lmh6550mm National Semiconductor Corporation, lmh6550mm Datasheet - Page 11

• Manufacturer Part Number: lmh6550mm
• Description: Differential, High Speed Op Amp
• Manufacturer: National Semiconductor Corporation

Application Section
The LMH6550 is a fully differential amplifier designed to
provide low distortion amplification to wide bandwidth differ-
ential signals. The LMH6550, though fully integrated for
ultimate balance and distortion performance, functionally
provides three channels. Two of these channels are the V
and V
inverting mode operational amplifiers and are the primary
signal paths. The third channel is the common mode feed-
back circuit. This is the circuit that sets the output common
mode as well as driving the V
magnitude and opposite phase, even when only one of the
two input channels is driven. The common mode feedback
circuit allows single ended to differential operation.
The LMH6550 is a voltage feedback amplifier with gain set
by external resistors. Output common mode voltage is set by
the V
reference and should be bypassed to ground with a 0.1 µF
ceramic capacitor. Any signal coupling into the V
passed along to the output and will reduce the dynamic
range of the amplifier.
The LMH6550 is equipped with a ENABLE pin to reduce
power consumption when not in use. The ENABLE pin floats
to logic high. If this pin is not used it can be left floating. The
amplifier output stage goes into a high impedance state
when the amplifier is disabled. The feedback and gain set
resistors will then set the impedance of the circuit. For this
reason input to output isolation will be poor in the disabled
state.
FULLY DIFFERENTIAL OPERATION
The LMH6550 will perform best when used with split sup-
plies and in a fully differential configuration. See Figure 1
and Figure 3 for recommend circuits.
The circuit shown in Figure 1 is a typical fully differential
application as might be used to drive an ADC. In this circuit
closed loop gain, (A
applications in this data sheet V
voltage presented to the circuit by the signal source. For
differential signals this will be the difference of the signals on
each input (which will be double the magnitude of each
individual signal), while in single ended inputs it will just be
the driven input signal.
CM
−
signal path channels, which function similarly to
pin. This pin should be driven by a low impedance
FIGURE 1. Typical Application
V
) = V
OUT
+
/ V
and V
IN
IN
is presumed to be the
−
= R
outputs to be equal
F
/R
G
. For all the
CM
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+
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The resistors R
sented with a load C
converter (ADC). When fed with a differential signal, the
LMH6550 provides excellent distortion, balance and com-
mon mode rejection provided the resistors R
are well matched and strict symmetry is observed in board
layout. With a DC CMRR of over 80 dB, the DC and low
frequency CMRR of most circuits will be dominated by the
external resistors and board trace resistance. At higher fre-
quencies board layout symmetry becomes a factor as well.
Precision resistors of at least 0.1% accuracy are recom-
mended and careful board layout will also be required.
With up to 15 V
mA of linear drive current the LMH6550 makes an excellent
cable driver as shown in Figure 2. The LMH6550 is also
suitable for driving differential cables from a single ended
source.
FIGURE 3. Single Ended in Differential Out
FIGURE 2. Fully Differential Cable Driver
O
PP
help keep the amplifier stable when pre-
differential output voltage swing and 80
L
as is typical in an analog to digital
F
, R
www.national.com
G
and R
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