LMH6553 Datasheet, PDF (18/24 Pages) National Semiconductor (TI) – 900 MHz Fully Differential Amplifier With Output Limiting Clamp

30043710
FIGURE 3. Single-Ended Input with Differential Output
When using the LMH6553 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 complementary 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 volt-
age to output differential voltage is commonly referred to as
output balance error. The output balance error response of
the LMH6553 over frequency is shown in the Typical Perfor-
mance Characteristics section.
To match the input impedance of the circuit in Figure 3 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 3. These equations,
along with the source matching condition, must be solved it-
eratively to achieve the desired gain with the proper input
termination. Component values for several common gain con-
figurations in a 50Ω environment are given in Table 1.
TABLE 1. Gain Component Values for 50Ω System
LLP Package
Gain
0 dB
6 dB
12 dB
R_F
275Ω
275Ω
275Ω
R_G
255Ω
127Ω
54.9Ω
R_T
59Ω
68.1Ω
107Ω
R_M
26.7Ω
28.7Ω
34Ω
TABLE 2. Gain Component Values for 50Ω System
PSOP Package
Gain
0 dB
6 dB
12 dB
R_F
325Ω
325Ω
325Ω
R_G
316Ω
150Ω
68.1Ω
R_T
56.2Ω
66.5Ω
110Ω
R_M
26.7Ω
28.7Ω
34.8Ω
30043754
FIGURE 4. Single Ended Input S-Parameter Test Circuit
(50Ω System)
The circuit shown in Figure 4 was used to measure S-param-
eters for a single-to-differential configuration. The S-parame-
ter plots in the Typical Performance Curves are taken using
the recommended component values for 0 dB gain.
SINGLE SUPPLY OPERATION
Single supply operation is possible on supplies from 5V to
10V; however, as discussed earlier, AC input coupling is rec-
ommended for low supplies due to input common mode limi-
tations. An example of an AC coupled, single supply, single-
to-differential circuit is shown in Figure 5. Note that when AC
coupling, both inputs need to be AC coupled irrespective of
single-to-differential or differential-to-differential configura-
tion. 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 out-
puts are within their specified operating ranges.
30043709
FIGURE 5. AC Coupled for Single Supply Operation
www.national.com
18