LMH6550_14 Datasheet, PDF (13/25 Pages) National Semiconductor (TI) – LMH6550 Differential, High Speed Op Amp

LMH6550
www.ti.com
SNOSAK0H – DECEMBER 2004 – REVISED MARCH 2013
SINGLE ENDED INPUT TO DIFFERENTIAL OUTPUT
The LMH6550 provides excellent performance as an active balun transformer. Figure 29 shows a typical
application where an LMH6550 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 23 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 23
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 for bypassing
recommendations and also see Figure 27 and Figure 28 for recommended supply bypassing configurations.
R_F
A_V, R_IN
R_{S VI}
R_{G VI1}
V_S
a ^RT
V_CM
R_M
R_G
V_I2
+-
V⁺
+
V_O1
-
V_O2
V^-
R_O
-
V_O
+
R_O
IN-
ADC
IN+
R_F
Conditions :
R_S R_T || R_IN
R_M R_T || R_S
A_v
V_O
V_I
2(1
1
1⁾
2
#
R_F
R_G
for R_M
R_G
Definitions :
R_G
1 _{RG RF}
R_G R_M
2 _{RG RM RF}
R_IN
2R_G R_M(1
1₂
2⁾
R_G(1
1
2₎
1
2
#
2R_G(1 A_v )
2 A_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
for R_M
R_G
Figure 29. Single Ended In to Differential Out
SINGLE SUPPLY OPERATION
The input stage of the LMH6550 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 29, 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 30.
In Figure 29 closed loop gain = V_O / V_I ≊ R_F / R_G, where V_I =V_S / 2, as long as R_M << R_G. Note that in single
ended to differential operation V_I is measured single ended while V_O is measured differentially. This means that
gain is really 1/2 or 6 dB less when measured on either of the output pins separately. Additionally, note that the
input signal at R_T (labeled as V_I) is 1/2 of V_S when R_T is chosen to match R_S to R_IN.
V_ICM = Input common mode voltage = (V_I1+V_I2) / 2.
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