LMH6554 Datasheet, PDF (13/24 Pages) National Semiconductor (TI) – 2.8 GHz Ultra Linear Fully Differential Amplifier

LMH6554
www.ti.com
SNOSB30O – OCTOBER 2008 – REVISED MARCH 2013
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 = ±2.5V.
To match the input impedance of the circuit in Figure 31 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 31.
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 50Ω 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
Single Supply Operation
Single 5V supply operation is possible: however, as discussed earlier, AC input coupling is recommended due to
input common mode limitations. An example of an AC coupled, single supply, single-to-differential circuit is
shown in Figure 32. Note that when AC coupling, both inputs need to be AC coupled irrespective of single-to-
differential or differential-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 operation ranges.
R_S
V_S
a
0.1 PF RG
R_T
V_CM
0.1 PF
R_G
R_M
R_F
R_O
+
LMH6554
-
C_L R_L V_O
R_O
R_F
V_EN
Figure 32. AC Coupled for Single Supply Operation
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