LMH6622MAX National Semiconductor, LMH6622MAX Datasheet - Page 15

• Manufacturer Part Number: LMH6622MAX
• Description: IC,Operational Amplifier,DUAL,BIPOLAR,SOP,8PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LMH6622MAX

• Dc: 06+
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

DSL Receive Channel Applications
The LMH6622 is a low noise, low distortion high speed
operational amplifier. The LMH6622 comes in either SOIC-8
or MSOP-8 packages. Because two channels are available
in each package the LMH6622 can be used as a high
dynamic range differential amplifier for the purpose of driving
a high speed analog-to-digital converter. Driving a 1kΩ load,
the differential amplifier of Figure 3 provides 20dB gain, a flat
frequency response up to 6MHz, and harmonic distortion
that is lower than 80dBc. This circuit makes use of a trans-
former to convert a single-ended signal to a differential sig-
nal. The input resistor R
tion,
The gain of this differential amplifier can be adjusted by R
and R
(Continued)
FIGURE 5. Total Output Referred Noise Density
F
,
FIGURE 4. Frequency Response
IN
is chosen by the following equa-
20029221
20029222
C
15
CIRCUIT LAYOUT CONSIDERATIONS
National Semiconductor suggests the copper patterns on the
evaluation boards listed below as a guide for high frequency
layout. These boards are also useful as an aid in device
testing and characterization. As is the case with all high-
speed amplifiers, accepted-practice R
the PCB layout is mandatory. Generally, a good high fre-
quency layout exhibits a separation of power supply and
ground traces from the inverting input and output pins. Para-
sitic capacitances between these nodes and ground will
cause frequency response peaking and possible circuit os-
cillations (see Application Note OA-15 for more information).
High quality chip capacitors with values in the range of
1000pF to 0.1µF should be used for power supply bypass-
ing. One terminal of each chip capacitor is connected to the
ground plane and the other terminal is connected to a point
that is as close as possible to each supply pin as allowed by
the manufacturer’s design rules. In addition, a tantalum ca-
pacitor with a value between 4.7µF and 10µF should be
connected in parallel with the chip capacitor. Signal lines
connecting the feedback and gain resistors should be as
short as possible to minimize inductance and microstrip line
effect. Input and output termination resistors should be
placed as close as possible to the input/output pins. Traces
greater than 1 inch in length should be impedance matched
to the corresponding load termination.
Symmetry between the positive and negative paths in the
layout of differential circuitry should be maintained so as to
minimize the imbalance of amplitude and phase of the dif-
ferential signal.
These free evaluation boards are shipped when a device
sample request is placed with National Semiconductor.
Component value selection is another important parameter
in working with high speed/high performance amplifiers.
Choosing external resistors that are large in value compared
to the value of other critical components will affect the closed
loop behavior of the stage because of the interaction of
these resistors with parasitic capacitances. These parasitic
capacitors could either be inherent to the device or be a
by-product of the board layout and component placement.
Moreover, a large resistor will also add more thermal noise to
the signal path. Either way, keeping the resistor values low
will diminish this interaction. On the other hand, choosing
very low value resistors could load down nodes and will
contribute to higher overall power dissipation and worse
distortion.
DRIVING CAPACITIVE LOAD
Capacitive Loads decrease the phase margin of all op amps.
The output impedance of a feedback amplifier becomes
inductive at high frequencies, creating a resonant circuit
when the load is capacitive. This can lead to overshoot,
ringing and oscillation. To eliminate oscillation or reduce
ringing, an isolation resistor can be placed between the load
and the output. In general, the bigger the isolation resistor,
the more damped the pulse response becomes. For initial
evaluation, a 50Ω isolation resistor is recommended.
LMH6622MM
LMH6622MA
Device
Package
MSOP-8
SOIC-8
F
Evaluation Board P/N
design technique on
CLC730036
CLC730123
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