LMH6551 Datasheet, PDF (16/19 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

Application Section (Continued)
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FIGURE 11. Transformer Out Low Impedance Load
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FIGURE 12. Driving 50Ω Test Equipment
CAPACITIVE DRIVE
As noted in the Driving ADC section, capacitive loads should
be isolated from the amplifier output with small valued resis-
tors. This is particularly the case when the load has a resis-
tive component that is 500Ω or higher. A typical ADC has
capacitive components of around 10 pF and the resistive
component could be 1000Ω or higher. If driving a transmis-
sion line, such as 50Ω coaxial or 100Ω twisted pair, using
matching resistors will be sufficient to isolate any subse-
quent capacitance. For other applications see the “Sug-
gested Rout vs. Cap Load” charts in the Typical Perfor-
mance Characteristics section.
POWER DISSIPATION
The LMH6551 is optimized for maximum speed and perfor-
mance in the small form factor of the standard SOIC pack-
age, and is essentially a dual channel amplifier. To ensure
maximum output drive and highest performance, thermal
shutdown is not provided. Therefore, it is of utmost impor-
tance to make sure that the T_JMAX is never exceeded due to
the overall power dissipation.
Follow these steps to determine the Maximum power dissi-
pation for the LMH6551:
1. Calculate the quiescent (no-load) power: P_AMP = I_CC*
(V_S), where V_S = V⁺ - V^−. (Be sure to include any current
through the feedback network if V_OCM is not mid rail.)
2. Calculate the RMS power dissipated in each of the
output stages: P_D (rms) = rms ((V_S - V^+OUT) * I^+OUT) +
rms ((V_S − V^−OUT) * I^−OUT) , where V_OUT and I_OUT are
the voltage and the current measured at the output pins
of the differential amplifier as if they were single ended
amplifiers and V_S is the total supply voltage.
3. Calculate the total RMS power: P_T = P_AMP + P_D.
The maximum power that the LMH6551 package can dissi-
pate at a given temperature can be derived with the following
equation:
P_MAX = (150˚ – T_AMB)/ θ_JA, where T_AMB = Ambient tempera-
ture (˚C) and θ_JA = Thermal resistance, from junction to
ambient, for a given package (˚C/W). For the SOIC package
θ_JA is 150˚C/W.
NOTE: If V_CM is not 0V then there will be quiescent current
flowing in the feedback network. This current should be
included in the thermal calculations and added into the qui-
escent power dissipation of the amplifier.
ESD PROTECTION
The LMH6551 is protected against electrostatic discharge
(ESD) on all pins. The LMH6551 will survive 2000V Human
Body model and 200V Machine model events. Under normal
operation the ESD diodes have no effect on circuit perfor-
mance. There are occasions, however, when the ESD di-
odes will be evident. If the LMH6551 is driven by a large
signal while the device is powered down the ESD diodes will
conduct . The current that flows through the ESD diodes will
either exit the chip through the supply pins or will flow
through the device, hence it is possible to power up a chip
with a large signal applied to the input pins. Using the
shutdown mode is one way to conserve power and still
prevent unexpected operation.
BOARD LAYOUT
The LMH6551 is a very high performance amplifier. In order
to get maximum benefit from the differential circuit architec-
ture board layout and component selection is very critical.
The circuit board should have low a inductance ground plane
and well bypassed broad supply lines. External components
should be leadless surface mount types. The feedback net-
work and output matching resistors should be composed of
short traces and precision resistors (0.1%). The output
matching resistors should be placed within 3-4 mm of the
amplifier as should the supply bypass capacitors. The
LMH730154 evaluation board is an example of good layout
techniques. Evaluation boards are available free of charge
through the product folder on National’s web site.
The LMH6551 is sensitive to parasitic capacitances on the
amplifier inputs and to a lesser extent on the outputs as well.
Ground and power plane metal should be removed from
beneath the amplifier and from beneath R_F and R_G.
With any differential signal path symmetry is very important.
Even small amounts of assymetery will contribute to distor-
tion and balance errors.
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