LMH6552_14 Datasheet, PDF (17/32 Pages) Texas Instruments – 1.5 GHz Fully Differential Amplifier

LMH6552
www.ti.com
SNOSAX9H – APRIL 2007 – REVISED MARCH 2013
The circuit shown in Figure 47 was used to measure S-parameters for a single-to-differential configuration.
Figure 39 and Figure 40 in Typical Performance Characteristics are taken using the recommended component
values for 0 dB gain.
SINGLE SUPPLY OPERATION
Single supply operation is possible on supplies from 5V to 10V; however, as discussed earlier, AC input coupling
is recommended for low supplies such as 5V due to input common mode limitations. An example of an AC
coupled, single supply, single-to-differential circuit is shown in Figure 48. Note that when AC coupling, both
inputs need to be AC coupled irrespective of single-to-differential or differential-to-differential configuration. For
higher supply voltages DC coupling of the inputs may be possible provided that the output common mode DC
level is set high enough so that the amplifier's inputs and outputs are within their specified operating ranges.
R_F
R_O
R_S
a ^VS
R_G
R_T ^VCM
R_G
R_M
V_I1
+
-
V_I2
V_O1
C_L R_L V_O
V_O2
R_O
R_F
ENABLE
V_O1 + V_O2
*V_CM =
2
*BY DESIGN
V_ICM = V_OCM
V_I1 + V_I2
V_ICM =
2
Figure 48. AC Coupled for Single Supply Operation
SPLIT SUPPLY OPERATION
For optimum performance, split supply operation is recommended using +5V and −5V supplies; however,
operation is possible on split supplies as low as +2.25V and −2.25V and as high as +6V and −6V. Provided the
total supply voltage does not exceed the 4.5V to 12V operating specification, non-symmetric supply operation is
also possible and in some cases advantageous. For example, if a 5V DC coupled operation is required for low
power dissipation but the amplifier input common mode range prevents this operation, it is still possible with split
supplies of (V⁺) and (V^−). Where (V⁺) - (V^−) = 5V and V⁺ and V^− are selected to center the amplifier input
common mode range to suit the application.
OUTPUT NOISE PERFORMANCE AND MEASUREMENT
Unlike differential amplifiers based on voltage feedback architectures, noise sources internal to the LMH6552
refer to the inputs largely as current sources, hence the low input referred voltage noise and relatively higher
input referred current noise. The output noise is therefore more strongly coupled to the value of the feedback
resistor and not to the closed loop gain, as would be the case with a voltage feedback differential amplifier. This
allows operation of the LMH6552 at much higher gain without incurring a substantial noise performance penalty,
simply by choosing a suitable feedback resistor.
Figure 49 shows a circuit configuration used to measure noise figure for the LMH6552 in a 50Ω system. An R_F
value of 275Ω is chosen for the SOIC package to minimize output noise while simultaneously allowing both high
gain (9 V/V) and proper 50Ω input termination. Refer to SINGLE ENDED INPUT TO DIFFERENTIAL OUTPUT
OPERATION for calculation of resistor and gain values. Noise figure values at various frequencies are shown
Figure 36 in Typical Performance Characteristics.
Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LMH6552
Submit Documentation Feedback
17