LMH6554_14 Datasheet, PDF (15/24 Pages) National Semiconductor (TI) – LMH6554 2.8 GHz Ultra Linear Fully Differential Amplifier

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LMH6554
SNOSB30O – OCTOBER 2008 – REVISED MARCH 2013
R_S = 50:
91: 200:
V_S
a
C
AC-Coupled
Source
91:
30:
76.8:
0.1 PF _VCM
V⁺
+-
LMH6554
-+
V^-
0.1 PF
0.1 PF
200:
50:
50:
V_EN
ADC
Up To 16-Bit
Data Converter
V_CMO
Figure 34. Driving a 10-bit Gigasample ADC
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 35 shows a circuit configuration used to measure noise figure for the LMH6554 in a 50Ω 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 50Ω 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 35. Noise Figure Circuit Configuration
Driving Capacitive Loads
As noted previously, capacitive loads should be isolated from the amplifier output with small valued resistors.
This is particularly the case when the load has a resistive component that is 500Ω or higher. A typical ADC has
capacitive components of around 10 pF and the resistive component could be 1000Ω or higher. If driving a
transmission line, such as 50Ω coaxial or 100Ω twisted pair, using matching resistors will be sufficient to isolate
any subsequent capacitance. For other applications see Figure 8 in Typical Performance Characteristics V_S =
±2.5V.
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