LMH6551 Datasheet, PDF (15/19 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

Application Section (Continued)
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 be-
fore choosing to use one. Compared to a differential ampli-
fier, the most serious limitations of a transformer are the
inability to pass DC and balance error (which causes distor-
tion and gain errors). For most applications the LMH6551 will
have adequate output swing and drive current and a trans-
former will not be desirable. Transformers are used primarily
to interface differential circuits to 50Ω single ended test
equipment to simplify diagnostic testing.
20133205
FIGURE 8. Driving an ADC
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.
20133207
FIGURE 9. Transformer Out High Impedance Load
20133232
FIGURE 10. Calculating Transformer Circuit Net Gain
15
www.national.com