lmh6551qmmx National Semiconductor Corporation, lmh6551qmmx Datasheet - Page 12

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

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With up to 15 V
of linear drive current the LMH6551Q makes an excellent ca-
ble driver as shown in
suitable for driving differential cables from a single ended
source.
The LMH6551Q requires supply bypassing capacitors as
shown in
pacitors should be leadless SMT ceramic capacitors and
should be no more than 3 mm from the supply pins. The SMT
capacitors should be connected directly to a ground plane.
Thin traces or small vias will reduce the effectiveness of by-
pass capacitors. Also shown in both figures is a capacitor from
the V
to a buffer which sets the output common mode voltage. Any
noise on this input is transferred directly to the output. Output
common mode noise will result in loss of dynamic range, de-
graded CMRR, degraded Balance and higher distortion. The
V
is an internal resistive divider on chip to set the output com-
mon mode voltage to the mid point of the supply pins. The
impedance looking into this pin is approximately 25 kΩ. If a
different output common mode voltage is desired drive this
pin with a clean, accurate voltage reference.
CM
pin should be bypassed even if the pin in not used. There
CM
FIGURE 3. Split Supply Bypassing Capacitors
pin to ground. The V
Figure 3
PP
differential output voltage swing and 80 mA
and
Figure
Figure
CM
4. The 0.01 µF and 0.1 µF ca-
pin is a high impedance input
2. The LMH6551Q is also
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SINGLE ENDED INPUT TO DIFFERENTIAL OUTPUT
The LMH6551Q provides excellent performance as an active
balun transformer.
an LMH6551Q is used to produce a differential signal from a
single ended source.
In single ended input operation the output common mode
voltage is set by the V
in this mode the common mode feedback circuit must recre-
ate the signal that is not present on the unused differential
input pin. The performance chart titled “Balance Error” is the
measurement of the effectiveness of this process. The com-
mon mode feedback circuit is responsible for ensuring bal-
anced output with a single ended input. Balance error is
defined as the amount of input signal that couples into the
output common mode. It is measured as a the undesired out-
put common mode swing divided by the signal on the input.
Balance error can be caused by either a channel to channel
gain error, or phase error. Either condition will produce a
common mode shift. The chart titled “Balance Error” mea-
sures the balance error with a single ended input as that is
the most demanding mode of operation for the amplifier.
Supply and V
operation. See the above section on
OPERATION
Figure 3
configurations.
FIGURE 4. Single Supply Bypassing Capacitors
and
CM
for bypassing recommendations and also see
Figure 4
pin bypassing are also critical in this mode of
Figure 5
CM
for recommended supply bypassing
pin as in fully differential mode. Also,
shows a typical application where
FULLY DIFFERENTIAL
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