LMH6554_14 Datasheet, PDF (11/24 Pages) National Semiconductor (TI) – LMH6554 2.8 GHz Ultra Linear Fully Differential Amplifier

LMH6554
www.ti.com
SNOSB30O – OCTOBER 2008 – REVISED MARCH 2013
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 differential signals. The common mode
feedback circuit sets the output common mode voltage independent of the input common mode, as well as
forcing the V⁺ and V^− outputs to be equal in magnitude and opposite in phase, even when only one of the inputs
is driven as in single to differential conversion.
The proprietary current feedback architecture of the LMH6554 offers gain and bandwidth independence with
exceptional gain flatness and noise performance, 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. 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_F value of 200Ω depending on 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 reference 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.
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⁺ = +2.5V and V^− = −2.5V. Operation 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 performance.
Enable / Disable Operation
The LMH6554 is equipped with an enable pin (V_EN) to reduce power consumption when not in use. The V_EN pin,
when not driven, floats high (on). When the V_EN 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.
With a 5V difference between V⁺ and V^-, the V_EN threshold is ½ way between the supplies (e.g. 2.5V with 5V
single supply) as shown in Figure 29. R2 ensures active (enable) mode with V_EN floating, and R1 provides input
current limiting. V_EN also has ESD diodes to either supply.
V⁺
R
Supply
Mid-Point
LMH6554
Bias
Circuitry
Q2 Q1
R2
20k
R1
10k
V_EN
R
I Tail
V^-
Figure 29. Enable Block Diagram
Fully Differential Operation
The LMH6554 will perform best in a fully differential configuration. The circuit shown in Figure 30 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 is 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 the Driving
Capacitive Loads section for details.
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