LMH6550_06 Datasheet, PDF (13/17 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

Application Section (Continued)
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FIGURE 7. AC Coupled for Single Supply Operation
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 56Ω 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.
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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.
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 50Ω single ended test
equipment to simplify diagnostic testing.
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