LMH6551MAX_NOPB Datasheet, PDF (14/28 Pages) Texas Instruments – LMH6551 Differential, High Speed Op Amp

LMH6551
SNOSAK7C – FEBRUARY 2005 – REVISED MARCH 2013
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SINGLE ENDED INPUT TO DIFFERENTIAL OUTPUT
The LMH6551 provides excellent performance as an active balun transformer. Figure 31 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 V_CM 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 26 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 a 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 26
measures the balance error with a single ended input as that is the most demanding mode of operation for the
amplifier.
Supply and V_CM 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 29 and Figure 30 for
recommended supply bypassing configurations.
A_V, R_IN
R_{S VI}
R_{G VI1}
V_S
a ^RT
V_CM
R_M
R_G
V_I2
+-
R_F
V⁺
+
V_O1
-
V_O2
V^-
R_O
-
V_O
+
R_O
R_F
IN-
ADC
IN+
Conditions :
Definitions :
R_S R_T||R_IN
R_M R_T || R_S
R_G
1 _{RG RF}
R_G R_M
2 _{RG RM RF}
A_v
V_O
V_I
2(1
1
1
2
# ^5F
5_G
for R_M
R_G
R_IN
2R_G R_M(1
1₂
2
R_G(1
2₎
1_#
2
5_G
$_v
$_v
for R_M
R_G
V_OCM
V_CM
V_O1 V_{O2 (by design)}
2
V_ICM
V_I1 V_I2
2
V_OCM.
2
#
V_OCM
1 A_v
IRU 5_M
5_G
Figure 31. Single Ended In to Differential Out
SINGLE SUPPLY OPERATION
The input stage of the LMH6551 has a built in offset of 0.7V towards the lower supply to accommodate single
supply operation with single ended inputs. As shown in Figure 31, the input common mode voltage is less than
the output common voltage. It is set by current flowing through the feedback network from the device output. The
input common mode range of 0.4V to 3.2V places constraints on gain settings. Possible solutions to this
limitation include AC coupling the input signal, using split power supplies and limiting stage gain. AC coupling
with single supply is shown in Figure 32.
14
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