LMH6554LE NSC [National Semiconductor], LMH6554LE Datasheet - Page 11

• Manufacturer Part Number: LMH6554LE
• Description: 2.8 GHz Ultra Linear Fully Differential Amplifier
• Manufacturer: NSC [National Semiconductor]

Application Information
The LMH6554 is a fully differential, current feedback amplifier
with integrated output common mode control, designed to
provide low distortion amplification to wide bandwidth differ-
ential signals. The common mode feedback circuit sets the
output common mode voltage independent of the input com-
mon mode, as well as forcing the V
equal in magnitude and opposite in phase, even when only
one of the inputs is driven as in single to differential conver-
sion.
The proprietary current feedback architecture of the
LMH6554 offers gain and bandwidth independence with ex-
ceptional gain flatness and noise performance, even at high
values of gain, simply with the appropriate choice of R
R
R
these resistors greatly affects CMRR, DC offset error, and
output balance. A maximum of 0.1% tolerance resistors are
recommended for optimal performance, and the amplifier is
internally compensated to operate with optimum gain flatness
with R
resistance.
The output common mode voltage is set by the V
a fixed gain of 1 V/V. This pin should be driven by a low
impedance reference and should be bypassed to ground with
a 0.1 µF ceramic capacitor. Any unwanted signal coupling into
the V
performance of the amplifier.
The LMH6554 can be configured to operate on a single 5V
supply connected to V+ with V- grounded or configured for a
split supply operation with V
ation on a single 5V supply, depending on gain, is limited by
the input common mode range; therefore, AC coupling may
be required. Split supplies will allow much less restricted AC
and DC coupled operation with optimum distortion perfor-
mance.
The LMH6554 is equipped with an enable pin (VEN) to reduce
power consumption when not in use. The VEN pin, when not
driven, floats high (on). When the VEN pin is pulled low the
amplifier is disabled and the amplifier output stage goes into
a high impedance state so the feedback and gain set resistors
determine the output impedance of the circuit. For this reason
input to output isolation will be poor in the disabled state and
the part is not recommended in multiplexed applications
where outputs are all tied together.
Fully Differential Operation
The LMH6554 will peform best in a fully differential configu-
ration. The circuit shown in
application circuit as might be used to drive an analog to dig-
ital converter (ADC). In this circuit the closed loop gain is
A
series output resistors, R
amplifier stable when presented with a capacitive load. Refer
to the Driving Capacitive Loads section for details.
When driven from a differential source, the LMH6554 pro-
vides low distortion, excellent balance, and common mode
rejection. This is true provided the resistors R
are well matched and strict symmetry is observed in board
layout. With an intrinsic device CMRR of greater than 70 dB,
using 0.1% resistors will give a worst case CMRR of around
50 dB for most circuits.
The circuit configuration shown in
sure differential S-parameters in a 100Ω environment at a
V
F2
G2
=V
. Generally R
, so that the gain is set by the ratio R
CM
OUT
F
value of 200Ω depending on PCB layout, and load
pin will be passed along to the outputs, reducing the
/V
IN
=R
F
/R
F1
G
, where the feedback is symmetric. The
is set equal to R
O
Figure 1
, are optional and help keep the
+
= +2.5V and V
Figure 1
is a typical fully differential
+
F2
and V
, and R
F
was used to mea-
/R
−
−
= −2.5V. Oper-
G
outputs to be
F
. Matching of
, R
G1
CM
G
equal to
pin with
and R
F1
and
O
11
gain of 1 V/V. Refer to the Differential S-Parameter vs. Fre-
quency Plots in the Typical Performance Characteristics sec-
tion for measurement results.
Single Ended Input To Differential
Output Operation
In many applications, it is required to drive a differential input
ADC from a single ended source. Traditionally, transformers
have been used to provide single to differential conversion,
but these are inherently bandpass by nature and cannot be
used for DC coupled applications. The LMH6554 provides
excellent performance as a single-ended input to differential
output converter down to DC.
cation circuit where an LMH6554 is used to produce a bal-
anced differential output signal from a single ended source.
When using the LMH6554 in single-to-differential mode, the
complimentary output is forced to a phase inverted replica of
the driven output by the common mode feedback circuit as
opposed to being driven by its own complimentary input. Con-
sequently, as the driven input changes, the common mode
feedback action results in a varying common mode voltage at
the amplifier's inputs, proportional to the driving signal. Due
to the non-ideal common mode rejection of the amplifier's in-
put stage, a small common mode signal appears at the out-
FIGURE 2. Single Ended Input with Differential Output
FIGURE 1. Differential S-Parameter Test Circuit
Figure 2
shows a typical appli-
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