LMH6550 datasheet(13/17 Pages) NSC

13 / 17 page

Application Section (Continued)

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 56

Ω 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 8 the cutoff frequency of the filter is 1/ (2*

π*56Ω *(39

pF + 14 pF)) = 53 MHz (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 differential input the effective input

capacitance is double. Also as shown in Figure 8 the input

capacitance to many ADCs is 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

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.

20130109

FIGURE 7. AC Coupled for Single Supply Operation

20130105

FIGURE 8. Driving an ADC

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