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THS770006_14 Datasheet, PDF (22/40 Pages) Texas Instruments – Broadband, Fully-Differential, 14-/16-Bit ADC DRIVER AMPLIFIER
THS770006
SBOS520B – JULY 2010 – REVISED JANUARY 2012
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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 bandpass filter). Other ADCs supply their
required input common-mode voltage from a reference voltage output pin (often called CM or VCM). 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 37 illustrates. However, the signal is attenuated because
of the voltage divider created by RCM and RO.
RO
Amp
RO
RCM
RCM
AIN+
AIN-
ADC
CM
Figure 37. 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.
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 2.4V input common-mode, while others such as the ADS5485 require a
nominal 3.1V input common-mode; still others such as the ADS6149and the ADS4149 require 1.5V and 0.95V,
respectively. As shown in Figure 38, 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 2.5V THS770006 output common-mode are
easier to dc-couple, and require little or no level shifting.
VREF
VAMP+
RO
RP
VADC+
ADC
Amp
RIN
CIN
VAMP-
RO
RP
VADC-
VREF
Figure 38. Resistor Network to DC Level-Shift Common-Mode Voltage
For common-mode analysis of the circuit in Figure 38, assume that VAMP± = VOCM and VADC± = VCM (the
specification for the ADC input common-mode voltage). VREF is chosen to be a voltage within the system higher
than VCM (such as the ADC or amplifier analog supply) or ground, depending on whether the voltage must be
pulled up or down, respectively; RO is chosen to be a reasonable value, such as 24.9Ω. With these known
values, RP can be found by using Equation 4:
RP = RO
VADC - VREF
VAMP - VADC
(4)
22
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