OP467_15 Datasheet, PDF (13/20 Pages) Analog Devices – Quad Precision, High Speed Operational Amplifier

OP467
APPLICATIONS INFORMATION
OUTPUT SHORT-CIRCUIT PERFORMANCE
To achieve a wide bandwidth and high slew rate, the OP467
output is not short-circuit protected. Shorting the output to
ground or to the supplies may destroy the device.
For safe operation, the output load current should be limited so
that the junction temperature does not exceed the absolute
maximum junction temperature.
The maximum internal power dissipation can be calculated by
P_D
=
T_J
max − T_A
θ _JA
where:
TJ and TA are junction and ambient temperatures, respectively.
PD is device internal power dissipation.
θJA is the packaged device thermal resistance given in the data sheet.
UNUSED AMPLIFIERS
On the other hand, ceramic chip capacitors have excellent ESR
and effective series inductance (ESL) performance at higher
frequencies, and because of their small size, they can be placed
very close to the device pin, further reducing the stray inductance.
Best results are achieved by using a combination of these two
capacitors. A 5 μF to 10 μF tantalum parallel capacitor with a
0.1 μF ceramic chip capacitor is recommended. If additional
isolation from high frequency resonances of the power supply is
needed, a ferrite bead should be placed in series with the supply
lines between the bypass capacitors and the power supply. Note
that addition of the ferrite bead introduces a new pole and zero
to the frequency response of the circuit and could cause unstable
operation if it is not selected properly.
+V_S
+
10µF TANTALUM
0.1µF CERAMIC CHIP
It is recommended that any unused amplifiers in the quad
package be connected as a unity-gain follower with a 1 kΩ
feedback resistor with noninverting input tied to the ground plain.
0.1µF CERAMIC CHIP
PCB LAYOUT CONSIDERATIONS
10µF TANTALUM
Satisfactory performance of a high speed op amp largely
depends on a good PCB layout. To achieve the best dynamic
performance, follow the high frequency layout technique.
GROUNDING
A good ground plain is essential to achieve the optimum
performance in high speed applications. It can significantly
reduce the undesirable effects of ground loops and IR drops by
providing a low impedance reference point. Best results are
obtained with a multilayer board design with one layer assigned
to the ground plain. To maintain a continuous and low impedance
ground, avoid running any traces on this layer.
POWER SUPPLY CONSIDERATIONS
In high frequency circuits, device lead length introduces an
inductance in series with the circuit. This inductance, combined
with stray capacitance, forms a high frequency resonance circuit.
Poles generated by these circuits cause gain peaking and additional
phase shift, reducing the phase margin of the op amp and leading
to an unstable operation.
A practical solution to this problem is to reduce the resonance
frequency low enough to take advantage of the power supply
rejection of the amplifier. This is easily done by placing capacitors
across the supply line and the ground plane as close as possible
to the device pin. Because capacitors also have internal parasitic
components, such as stray inductance, selecting the right capacitor
is important. To be effective, they should have low impedance
over the frequency range of interest. Tantalum capacitors are an
excellent choice for their high capacitance/size ratio, but their
effective series resistance (ESR) increases with frequency
making them less effective.
–V_S
Figure 40. Recommended Power Supply Bypass
SIGNAL CONSIDERATIONS
Input and output traces need special attention to assure a
minimum stray capacitance. Input nodes are very sensitive to
capacitive reactance, particularly when connected to a high
impedance circuit. Stray capacitance can inject undesirable
signals from a noisy line into a high impedance input. Protect
high impedance input traces by providing guard traces around
them, which also improves the channel separation significantly.
Additionally, any stray capacitance in parallel with the input
capacitance of the op amp generates a pole in the frequency
response of the circuit. The additional phase shift caused by this
pole reduces the gain margin of the circuit. If this pole is within
the gain range of the op amp, it causes unstable performance. To
reduce these undesirable effects, use the lowest impedance
where possible. Lowering the impedance at this node places the
poles at a higher frequency, far above the gain range of the
amplifier. Stray capacitance on the PCB can be reduced by making
the traces narrow and as short as possible. Further reduction
can be realized by choosing a smaller pad size, increasing the
spacing between the traces, and using PCB material with a low
dielectric constant insulator (dielectric constant of some common
insulators: air = 1, Teflon® = 2.2, and FR4 = 4.7, with air being
an ideal insulator).
Removing segments of the ground plane directly under the
input and output pads is recommended.
Rev. * | Page 13 of 20