lmh6551qmmx National Semiconductor Corporation, lmh6551qmmx Datasheet - Page 11

• Manufacturer Part Number: lmh6551qmmx
• Description: Differential, High Speed Op Amp
• Manufacturer: National Semiconductor Corporation

Application Section
The LMH6551Q is a fully differential amplifier designed to
provide low distortion amplification to wide bandwidth differ-
ential signals. The LMH6551Q, though fully integrated for
ultimate balance and distortion performance, functionally pro-
vides three channels. Two of these channels are the V
V
mode operational amplifiers and are the primary signal paths.
The third channel is the common mode feedback circuit. This
is the circuit that sets the output common mode as well as
driving the V
posite phase, even when only one of the two input channels
is driven. The common mode feedback circuit allows single
ended to differential operation.
The LMH6551Q 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.
FULLY DIFFERENTIAL OPERATION
The LMH6551Q will perform best when used with split sup-
plies and in a fully differential configuration. See
Figure 2
−
signal path channels, which function similarly to inverting
CM
pin. This pin should be driven by a low impedance
for recommend circuits.
+
FIGURE 1. Typical Application
and V
−
outputs to be equal magnitude and op-
CMRR
Figure 1
30157933
CM
30157904
will be
+
and
and
11
The circuit shown in
plication as might be used to drive an ADC. In this circuit
closed loop gain, (A
cations in this data sheet V
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 sig-
nal.
The resistors R
sented with a load C
converter (ADC). When fed with a differential signal, the
LMH6551 provides excellent distortion, balance and common
mode rejection provided the resistors R
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 re-
sistors and board trace resistance. At higher frequencies
board layout symmetry becomes a factor as well. Precision
resistors of at least 0.1% accuracy are recommended and
careful board layout will also be required.
FIGURE 2. Fully Differential Cable Driver
O
help keep the amplifier stable when pre-
V
Balance Error
Figure 1
) = V
L
as is typical in an analog to digital
OUT
IN
/ V
is a typical fully differential ap-
is presumed to be the voltage
IN
= R
F
/R
F
, R
G
. For all the appli-
G
and R
30157913
O
are well
30157902
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