal
integrity as well as a doubly-terminated line. If the input impedance of the
destination device is low, some signal attenuation occurs due to the voltage
divider formed by the series output into the terminating impedance.
- Socketing a high-speed
part like the OPA695 is not recommended. The additional lead length
and pin-to-pin capacitance introduced by the socket can create a troublesome
parasitic network, which makes achieving a smooth, stable frequency response
almost impossible. Best results are obtained by soldering the OPA695
directly onto the board.