LMH6550_06 Datasheet, PDF (12/17 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

Application Section (Continued)
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FIGURE 4. Split Supply Bypassing Capacitors
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FIGURE 5. Single Supply Bypassing Capacitors
The LMH6550 requires supply bypassing capacitors as
shown in Figure 4 and Figure 5. The 0.01 µF and 0.1 µF
capacitors should be leadless SMT ceramic capacitors and
should be no more than 3 mm from the supply pins. The
SMT capacitors should be connected directly to a ground
plane. Thin traces or small vias will reduce the effectiveness
of bypass capacitors. Also shown in both figures is a capaci-
tor from the V_CM pin to ground. The V_CM pin is a high
impedance input to a buffer which sets the output common
mode voltage. Any noise on this input is transferred directly
to the output. Output common mode noise will result in loss
of dynamic range, degraded CMRR, degraded Balance and
higher distortion. The V_CM pin should be bypassed even if
the pin in not used. There is an internal resistive divider on
chip to set the output common mode voltage to the mid point
of the supply pins. The impedance looking into this pin is
approximately 25 kΩ. If a different output common mode
voltage is desired drive this pin with a clean, accurate volt-
age reference.
SINGLE ENDED INPUT TO DIFFERENTIAL OUTPUT
The LMH6550 provides excellent performance as an active
balun transformer. Figure 3 shows a typical application
where an LMH6550 is used to produce a differential signal
from a single ended source. It should be noted that com-
pared to differential input, using a single ended input will
reduce gain by 1/2. So that the closed loop gain will be; Gain
= A_V = 0.5 * R_F/R_G.
In single ended input operation the output common mode
voltage is set by the V_CMpin 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 differ-
ential input pin. The performance chart titled “Balance Error”
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 pro-
duce a common mode shift. The chart titled “Balance Error”
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 DIFFEREN-
TIAL OPERATION for bypassing recommendations also see
Figure 4 and Figure 5 for recommended supply bypassing
configurations.
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 6, 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 7.
In Figure 6 below closed loop gain = A_V= R_F/R_G. Please note
that in single ended to differential operation V_IN is measured
single ended while V_OUT is measured differentially. This
means that gain is really 1/2 or 6 dB less when measured on
either of the output pins separately.
V_ICM= Input common mode voltage = (V^+IN+V^−IN)/2.
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FIGURE 6. Relating A_V to Input/Output Common Mode
Voltages
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