lmh6551qmmx National Semiconductor Corporation, lmh6551qmmx Datasheet - Page 14

• Manufacturer Part Number: lmh6551qmmx
• Description: Differential, High Speed Op Amp
• Manufacturer: National Semiconductor Corporation

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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 ca-
pacitor or sample and hold circuits.
circuit for driving an ADC. The two 56Ω resistors serve to iso-
late 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 reduc-
tion 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
frequency of the filter is 1/ (2*
53MHz (which is slightly less than the sampling frequency).
Note that the ADC input capacitance must be factored into the
frequency response of the input filter, and that being a differ-
ential input the effective input capacitance is double. Also as
shown in
variable based on the clock cycle. See the data sheet for your
particular ADC for details.
The amplifier and ADC should be located as closely together
as possible. Both devices require that the filter components
Figure 7
FIGURE 7. Driving an ADC
the input capacitance to many ADCs is
π
*56Ω *(39 pF + 14pF)) =
Figure 7
Figure 7
shows a typical
the cutoff
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be in close proximity to them. The amplifier needs to have
minimal parasitic loading on the output traces and the ADC is
sensitive to high frequency noise that may couple in on its
input lines. Some high performance ADCs have an input
stage that has a bandwidth of several times its sample rate.
The sampling process results in all input signals presented to
the input stage mixing down into the Nyquist range (DC to Fs/
2). See AN-236 for more details on the subsampling process
and the requirements this imposes on the filtering necessary
in your system.
USING TRANSFORMERS
Transformers are useful for impedance transformation as well
as for single to differential, and differential to single ended
conversion. A transformer can be used to step up the output
voltage of the amplifier to drive very high impedance loads as
shown in
the output voltage is stepped down to drive a low impedance
load.
Transformers have limitations that must be considered before
choosing to use one. Compared to a differential amplifier, the
most serious limitations of a transformer are the inability to
pass DC and balance error (which causes distortion and gain
errors). For most applications the LMH6551Q will have ade-
quate output swing and drive current and a transformer will
not be desirable. Transformers are used primarily to interface
differential circuits to 50Ω single ended test equipment to
simplify diagnostic testing.
FIGURE 8. Transformer Out High Impedance Load
Figure
8.
Figure 10
shows the opposite case where
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