THS770006_10 Datasheet, PDF (21/36 Pages) Texas Instruments – Broadband, Fully-Differential, 14-/16-Bit ADC DRIVER AMPLIFIER

THS770006
www.ti.com
SBOS520 – JULY 2010
Common-mode phase shift introduced by the filter nullifies the basic assumption that the spur sources are in
phase. This phase shift can lead to better performance than predicted as the spurs become phase shifted, and
there is the potential for cancellation as the phase shift reaches 180°.
Differential phase shift in the filter as a result of mismatched components caused by nominal tolerance can
severely degrade the second-order distortion of the ADC. Single-order RC filters cause very little differential
phase shift with nominal tolerances of 5% or less, but higher-order LC filters are very sensitive to component
mismatch. For instance, a third-order Butterworth bandpass filter with 100MHz center frequency and 20MHz
bandwidth shows up to 20° differential phase imbalance in a Spice Monte Carlo analysis with 2% component
tolerances. Therefore, while a prototype may work, production variance is unacceptable. A transformer or balun
is recommended at the ADC input in these applications to restore the phase balance in the input signal to the
ADC.
ADC Input Common-Mode Voltage Considerations
The input common-mode voltage range of the ADC must be observed 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 use internal bias networks and others use external
components, such as resistors, from each input to the CM output of the ADC. When ac coupling, the output
common-mode voltage of the amplifier is a don’t care for the ADC, and V_OCM should be set for optimum
performance of the amplifier.
DC-coupled applications vary in complexity and requirements, depending on 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, and still others like the ADS6149 require 1.5V and the ADS4149 require 0.95V. Given
the THS770006 output common-mode range, ADCs with input common-mode closer to 2.5V are easier to
dc-couple to, and require little or no level shifting. For applications that require a different common-mode voltage
between the amplifier and the ADC, a resistor network can be used, as shown in Figure 36. With ADCs that have
internal resistors (R_INT) that bias the ADC input to V_CM, the bias resistors do not affect the desired value of R_P,
but do cause more attenuation of the differential input signal. Knowing the differential input resistance is required
and sometimes, that is all that is provided.
V_REF
V_AMP+
R_O
R_P
V_ADC+
R_INT
ADC
Amp
V_CM
V_AMP-
R_O
R_P
V_ADC-
R_INT
V_REF
Figure 36. Resistor Network to DC Level Shift Common-Mode Voltage
For common-mode analysis, assume that V_AMP± = V_OCM and V_ADC± = V_ADC (the specification for the ADC input
common-mode voltage). V_REF is chosen to be a voltage within the system (such as the ADC or amplifier analog
supply) or ground, depending on whether the voltage must be pulled up or down, and R_O is chosen to be a
reasonable value, such as 49.9Ω. With these known values, R_P can be found by using Equation 1:
1=
1
V_AMP - V_{ADC +} V_CM - V_ADC
R_P ^V_ADC ^{- V}_REF
R_O
R_INT
(1)
The insertion of this resistor network also attenuates the amplifier output signal. The gain (or loss) can be
calculated by Equation 2:
GAIN =
R_P || R_INT
R_O + (R_P || R_INT)
(2)
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