OPA4684M Datasheet, PDF (29/32 Pages) Texas Instruments – QUAD LOW-POWER CURRENT-FEEDBACK OPERATIONAL AMPLIFIER

OPA4684M
QUAD LOWĆPOWER CURRENTĆFEEDBACK
OPERATIONAL AMPLIFIER
SGLS145B − AUGUST 2003 − REVISED FEBRUARY 2004
− 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 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. The power-supply connections should always be decoupled with these
capacitors. An optional supply de-coupling capacitor (0.01 µF) across the two power supplies (for
bipolar operation) will improve 2nd-harmonic distortion performance. Larger (2.2 µF to 6.8 µF)
decoupling capacitors, effective at lower frequencies, should also be used on the main supply pins.
These may be placed somewhat farther from the device and may be shared among several devices in
the same area of the PC board.
− Careful selection and placement of external components will preserve the high-frequency performance
of the OPA4684. Resistors should be 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 their leads and PC-board trace length as short
as possible. Never use wirewound type resistors in a high-frequency application. Since 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. The quad amplifier pinout allows
each output and inverting input to be connected by the feedback element with virtually no trace length.
Other network components, such as noninverting input termination resistors, should also be placed
close to the package. The frequency response is primarily determined by the feedback resistor value as
described previously. Increasing its value will reduce the peaking at higher gains, while decreasing it will
give a more peaked frequency response at lower gains. The 800-Ω feedback resistor used in the Typical
Characteristics at a gain of +2 on ±5-V supplies is a good starting point for design. Note that a 800-Ω
feedback resistor, rather than a direct short, is required for the unity-gain follower application. A
current-feedback op amp requires a feedback resistor even in the unity-gain follower configuration to
control stability.
− 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) should be used, preferably with
ground and power planes opened up around them. Estimate the total capacitive load and set RS from
the plot of recommended RS vs CLOAD. Low parasitic capacitive loads (< 5 pF) may not need an RS
since the OPA4684 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). A 50-Ω environment is normally
not necessary on board, and in fact a higher impedance environment will improve distortion, see the
distortion versus load plots. 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 OPA4684
is used, as well as a terminating shunt resistor at the input of the destination device. Remember also that
the terminating impedance will be the parallel combination of the shunt resistor and the input impedance
of the destination device; this total effective impedance should be set to match the trace impedance.
The high output voltage and current capability of the OPA4684 allows multiple destination devices to be
handled as separate transmission lines, each with their own 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 resistor value as shown in the plot of RS vs CLOAD. This will not preserve signal integrity as well
as a doubly-terminated line. If the input impedance of the destination device is LOW, there will be some
signal attenuation due to the voltage divider formed by the series output into the terminating impedance.
− Socketing a high-speed part like the OPA4684 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 OPA4684 onto the board.
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265
29