LMH6552 Datasheet, PDF (21/38 Pages) Texas Instruments – 1.5-GHz Fully Differential Amplifier

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LMH6552
SNOSAX9I – APRIL 2007 – REVISED JANUARY 2015
9.2.2 Single-Ended Input to Differential Output Operation
In many applications, it is required to drive a differential input ADC from a single-ended source. Traditionally,
transformers have been used to provide single to differential conversion, but these are inherently bandpass by
nature and cannot be used for DC coupled applications. The LMH6552 provides excellent performance as a
single-to-differential converter down to DC. Figure 45 shows a typical application circuit where an LMH6552 is
used to produce a differential signal from a single-ended source.
A_V, R_IN
R_S
R_G
a VS
R_T
V_CM
R_F
V⁺
+
LMH6552
R_M
R_G
+-
-
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 45. Single-Ended Input with Differential Output
When using the LMH6552 in single-to-differential mode, the complementary 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 LMH6552 over frequency is shown in Typical Performance Characteristics.
To match the input impedance of the circuit in Figure 45 to a specified source resistance, R_S, requires that R_T ||
R_IN = R_S. The equations governing R_IN and A_V for single-to-differential operation are also provided in Figure 45.
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 configurations in a 50-Ω
environment are given in Table 1. Typically R_S=50Ω while R_M=R_S||R_T.
9.2.3 Single Supply Operation
Single supply operation is possible on supplies from 5 V to 10 V; however, as discussed earlier, AC input
coupling is recommended for low supplies such as 5 V due to input common mode limitations. An example of an
AC coupled, single supply, single-to-differential circuit is shown in Figure 46. Note that when AC coupling, both
inputs need to be AC coupled irrespective of single-to-differential or differential-to-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 operating ranges.
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