LMH6552MA NSC [National Semiconductor], LMH6552MA Datasheet - Page 12

• Manufacturer Part Number: LMH6552MA
• Description: 1 GHz Fully Differential Amplifier
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
The LMH6552 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
LMH6552 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 minimum of 0.1% tolerance resistors are
recommended for optimal performance, and the amplifier is
internally compensated to operate with optimum gain flatness
with values of R
package selection and PCB layout.
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. This pin must not be left floating.
The LMH6552 can be operated on a supply range as either a
single 5V supply or as a split +5V and −5V. Operation on a
single 5V supply, depending on gain, is limited by the input
common mode range: therefore, AC coupling may be re-
quired. For example, in a DC coupled input application on a
single 5V supply, with a V
age at a gain of 1 will be 0.75V which is outside the minimum
1.2V to 3.8V input common mode range of the amplifier. The
minimum V
depending on output signal swing. Alternatively, AC coupling
of the inputs in this example results in equal input and output
common mode voltages, so a 1.5V V
Split supplies will allow much less restricted AC and DC cou-
pled operation with optimum distortion performance.
The LMH6552 is equipped with an ENABLE pin to reduce
power consumption when not in use. The ENABLE pin, when
not driven, floats high (on). When the ENABLE 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 LMH6552 will perform best in a fully differential configu-
ration. The circuit shown in Figure 1 is a typical fully differential
application circuit as might be used to drive an analog to dig-
ital converter (ADC). In this circuit the closed loop gain,
A
The series output resistors, R
the amplifier stable when presented with a capacitive load.
Refer to the Driving Capacitive Loads section for details.
F2
G2
V
= V
. Generally R
, so that the gain is set by the ratio R
CM
OUT
pin will be passed along to the outputs, reducing the
/ V
CM
IN
for this application should be greater than 2.5V
= R
F
F1
between 270Ω and 390Ω depending on
F
is set equal to R
/R
G
, where the feedback is symmetric.
CM
of 1.5V, the input common volt-
O
, are optional and help keep
CM
+
F2
and V
would be achievable.
, and R
F
/R
−
G
outputs to be
. Matching of
G1
CM
equal to
pin with
F1
and
12
When driven from a differential source, the LMH6552 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 80 dB, using 0.1%
resistors will give a worst case CMRR of around 60 dB for
most circuits.
The circuit configuration shown in Figure 2 was used to mea-
sure differential S parameters in a 50Ω environment at a gain
of 1 V/V. Refer to the Differential S-Parameter vs. Frequency
plots in the Typical Performance Characteristics section 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 LMH6552 provides
excellent performance as a single-to-differential converter
down to DC. Figure 3 shows a typical application circuit where
an LMH6552 is used to produce a differential signal from a
single ended source.
FIGURE 2. Differential S-Parameter Test Circuit
FIGURE 1. Typical Application
F
, R
G
30003553
30003504
and R
O