LMH6551 Datasheet, PDF (19/32 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

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LMH6551
SNOSAK7D – FEBRUARY 2005 – REVISED JANUARY 2015
Typical Applications (continued)
9.2.4 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 31 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 31the cutoff frequency of the filter is 1/ (2*π*56 Ω *(39 pF + 14pF)) = 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 31 the input capacitance to many ADCs is variable based on the clock cycle. See the data sheet for your
particular ADC for details.
R_G1
= ^VI
R_G2
R_F1
56
+
V_CM
-
39 pF
39 pF
ADC12LO66
7 - 8 pF
56
R_F2
V_REF
1V LOW IMPEDANCE
VOLTAGE REFERENCE
Figure 31. 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.
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