Datasheet
L2
L1
C1
C2
C3
L5
ADC Z
IN
R4
ADC V
IN
+
ADC V
IN
-
ADC V
CM
AMP V
OUT
-
AMP V
OUT
+
R3
R1
R2
0 100 200 300 400
12
14
16
18
20
22
24
POWER GAIN AT LOAD (dB)
LOAD IMPEDANCE ()
LMH6881
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SNOSC72E –JUNE 2012–REVISED MARCH 2013
Figure 41. Power Gain as a Function of the Load
Printed circuit board (PCB) design is critical to high-frequency performance. In order to ensure output stability the
load-matching resistors should be placed as close to the amplifier output pins as possible. This allows the
matching resistors to mask the board parasitics from the amplifier output circuit. An example of this is shown in
Figure 36. Also note that the low-pass filters in Figure 43 and Figure 44 use center-tapped capacitors. Having
capacitors to ground provides a path for high-frequency, common-mode energy to dissipate. This is equally
valuable for the ADC, so there are also capacitors to ground on the ADC side of the filter. The LMH6881EVAL
evaluation board is available to serve as a guide for system board layout. See also application note AN-2235 for
more details.
INTERFACING TO AN ADC
The LMH6881 is an excellent choice for driving high-speed ADCs such as the ADC12D1800RF,
ADC12D1600RF or the ADS5400. The following sections will detail several elements of ADC system design,
including noise filters, AC-, and DC-coupling options.
ADC NOISE FILTER
When connecting a broadband amplifier to an analog-to-digital converter, it is nearly always necessary to filter
the signal before sampling it with the ADC. Figure 42 shows a schematic of a second order Butterworth filter, and
Table 1 shows component values for some common IF frequencies. These filters offer a good compromise
between bandwidth, noise rejection and cost. This filter topology is the same as is used on the
ADC14V155KDRB High IF Receiver reference design board. This filter topology is adequate for reducing aliasing
of broadband noise and will also provide rejection of harmonic distortion and many of the images that are
commonly created by mixers.
Figure 42. ADC Noise Filter Schematic
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