LMH6554_15 Datasheet, PDF (15/29 Pages) Texas Instruments – LMH6554 2.8-GHz Ultra Linear Fully Differential Amplifier

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Typical Applications (continued)
LMH6554
SNOSB30P – OCTOBER 2008 – REVISED JANUARY 2015
A_V, R_IN
R_S
R_G
a VS
R_T
V_CM
R_M
R_G
+-
R_F
V⁺
+
LMH6554
-
V^-
R_F
R_O
-
V_O
+
R_O
IN-
ADC
IN+
A_V =
¨©§¨
2(1 - E1)
E₁ + E₂
R_IN
=
¨¨©§^2RG
+
1
R_M (1-E₂)
+ E₂
E₁ = _{¨¨©§RG}^R+G_RF
E₂ = _{¨¨©§RG}^R+G_R+F^R+M_RM
R_S = R_T || R_IN
R_M = R_T || R_S
Figure 27. Single-Ended Input with Differential Output
When using the LMH6554 in single-to-differential mode, the complimentary output is forced to a phase inverted
replica of the driven output by the common mode feedback circuit as opposed to being driven by its own
complimentary input. Consequently, as the driven input changes, the common mode feedback action results in a
varying common mode voltage at the amplifier's inputs, proportional to the driving signal. Due to the non-ideal
common mode rejection of the amplifier's input stage, a small common mode signal appears at the outputs which
is superimposed on the differential output signal. The ratio of the change in output common mode voltage to
output differential voltage is commonly referred to as output balance error. The output balance error response of
the LMH6554 over frequency is shown in the Typical Performance Characteristics V_S = ±2.5 V.
To match the input impedance of the circuit in Figure 27 to a specified source resistance, R_S, requries that R_T ||
R_IN = R_S. The equations governing R_IN and A_V for single-to-differential operation are also provide in Figure 27.
These equations, along with the source matching condition, must be solved iteratively to achieve the desired gain
with the proper input termination. Component values for several common gain configuration in a 50Ω
environment are given in Table 1.
GAIN
0dB
6dB
12dB
Table 1. Gain Component Values for 50 Ω System
R_F
200Ω
200Ω
200Ω
R_G
191Ω
91Ω
35.7Ω
R_T
62Ω
76.8Ω
147Ω
R_M
27.7Ω
30.3Ω
37.3Ω
9.2.1.2.3 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 500 Ω or higher. A typical ADC has
capacitive components of around 10 pF and the resistive component could be 1000 Ω or higher. If driving a
transmission line, such as 50-Ω coaxial or 100-Ω twisted pair, using matching resistors will be sufficient to isolate
any subsequent capacitance. For other applications, see Figure 29 in Typical Performance Characteristics V_S =
±2.5 V.
9.2.1.3 Application Curves
Many application circuits will have capacitive loading. As shown in Figure 28, amplifier bandwidth is reduced with
increasing capacitive load, so parasitic capacitance should be strictly limited.
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