LMH6401_16 Datasheet, PDF (33/48 Pages) Texas Instruments – LMH6401 DC to 4.5 GHz, Fully-Differential, Digital Variable-Gain Amplifier

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LMH6401
SBOS730A – APRIL 2015 – REVISED MAY 2015
Common-mode phase shift is the phase shift detected equally in both branches of the differential signal path
including the filter. Common-mode phase shift nullifies the basic assumption that the amplifier, filter, and ADC
spur sources are in phase. This phase shift can lead to better performance than predicted when the spurs
become phase shifted, and there is the potential for cancellation when the phase shift reaches 180°. However,
there is a significant challenge in designing an amplifier-ADC interface circuit to take advantage of a common-
mode phase shift for cancellation: the phase characteristics of the ADC spur sources are unknown, thus the
necessary phase shift in the filter and signal path for cancellation is also unknown.
Differential phase shift is the difference in the phase response between the two branches of the differential filter
signal path. Differential phase shift in the filter is a result of mismatched components caused by nominal
tolerances and can severely degrade the even harmonic distortion of the amplifier-ADC chain. This effect has the
same result as mismatched path lengths for the two differential traces, and causes more phase shift in one path
than the other. Ideally, the phase responses over frequency through the two sides of a differential signal path are
identical, such that even harmonics remain optimally out of phase and cancel when the signal is taken
differentially. However, if one side has more phase shift than the other, then the even harmonic cancellation is
not as effective.
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 band-pass
filter with a 100-MHz center frequency and a 20-MHz bandwidth shows as much as 20° of differential phase
imbalance in a SPICE Monte Carlo analysis with 2% component tolerances. Therefore, although a prototype may
work, production variance is unacceptable. For ac-coupled or dc-coupled applications where a transformer or
balun cannot be used, using first- or second-order filters is recommended to minimize the effect of differential
phase shift.
10.2.2.1.3 ADC Input Common-Mode Voltage Considerations—AC-Coupled Input
When interfacing to an ADC, the input common-mode voltage range of the ADC must be taken into account for
proper operation. In an ac-coupled application between the amplifier and the ADC, the input common-mode
voltage bias of the ADC can be accomplished in different ways. 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 termed 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 shown in Figure 67. AC coupling provides dc common-
mode isolation between the amplifier and the ADC; thus, the output common-mode voltage of the amplifier is a
don’t care for the ADC.
R_O
Amp
R_O
R_CM
R_CM
A_IN+
A_IN-
ADC
CM
Figure 67. Biasing AC-Coupled ADC Inputs Using the ADC CM Output
10.2.2.1.4 ADC Input Common-Mode Voltage Considerations—DC-Coupled Input
DC-coupled applications vary in complexity and requirements, depending on the ADC (a split supply for the CMV
is applicable). One typical requirement is resolving the mismatch between the common-mode voltage of the
driving amplifier and the ADC. Devices such as the ADC12J4000 require a nominal 1.23-V input common-mode,
whereas other devices such as the ADS54J60 require a nominal 2.1-V input common-mode. The simplest
approach when dc-coupling the LMH6401 with the input common-mode voltage of the ADC is to select the
supply voltages (VS+) and (VS–) such that the default output common-mode voltage being set is equal to the
input common-mode voltage of the ADC; see Figure 66. The default common-mode voltage being set can be
controlled externally using the VOCM pin. The device performance is optimal when the output common-mode
voltage is within ±0.5 V of mid-supply and degrades outside the range when the output swing approaches
clipping levels.
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