LMH3401 Datasheet, PDF (33/48 Pages) Texas Instruments – LMH3401 7-GHz, Ultra-Wideband, Fixed-Gain, Fully-Differential Amplifier

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LMH3401
SBOS695A – AUGUST 2014 – REVISED DECEMBER 2014
10.2.2.3.3 ADC Input Common-Mode Voltage Considerations—AC-Coupled Input
The input common-mode voltage range of the ADC must be respected for proper operation. In an ac-coupled
application between the amplifier and the ADC, the input common-mode voltage bias of the ADC is
accomplished in different ways depending on the ADC. Some ADCs use internal bias networks such that the
analog inputs are automatically biased to the required input common-mode voltage if the inputs are ac-coupled
with capacitors (or if the filter between the amplifier and ADC is a band-pass filter). Other ADCs supply their
required input common-mode voltage from a reference voltage output pin (often called CM or V_CM). With these
ADCs, the ac-coupled input signal can be re-biased to the input common-mode voltage by connecting resistors
from each input to the CM output of the ADC, as Figure 63 shows. However, the signal is attenuated because of
the voltage divider created by R_CM and R_O.
R_O
Amp
R_O
R_CM
R_CM
A_IN+
A_IN-
ADC
CM
Figure 63. Biasing AC-Coupled ADC Inputs Using the ADC CM Output
The signal can be re-biased when ac coupling; thus, the output common-mode voltage of the amplifier is a don’t
care for the ADC.
10.2.2.3.4 ADC Input Common-Mode Voltage Considerations—DC-Coupled Input
DC-coupled applications vary in complexity and requirements, depending on the ADC. One typical requirement is
resolving the mismatch between the common-mode voltage of the driving amplifier and the ADC. Devices such
as the ADS5424 require a nominal 2.4-V input common-mode, while other devices such as the ADS5485 require
a nominal 3.1-V input common-mode; still others such as the ADS6149 and the ADS4149 require 1.5 V and
0.95 V, respectively. As shown in Figure 64, a resistor network can be used to perform a common-mode level
shift. This resistor network consists of the amplifier series output resistors and pull-up or pull-down resistors to a
reference voltage. This resistor network introduces signal attenuation that may prevent the use of the full-scale
input range of the ADC. ADCs with an input common-mode closer to the typical 2.5-V LMH3401 output common-
mode are easier to dc-couple, and require little or no level shifting.
V_REF
V_AMP+
R_O
R_P
V_ADC+
ADC
Amp
R_IN
C_IN
V_AMP-
R_O
R_P
V_ADC-
V_REF
Figure 64. Resistor Network To DC Level-Shift Common-Mode Voltage
For common-mode analysis of the circuit in Figure 64, assume that V_AMP± = V_CM and V_ADC± = V_CM (the
specification for the ADC input common-mode voltage). V_REF is chosen to be a voltage within the system higher
than V_CM (such as the ADC or amplifier analog supply) or ground, depending on whether the voltage must be
pulled up or down, respectively; R_O is chosen to be a reasonable value, such as 24.9 Ω. With these known
values, R_P can be found by using Equation 11:
R_P = R_O
V_ADC - V_REF
V_AMP - V_ADC
(11)
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