THS770006_14 Datasheet, PDF (21/40 Pages) Texas Instruments – Broadband, Fully-Differential, 14-/16-Bit ADC DRIVER AMPLIFIER

THS770006
www.ti.com
SBOS520B – JULY 2010 – REVISED JANUARY 2012
This formula shows that if the SNR of the amplifier and filter equals the SNR of the ADC, the combined SNR is
3dB lower (worse). Thus, for minimal impact (< 1dB) on the ADC SNR, the SNR of the amplifier and filter should
be ≥ 10dB greater than the ADC SNR. The combined SNR calculated in this manner is usually accurate to within
±1dB of actual implementation.
SFDR Considerations
The SFDR of the amplifier is usually set by second-order or third-order harmonic distortion for single-tone inputs,
and by second-order or third-order intermodulation distortion for two-tone inputs. Harmonics and second-order
intermodulation distortion can be filtered to some degree, but third-order intermodulation spurs cannot be filtered.
The ADC generates the same distortion products as the amplifier, but as a result of the sampling and clock
feedthrough, additional spurs (not linearly related to the input signal) are included.
When the spurs from the amplifier and filter are known, each individual spur can be directly added to the same
spur from the ADC, as shown in Equation 3, to estimate the combined spur (spur amplitudes in dBc):
-HDx_AMP+FILTER
-HDx_ADC
× HDx_SYSTEM = -20 log 10 ²⁰
+ 10 ²⁰
(3)
This calculation assumes the spurs are in phase, but usually provides a good estimate of the final combined
distortion.
For example, if the spur of the amplifier and filter equals the spur of the ADC, then the combined spur is 6dB
higher. To minimize the amplifier contribution (< 1dB) to the overall system distortion, it is important that the spur
from the amplifier and filter be ~15dB better than the converter. The combined spur calculated in this manner is
usually accurate to within ±6dB of actual implementation; however, higher variations have been observed as a
result of phase shift in the filter, especially in second-order harmonic performance.
This worst-case spur calculation assumes that the amplifier/filter spur of interest is in phase with the
corresponding spur in the ADC, such that the two spur amplitudes can be added linearly. There are two
phase-shift mechanisms that cause the measured distortion performance of the amplifier-ADC chain to deviate
from the expected performance calculated using Equation 3: common-mode phase shift and differential phase
shift.
Common-mode phase shift is the phase shift seen 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 as the spurs become phase
shifted, and there is the potential for cancellation as the phase shift reaches 180°. However, there is a significant
challenge in designing an amplifier-ADC interface circuit to take advantage of common-mode phase shift for
cancellation: the phase characteristic 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 as a result of mismatched components caused by nominal
tolerance can severely degrade the even-order 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 response over frequency through the two sides of a differential signal path are
identical, such that even-order 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-order 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
bandpass filter with 100MHz center frequency and 20MHz bandwidth shows up to a 20° differential phase
imbalance in a Spice Monte Carlo analysis with 2% component tolerances. Therefore, while a prototype may
work, production variance is unacceptable. In ac-coupled applications that require second- and higher-order
filters between the THS770006 and ADC, a transformer or balun is recommended at the ADC input to restore the
phase balance. For dc-coupled applications where a transformer or balun at the ADC input cannot be used, it is
recommended to use first- or second-order filters to minimize the effect of differential phase shift because of
component tolerance.
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