Datasheet

V
IN
R
G
R
G
V
CM
R
L
V
O
R
F
R
F
C
L
R
O
R
O
V
CLAMP
a
+
-
LMH6553
www.ti.com SNOSB07H –SEPTEMBER 2008–REVISED MARCH 2013
APPLICATION INFORMATION
The LMH6553, a fully differential current feedback amplifier with integrated output common mode control and
output limiting clamp, is designed to provide protection of following input stages. The common mode feedback
circuit sets the output common mode voltage independent of the input common mode, as well as forcing the
outputs to be equal in magnitude and opposite in phase, even when only one of the inputs is driven as in single
ended to differential conversion.
The proprietary current feedback architecture of the LMH6553 offers gain and bandwidth independence even at
high values of gain, simply with the appropriate choice of R
F1
and R
F2
. Generally R
F1
is set equal to R
F2
, and R
G1
equal to R
G2
, so that the gain is set by the ratio R
F
/R
G
. Matching of these resistors greatly affects CMRR, DC
offset error, and output balance. Resistors with 0.1% tolerances are recommended for optimal performance, and
the amplifier is internally compensated to operate with optimum gain flatness with values of R
F
between 250Ω
and 350Ω depending on package selection, PCB layout, and load resistance.
The output common mode voltage is set by the V
CM
pin with a fixed gain of 1 V/V. This pin should be driven by a
low impedance source and should be bypassed to ground with a 0.1 µF ceramic capacitor. Any unwanted signal
coupling into the V
CM
pin will be passed along to the outputs, reducing the performance of the amplifier. This pin
must not be left floating.
The LMH6553 can be operated with either a single 5V supply or split +5V and −5V supplies. Operation on a
single 5V supply, depending on gain, is limited by the input common mode range; therefore, AC coupling may be
required. For example, in a DC coupled input application on a single 5V supply, with a V
CM
of 1.5V, the input
common voltage at a gain of 1 will be 0.75V which is outside the minimum 1.5V to 3.5V input common mode
range of the amplifier. The minimum V
CM
for this application should be greater than 1.5V 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 input common mode would result. Split supplies allow much less restricted AC and DC
coupled operation with optimum distortion performance.
The LMH6553 has a V
CLAMP
input which allows control of the maximum amplifier output swing to prevent
overdriving of following stages such as sensitive ADC inputs and also provides fast recovery from transients that
would otherwise saturate the signal path.
RECOMMENDED FEEDBACK RESISTOR
The LMH6553 is available in both an 8-pin WSON and SO PowerPAD package. The recommended feedback
resistor, R
F
, for the WSON package is 275Ω and 325Ω for the SO PowerPAD to give a flat frequency response
with minimal peaking.
FULLY DIFFERENTIAL OPERATION
The LMH6553 is ideal for a fully differential configuration. The circuit shown in Figure 54 is a typical fully
differential application circuit as might be used to drive an analog to digital converter (ADC). In this circuit the
closed loop gain A
V
= V
OUT
/ V
IN
= R
F
/R
G
, where the feedback is symmetric. The series output resistors, R
O
, are
optional and help keep the amplifier stable when presented with a capacitive load. Refer to DRIVING
CAPACITIVE LOADS for details.
Figure 54. Typical Application
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