91:

+

-

LMH6554

V

+

V

-

VCMO

VS

91:

-

+

50:

50:

200:

200:

ADC

Up To 16-Bit

Data Converter

a

76.8:

RS = 50:

VEN

30:

VCM

0.1 PF

AC-Coupled

Source

0.1 PF

C

0.1 PF

LMH6554

www.ti.com

SNOSB30P – OCTOBER 2008 – REVISED JANUARY 2015

9 Application and Implementation

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 RF1 and RF2. Generally RF1 is set equal to RF2, and RG1 equal to RG2, so that the gain is set by the ratio RF/RG.

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 RF value of 200 Ω depending on PCB layout, and load resistance.

The output common mode voltage is set by the VCM 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 VCM 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

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

Submit Documentation Feedback

13

Product Folder Links: LMH6554