LMH6881 Datasheet, PDF (21/35 Pages) Texas Instruments – Small Signal Bandwidth: 2400 MHz

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LMH6881
SNOSC72F – JUNE 2012 – REVISED FEBRUARY 2015
Application Information (continued)
8.1.3.1 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 44 shows a schematic of a second order Butterworth filter, and
Table 6 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.
R1
L1
AMP V_OUT -
C1
L5
ADC V_IN +
C2
L2
AMP V_OUT + _R2
ADC V_IN -
ADC V_CM
Figure 44. ADC Noise Filter Schematic
Table 6. Filter Component Values⁽¹⁾
CENTER
FREQUENCY
75 MHz
150 MHz
180 MHz
250 MHz
BANDWIDTH
40 MHz
60 MHz
75 MHz
100 MHz
R1, R2
90 Ω
90 Ω
90 Ω
90 Ω
L1, L2
390 nH
370 nH
300 nH
225 nH
C1, C2
10 pF
3 pF
2.7 pF
1.9 pF
C3
22 pF
19 pF
15 pF
11 pF
L5
220 nH
62 nH
54 nH
36 nH
(1) Resistor values are approximate, but have been reduced due to the internal 10 Ω of output resistance per pin.
R3, R4
100 Ω
100 Ω
100 Ω
100 Ω
8.1.3.2 AC Coupling to ADC
AC coupling is an effective method for interfacing to an ADC for many communications systems. In many
applications this will be the best choice. The LMH6881 evaluation board is configured for AC coupling as shipped
from the factory. Coupling with capacitors is usually the most cost-effective method. Transformers can provide
both AC coupling and impedance transformation as well as single-ended to differential conversion. One of the
key benefits to AC coupling is that each stage of the system can be biased to the ideal DC operating point. Many
systems operate with lower overall power dissipation when DC bias currents are eliminated between stages.
8.1.3.3 DC Coupling to ADC
The LMH6881 supports DC-coupled signals. In order to successfully implement a DC-coupled signal chain the
common-mode voltage requirements of every stage need to be met. This will require careful planning, and in
some cases there will be signal level, gain or termination compromises required to meet the requirements of
every part. Figure 45 and Figure 46 show a method using resistors to change the 2.5-V common mode of the
amplifier output to a common mode compatible for the input of a low-input-voltage ADC such as the
ADC12D1800RF. This DC level shift is achieved while maintaining an AC impedance match with the filter in
Figure 45, while in Figure 46 there is a small mismatch between the amplifier termination resistors and the ADC
input. Because there is no universal ADC input common mode and some ADCs have impedance controlled
input, each design will require a different resistor ratio. For high-speed data conversion systems it is very
important to keep the physical distance between the amplifier and the ADC electrically short. When connections
between the amplifier and the ADC are electrically short, termination mismatches are not critical.
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