LMH6551Q Datasheet, PDF (15/20 Pages) Texas Instruments – Differential, High Speed Op Amp

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 Figure 9. Figure 11 shows the opposite case where
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. Driving an ADC
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The amplifier and ADC should be located as closely together
as possible. Both devices require that the filter components
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.
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FIGURE 9. Transformer Out High Impedance Load
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FIGURE 10. Calculating Transformer Circuit Net Gain
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