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LMH6551 Datasheet, PDF (17/32 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp
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LMH6551
SNOSAK7D – FEBRUARY 2005 – REVISED JANUARY 2015
Typical Applications (continued)
9.2.2 Single-Ended Input to Differential Output
The LMH6551 provides excellent performance as an active balun transformer. Figure 29 shows a typical
application where an LMH6551 is used to produce a differential signal from a single-ended source.
In single-ended input operation the output common-mode voltage is set by the VCM pin as in fully differential
mode. Also, in this mode the common-mode feedback circuit must recreate the signal that is not present on the
unused differential input pin. Figure 22 is the measurement of the effectiveness of this process. The common-
mode feedback circuit is responsible for ensuring balanced output with a single-ended input. Balance error is
defined as the amount of input signal that couples into the output common mode. It is measured as the
undesired output common-mode swing divided by the signal on the input. Balance error can be caused by either
a channel to channel gain error, or phase error. Either condition will produce a common-mode shift. Figure 22
measures the balance error with a single-ended input as that is the most demanding mode of operation for the
amplifier.
Supply and VCM pin bypassing are also critical in this mode of operation. See the above section on Fully
Differential Operation for bypassing recommendations and also see Figure 25 and Figure 26 for recommended
supply bypassing configurations.
AV, RIN
RS VI
RG VI1
VS
a RT
VCM
RM
RG
VI2
+-
RF
V+
+
VO1
-
VO2
V-
RO
-
VO
+
RO
RF
IN-
ADC
IN+
Conditions :
Definitions :
RS RT||RIN
RM RT || RS
1
RG
RG  RF
2
RG  RM
RG  RM  RF
Av
VO
VI
2(1 1 # 5F
1  2 5G
for RM

RG
RIN
2RG  RM(1 2
1 2
RG(1
2
1
)
#
5G 
$v
 2
  $v
for RM
 RG
VOCM
VCM
VO1  VO2 (by design)
2
VICM
VI1  VI2
2
VOCM.2
#
VOCM
1 Av
IRU5M 

5G 
Figure 29. Single-Ended In to Differential Out
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