LMH6550 datasheet(14/17 Pages) NSC

14 / 17 page

Application Section (Continued)

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.

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 LMH6550 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 50

Ω single ended test

equipment to simplify diagnostic testing.

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FIGURE 8. Driving an ADC

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FIGURE 9. Transformer Out High Impedance Load

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FIGURE 10. Calculating Transformer Circuit Net Gain

LMH6550 Datasheet(PDF) 14 Page - National Semiconductor (TI)