LMH6550 Datasheet, PDF (14/33 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

LMH6550
SNOSAK0I – DECEMBER 2004 – REVISED JANUARY 2015
9 Application and Implementation
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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 LMH6550 is a fully differential amplifier designed to provide low distortion amplification to wide bandwidth
differential signals. The LMH6550, 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 LMH6550 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.
The LMH6550 is equipped with a ENABLE pin to reduce power consumption when not in use. The ENABLE pin
floats to logic high. If this pin is not used it can be left floating. The amplifier output stage goes into a high
impedance state when the amplifier is disabled. The feedback and gain set resistors will then set the impedance
of the circuit. For this reason input to output isolation will be poor in the disabled state.
9.2 Typical Applications
9.2.1 Typical Fully Differential Application
The LMH6550 performs best when used with split supplies and in a fully differential configuration. See Figure 21
and Figure 22 for recommend circuits.
R_F1
R_O
R_G1
+
= ^VI
V_CM
-
R_G2
C_L
R_L V_O
R_O
R_F2
ENABLE
Figure 21. 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 (R_F=500/ R_G=250) and the distortion criteria is better than -90 dBc at a
frequency of 5 Mhz. The supply voltages are set to +5 V and -5 V and the output common mode is 0 V. The
LMH6550 can be placed into shutdown to reduce power dissipation to 10 mW.
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