LMH6551 Datasheet, PDF (14/19 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

Application Section (Continued)
SINGLE ENDED INPUT TO DIFFERENTIAL OUTPUT
The LMH6551 provides excellent performance as an active
balun transformer. Figure 3 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_CMpin as in fully differential mode. In
this mode the common mode feedback circuit must also,
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 the amplifier as a
transformer. The common mode feedback circuit is respon-
sible 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 when the amplifier is
driven with a differential signal is nearly unmeasurable if the
resistors and board are well matched. Balance error can be
caused by either a channel to channel gain error, or phase
error. Either condition will produce 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_CMpin bypassing is also critical in this mode of
operation. See the above section on FULLY DIFFERENTIAL
OPERATION for bypassing recommendations.
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 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 7. AC Coupled for Single Supply Operation
DRIVING ANALOG TO DIGITAL CONVERTERS
Analog to digital converters (ADC) present challenging load
conditions. They typically have high impedance inputs with
large and often variable capacitive components. As well,
there are usually current spikes associated with switched
capacitor or sample and hold circuits. Figure 8 shows a
typical circuit for driving an ADC. The two 56Ω resistors
serve to isolate the capacitive loading of the ADC from the
amplifier and ensure stability. In addition, the resistors form
part of a low pass filter which helps to provide anti alias and
noise reduction functions. The two 39 pF capacitors help to
smooth the current spikes associated with the internal
switching circuits of the ADC and also are a key component
in the low pass filtering of the ADC input. In the circuit of
Figure 8the cutoff frequency of the filter is 1/ (2*π*56Ω *(39
pF + 14pF)) = 53MHz (which is slightly less than the sam-
pling frequency). Note that the ADC input capacitance must
be factored into the frequency response of the input filter,
and that being a differential input the effective input capaci-
tance is double. Also as shown in Figure 8 the input capaci-
tance to many ADCs is variable based on the clock cycle.
See the data sheet for your particular ADC for details.
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FIGURE 6. Relating A_Vto Input/Output Common Mode
Voltages
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