Application Section
The LMH6551 is a fully differential amplifier designed to
provide low distortion amplification to wide bandwidth differ-
ential signals. The LMH6551, though fully integrated for
ultimate balance and distortion performance, functionally
provides three channels. Two of these channels are the V
+
and V
signal path channels, which function similarly to
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
+
and V
outputs to be equal
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 LMH6551 is a voltage feedback amplifier with gain set
by external resistors. Output common mode voltage is set by
the V
pin. This pin should be driven by a low impedance
reference and should be bypassed to ground with a 0.1 μF
ceramic capacitor. Any signal coupling into the V
will be
passed along to the output and will reduce the dynamic
range of the amplifier.
FULLY DIFFERENTIAL OPERATION
The LMH6551 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
) = V
/ V
IN
= R
/R
. For all the
applications in this data sheet V
is presumed to be the
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.
The resistors R
O
help keep the amplifier stable when pre-
sented with a load C
L
as is typical in an analog to digital
converter (ADC). When fed with a differential signal, the
LMH6551 provides excellent distortion, balance and com-
mon mode rejection provided the resistors R
F
, R
G
and R
O
are well matched and strict symmetry is observed in board
layout. With a DC CMRR of over 80dB, 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
differential output voltage swing and 80
mA of linear drive current the LMH6551 makes an excellent
cable driver as shown in
Figure 2
. The LMH6551 is also
suitable for driving differential cables from a single ended
source.
20133204
FIGURE 1. Typical Application
20133202
FIGURE 2. Fully Differential Cable Driver
20133210
FIGURE 3. Single Ended in Differential Out
L
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