LMH6551 Datasheet, PDF (14/32 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

LMH6551
SNOSAK7D – FEBRUARY 2005 – REVISED JANUARY 2015
9 Application and Implementation
www.ti.com
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 LMH6551 is a fully differential amplifier designed to provide low distortion amplification to wide bandwidth
differential signals. The LMH6551, though fully integrated for ultimate balance and distortion performance,
functionally provides three channels. Two of these channels are the V⁺ and V^− signal path channels, which
function similarly to inverting mode operational amplifiers and are the primary signal paths. The third channel is
the common-mode feedback circuit. This is the circuit that sets the output common mode as well as driving the
V⁺ and V^− outputs to be equal magnitude and opposite phase, even when only one of the two input channels is
driven. The common-mode feedback circuit allows single-ended to differential operation.
The LMH6551 is a voltage feedback amplifier with gain set by external resistors. Output common-mode voltage
is set by the V_CM pin. This pin should be driven by a low impedance reference and should be bypassed to ground
with a 0.1 µF ceramic capacitor. Any signal coupling into the V_CM will be passed along to the output and will
reduce the dynamic range of the amplifier.
9.2 Typical Applications
9.2.1 Typical Fully Differential Application
R_F1
R_O
R_G1
= ^VI
+
V_CM
-
R_G2
C_L
R_L V_O
R_O
R_F2
Figure 23. Typical Fully Differential Application Schematic
9.2.1.1 Design Requirements
Applications using fully differential amplifiers have several requirements. The main requirements are high linearity
and good signal amplitude. Linearity is accomplished by using well matched feedback and gain set resistors as
well as an appropriate supply voltage. The signal amplitude can be tailored by using an appropriate gain. In this
design, the gain is set for a gain of 2 (Rf=500/ RG=250) and the distortion criteria is better than -90 dBc at a
frequency of 5 Mhz for HD2 and HD3. The supply voltage is set for ±5 V and the output common mode is 0 V.
9.2.1.2 Detailed Design Procedure
9.2.1.2.1 Fully Differential Operation
The LMH6551 will perform best when used with split supplies and in a fully differential configuration. See
Figure 23 and Figure 24 for recommend circuits.
14
Submit Documentation Feedback
Product Folder Links: LMH6551
Copyright © 2005–2015, Texas Instruments Incorporated