When driving a direct-sampling high-speed ADC, the interface between the final amplifier and the ADC is often the most critical area where performance can degrade. This is because direct sampling ADCs generate nonlinear charge during the sampling process. This charge gets reflected back into the input network each time the sampling switch closes. If not properly managed, it can cause distortion or intermodulation distortion in the ADC.
To minimize this issue, the ADC's input network should be as close to 50 Ω as possible, allowing the nonlinear charge to be absorbed efficiently. A highly absorptive filter plays a key role here by suppressing these nonlinear components, which improves the ADC’s Spurious-Free Dynamic Range (SFDR).
In this application, the LTC6409 amplifier is used to drive the AD9265 ADC. The LTC6409 is a differential amplifier with excellent linearity, making it an ideal choice for driving high-performance ADCs like the AD9265. The AD9265 is a 16-bit, 125 Msps ADC that delivers an SNR better than 77 dB at 100 MHz and an SFDR better than 89 dB.
However, even with such high-performance components, improper design of the input network can lead to suboptimal results. A filter is typically required between the amplifier and the ADC to reduce broadband noise and suppress nonlinearities introduced during sampling. The design and layout of this filter are crucial—ideally, it should be an absorption-type filter that dissipates high-frequency nonlinear components into a 50 Ω termination resistor, preventing them from being reflected back to the ADC.
Figure 1 shows an absorption filter network designed for use between the LTC6409 and the AD9265. As shown in Figure 2, this filter doesn’t need to have high selectivity. Its main purpose is to attenuate broadband noise and nonlinear components generated during sampling. At higher frequencies, the inductor acts as an open circuit while the capacitor becomes a short, directing the high-frequency components into the 50 Ω resistor. This prevents any loop reflections that could otherwise reduce the ADC’s SFDR.
Another common source of distortion is an asymmetrical input network layout. In an ideal setup, the differential signal provides strong common-mode rejection and low second-harmonic distortion. However, any deviation from symmetry can introduce mismatches, leading to increased second-harmonic distortion. Even simple design choices, like uneven copper fill near one side of the differential signal, can create imbalances in ground currents, increasing overall system distortion.
To ensure optimal performance, absolute symmetry must be maintained in both the layout and component placement. Figure 3 illustrates the PCB layout for the LTC6409 driving the AD9265 along with the absorption filter. The layout includes measures to preserve symmetry and position absorbing components optimally. The first set of absorbing elements immediately captures high-frequency products, while the signal path is routed around grounded copper until reaching the second set of absorbers, finally terminating at a 50 Ω resistor on the amplifier side. This configuration maximizes the performance of the LTC6409 and AD9265.
To evaluate the performance difference between the absorption and reflection filters, a test board was designed using the DC890 interface with PScope software. Both the absorption filter from Figure 1 and a reflection filter from Figure 4 were tested across various frequencies.
The AD9265 was driven with a filtered sine wave from 48.1 MHz to 178.1 MHz at 125 Msps. SNR and SFDR were measured using PScope, and the collected data is shown in Figure 5. The results clearly show that the absorption filter outperforms the reflection filter in both SNR and SFDR. Specifically, the SFDR improvement is over 10 dB at certain points, and the SNR remains consistently better across all tested frequencies.
Figures 6 and 7 compare the SNR and SFDR performance of both filter types. The absorption network consistently delivers superior results, especially at higher frequencies where other factors may begin to dominate. Systems using absorption networks perform significantly better than those using reflective networks.
This study highlights the importance of using a highly absorptive and symmetric input network when driving high-speed ADCs. While the results are specific to the LTC6409 and AD9265, the principles apply broadly to any direct-sampling ADC and differential amplifier. Optimizing the interface between the amplifier and ADC is essential for achieving the best possible performance.
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