LMH6552_14 Datasheet, PDF (14/32 Pages) Texas Instruments – 1.5 GHz Fully Differential Amplifier

LMH6552
SNOSAX9H – APRIL 2007 – REVISED MARCH 2013
APPLICATION INFORMATION
www.ti.com
The LMH6552 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 LMH6552 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 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_F between 270Ω and 390Ω 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 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.
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 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 1.2V to 3.8V input
common mode range of the amplifier. The minimum V_CM for this application should be greater than 2.5V
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 V_CM would be achievable. Split supplies will allow much less
restricted AC and DC coupled 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.
WSON PACKAGE
Due to it's size and lower parasitics, the WSON requires the lower optimum value of 275Ω for R_F. This will give a
flat frequency response with minimal peaking. With a lower R_F value the WSON package will have a reduction in
noise compared to the SOIC with its optimum R_F = 360Ω.
FULLY DIFFERENTIAL OPERATION
The LMH6552 will perform best in a fully differential configuration. The circuit shown in Figure 44 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.
14
Submit Documentation Feedback
Product Folder Links: LMH6552
Copyright © 2007–2013, Texas Instruments Incorporated