LMH6683 Datasheet, PDF (18/30 Pages) Texas Instruments – 190MHz Single Supply, Dual and Triple Operational Amplifiers

LMH6682, LMH6683
SNOSA43A – MAY 2004 – REVISED APRIL 2013
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OUTPUT PHASE REVERSAL
This is a problem with some operational amplifiers. This effect is caused by phase reversal in the input stage due
to saturation of one or more of the transistors when the inputs exceed the normal expected range of voltages.
Some applications, such as servo control loops among others, are sensitive to this kind of behavior and would
need special safeguards to ensure proper functioning. The LMH6682/6683 is immune to output phase reversal
with input overload. With inputs exceeded, the LMH6682/6683 output will stay at the clamped voltage from the
supply rail. Exceeding the input supply voltages beyond the Absolute Maximum Ratings of the device could
however damage or otherwise adversely effect the reliability or life of the device.
DRIVING CAPACITIVE LOADS
The LMH6682/6683 can drive moderate values of capacitance by utilizing a series isolation resistor between the
output and the capacitive load. Capacitive load tolerance will improve with higher closed loop gain values.
Applications such as ADC buffers, among others, present complex and varying capacitive loads to the Op Amp;
best value for this isolation resistance is often found by experimentation and actual trial and error for each
application.
DISTORTION
Applications with demanding distortion performance requirements are best served with the device operating in
the inverting mode. The reason for this is that in the inverting configuration, the input common mode voltage
does not vary with the signal and there is no subsequent ill effects due to this shift in operating point and the
possibility of additional non-linearity. Moreover, under low closed loop gain settings (most suited to low
distortion), the non-inverting configuration is at a further disadvantage of having to contend with the input
common voltage range. There is also a strong relationship between output loading and distortion performance
(i.e. 2kΩ vs. 100Ω distortion improves by about 15dB @1MHz) especially at the lower frequency end where the
distortion tends to be lower. At higher frequency, this dependence diminishes greatly such that this difference is
only about 5dB at 10MHz. But, in general, lighter output load leads to reduced HD3 term and thus improves
THD. (See Harmonic Distortion plots, Figures 19 through 23).
PRINTED CIRCUIT BOARD LAYOUT AND COMPONENT VALUES SELECTION
Generally it is a good idea to keep in mind that for a good high frequency design both the active parts and the
passive ones are suitable for the purpose you are using them for. Amplifying frequencies of several hundreds of
MHz is possible while using standard resistors but it makes life much easier when using surface mount ones.
These resistors (and capacitors) are smaller and therefore parasitics have lower values and will have less
influence on the properties of the amplifier. Another important issue is the PCB, which is no longer a simple
carrier for all the parts and a medium to interconnect them. The board becomes a real part itself, adding its own
high frequency properties to the overall performance of the circuit. It's good practice to have at least one ground
plane on a PCB giving a low impedance path for all decouplings and other ground connections. Care should be
taken especially that on board transmission lines have the same impedance as the cables they are connected to
(i.e. 50Ω for most applications and 75Ω in case of video and cable TV applications). These transmission lines
usually require much wider traces on a standard double sided PCB than needed for a 'normal' connection.
Another important issue is that inputs and outputs must not 'see' each other or are routed together over the PCB
at a small distance. Furthermore it is important that components are placed as flat as possible on the surface of
the PCB. For higher frequencies a long lead can act as a coil, a capacitor or an antenna. A pair of leads can
even form a transformer. Careful design of the PCB avoids oscillations or other unwanted behavior. When
working with really high frequencies, the only components which can be used will be the surface mount ones (for
more information see OA-15 SNOA367).
As an example of how important the component values are for the behavior of your circuit, look at the following
case: On a board with good high frequency layout, an amplifier is placed. For the two (equal) resistors in the
feedback path, 5 different values are used to set the gain to +2. The resistors vary from 200Ω to 3kΩ.
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