Datasheet

LMH6881
ROUT
ROUT
OCM
x2
50:
+1.25V
VCM = 2.5V
N/C
N/C
+2.5V
LPF
INPS
INMS
INPD
INMD
VCM =1.5V
ADC
Parallel termination = 2* RT || RL = 150 || 300 =
100:
RT
RT
RL
VCM voltage divider = 2.5V * RT/(ROUT + RT) =
2.5 * 75/125 = 1.5 V
50:
75:
75:
50:
50:
R
IN
= 50:
300:
LMH6881
SNOSC72E –JUNE 2012–REVISED MARCH 2013 www.ti.com
Table 1. 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
R3, R4 100 Ω 100 Ω 100 Ω 100 Ω
(1) Resistor values are approximate, but have been reduced due to the internal 10 Ω of output resistance per pin.
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.
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. Shown in Figure 43 and Figure 44 is 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 43, while in Figure 44 there is a small mismatch between the amplifier termination resistors and the ADC
input. Since there is no universal ADC input common mode and some ADC’s 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.
Figure 43. DC-Coupled ADC Driver Example 1, High Input Impedance ADC
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