LMH6552 Datasheet, PDF (22/38 Pages) Texas Instruments – 1.5-GHz Fully Differential Amplifier

LMH6552
SNOSAX9I – APRIL 2007 – REVISED JANUARY 2015
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R_F
R_O
R_S
= ^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 46. AC Coupled for Single Supply Operation
9.2.4 Split Supply Operation
For optimum performance, split supply operation is recommended using +5 V and −5 V supplies; however,
operation is possible on split supplies as low as +2.25 V and −2.25 V and as high as +6 V and −6 V. Provided
the total supply voltage does not exceed the 4.5-V to 12-V operating specification, non-symmetric supply
operation is also possible and in some cases advantageous. For example, if a 5-V 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.
50:
Single-Ended
AC-coupled
Source
61.8:
169:
49.9:
0.1 PF
169:
61.8:
357:
V⁺
-+
LMH6552
-
+
V^-
125:
ADC12DL080
12-Bit
80 MSPS
2.2 pF
C_IN
~ 7- 8 pF
125:
V_REF
357:
Figure 47. Split Supply
9.2.5 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 48 shows a circuit configuration used to measure noise figure for the LMH6552 in a 50-Ω system. An R_F
value of 275 Ω is chosen for the SOIC package to minimize output noise while simultaneously allowing both high
gain (9 V/V) and proper 50-Ω 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 31 in Typical
Performance Characteristics.
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