LMH6554_15 Datasheet, PDF (13/29 Pages) Texas Instruments – LMH6554 2.8-GHz Ultra Linear Fully Differential Amplifier

www.ti.com
9 Application and Implementation
LMH6554
SNOSB30P – OCTOBER 2008 – REVISED JANUARY 2015
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 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 200 Ω 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 5-V supply connected to V+ with V- grounded or
configured for a split supply operation with V⁺ = +2.5 V and V^− = −2.5 V. Operation on a single 5-V 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.
9.2 Typical Applications
9.2.1 Single-Ended Input to Differential Output Operation
R_S = 50:
91: 200:
V_S
a
C
AC-Coupled
Source
91:
30:
76.8:
0.1 PF _VCM
V⁺
+-
LMH6554
-+
V^-
0.1 PF
0.1 PF
200:
50:
50:
V_EN
ADC
Up To 16-Bit
Data Converter
V_CMO
Figure 25. Single-Ended Input to Differential Output Schematic
9.2.1.1 Design Requirements
One typical application for the LMH6554 is to drive an ADC as shown in Figure 25. The following design is a
single-ended to differential circuit with an input impedance of 50 Ω and an output impedance of 100 Ω. The VCM
voltage of the amplifier needs to be set to the same voltage as the ADC reference voltage, which is typically 1.2
V. Figure 27 shows the design equations required to set the external resistor values. This design also requires a
gain of 2 and -96 dBc THD at 75 MHz.
Copyright © 2008–2015, Texas Instruments Incorporated
Product Folder Links: LMH6554
Submit Documentation Feedback
13