LMH6612MA Datasheet, PDF (23/32 Pages) Texas Instruments – Single Supply 345 MHz Rail-to-Rail Output Amplifiers

Because SINAD compares all undesired frequency compo-
nents with the input frequency, it is an overall measure of an
ADC’s dynamic performance. The following sections will dis-
cuss the three different ADC driver architectures in detail.
SINGLE TO SINGLE ADC DRIVER
This architecture has a single-ended input source connected
to the input of the op amp and the single-ended output of the
op amp is then fed to the single-ended input of the ADC. The
low noise of only 10 nV/ and a wide bandwidth of 345 MHz
make the LMH6611 an excellent choice for driving the 12-bit
ADC121S101 500 KSPS to 1 MSPS ADC, which has a suc-
cessive approximation architecture with internal sample and
hold circuits. Figure 2 shows the schematic of the LMH6611
in a 2nd order multiple-feedback with gain of −1 (inverting)
configuration, driving an ADC121S101. The inverting config-
uration is preferred over the non-inverting configuration, as it
offers more linear output response. Table 1 shows the per-
formance data of the LMH6611 combined with the AD-
C121S101. The ADC driver’s cutoff frequency of 500 kHz is
found from the equation:
The op amp’s gain is set by the equation:
FIGURE 4. Single to Single ADC Driver
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TABLE 1. Performance of the LMH6611 Combined with the ADC121S101
Amplifier
Output/ADC Input
4
SINAD
(dB)
70.2
SNR
(dB)
71.6
THD
(dB)
−75.7
SFDR
(dBc)
77.6
ENOB
11.4
Notes
ADC121S101 @ f = 200 kHz
When the op amp and the ADC are using the same supply, it
is important that both devices are well bypassed. A 0.1 µF
ceramic capacitor and a 10 µF tantalum capacitor should be
located as close as possible to each supply pin. A sample
layout is shown in Figure 5. The 0.1 µF capacitors (C13 and
C6) and the 10 µF capacitors (C11 and C5) are located very
close to the supply pins of the LMH6611 and the AD-
C121S101.
The following are recommendations for the design of PCB
layout in order to obtain the optimum high frequency perfor-
mance:
• Place ADC and amplifier as close together as possible.
• Put the supply bypassing capacitors as close as possible
to the device (<1”).
• Utilize surface mount instead of through-hole components
and ground and power planes.
• Keep the traces short where possible.
• Use terminated transmission lines for long traces.
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