LMH6554_15 Datasheet, PDF (19/29 Pages) Texas Instruments – LMH6554 2.8-GHz Ultra Linear Fully Differential Amplifier

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LMH6554
SNOSB30P – OCTOBER 2008 – REVISED JANUARY 2015
9.2.5 Output Noise Performance and Measurement
Unlike differential amplifiers based on voltage feedback architectures, noise sources internal to the LMH6554
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 LMH6554 at much higher gain without incurring a substantial noise performance penalty,
simply by choosing a suitable feedback resistor.
Figure 34 shows a circuit configuration used to measure noise figure for the LMH6554 in a 50-Ω system. A
feedback resistor value of 200Ω is chosen for the UQFN package to minimize output noise while simultaneously
allowing both high gain (7 V/V) and proper 50-Ω input termination. Refer to Single-Ended Input to Differential
Output Operation for the calculation of resistor and gain values.
R_S = 50:
a ^VS
50:
200:
V⁺
8:
V_CM
+
LMH6554
-
8:
V^-
1 PF _{2:1 (TURNS)}
-
V_O
50:
+
1 PF
200:
A_V = 7 V/V
Figure 34. Noise Figure Circuit Configuration
9.2.6 Balanced Cable Driver
With up to 5.68 V_PP differential output voltage swing the LMH6554 can be configured as a cable driver. The
LMH6554 is also suitable for driving differential cables from a single ended source as shown in Figure 35.
R_S = 50:
V_S
a
Input
Source
91:
76.8: ^VCM
91:
30.3:
200:
50:
+
LMH6554
-
2 V_PP
V_EN
200:
50:
100:
TWISTED PAIR
Figure 35. Fully Differential Cable Driver
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