When driving a direct-sampling high-speed ADC, the most critical area that can affect performance is the interface between the final amplifier and the ADC. This is where signal integrity and linearity are most vulnerable. Direct sampling ADCs inherently generate nonlinear charge during the sampling process. This charge is injected into the input network each time the sampling switch closes. If not properly managed, this charge can reflect back to the ADC, causing distortion or intermodulation issues.
To minimize this effect, the ADC’s input network should be as close as possible to 50 Ω. This helps maximize the dissipation of nonlinear charge. Additionally, using a highly absorptive filter can suppress these nonlinear components, improving the overall SFDR (Spurious Free Dynamic Range) of the system.
Driving the AD9265 with the LTC6409 amplifier is an effective approach. The LTC6409 is a differential amplifier known for its excellent linearity, making it ideal for driving high-performance ADCs like the AD9265. The AD9265 itself is a 16-bit, 125 Msps ADC with a SNR better than 77 dB at 100 MHz and an SFDR exceeding 89 dB.
However, improper design of the input network can compromise these performance metrics. A filter is typically required between the amplifier and the ADC to reduce broadband noise and nonlinearities from the sampling process. This filter must be designed carefully—ideally, it should be an absorptive type that terminates high-frequency nonlinear components in a 50 Ω resistor without allowing reflections back to the ADC.
Figure 1 shows an absorption filter network that can be used between the LTC6409 and the AD9265. The filter's purpose is not to provide sharp selectivity but to attenuate broadband noise and nonlinear artifacts generated during sampling. At high frequencies, the inductor acts as an open circuit while the capacitor shorts out, directing the high-frequency components into the 50 Ω termination. This prevents unwanted reflections that could otherwise degrade the ADC's performance.
Figure 2 illustrates the simulated response of the filter. It confirms that the design effectively absorbs high-frequency energy without introducing additional distortion.
Another potential source of distortion is an asymmetrical input layout. In an ideal setup, the differential nature of the signal provides strong common-mode rejection and low second-harmonic distortion. Any deviation from symmetry can lead to mismatches, resulting in increased second-harmonic distortion. Even small design choices, such as uneven copper filling near one side of a differential pair, can cause ground current imbalances, which increase system distortion.
For optimal performance, absolute symmetry in layout is essential. Figure 3 shows a PCB layout of the LTC6409 driving the AD9265 along with the absorption filter. Measures were taken to maintain symmetry and optimize component placement. The first set of absorbing elements is positioned to immediately capture any high-frequency products, while the main signal path is routed around grounded copper until it reaches the second set of absorbing elements, finally terminating at a 50 Ω resistor on the amplifier side. This configuration maximizes the performance of both the LTC6409 and the AD9265.
To evaluate the performance difference between an absorptive and a reflective filter, a test board was designed using the DC890 interface with PScope software. Both the absorptive filter from Figure 1 and a reflective filter from Figure 4 were tested across a range of frequencies.
The AD9265 was driven with a filtered sinusoidal signal from 48.1 MHz to 178.1 MHz at 125 Msps. SNR and SFDR were measured using PScope, and the results are shown in Figure 5. The data collected at 58.1 MHz demonstrates the effectiveness of the absorptive filter.
Figures 6 and 7 show the SNR and SFDR comparisons between the two filter types. The absorptive filter consistently outperforms the reflective one, with SFDR improvements of over 10 dB in some cases. The SNR is also significantly better, especially at higher frequencies where other factors begin to dominate.
In conclusion, systems using an absorptive network perform better than those using a reflective one. When using a reflective network, even high-performance components like the LTC6409 and AD9265 may not reach their full potential. These findings highlight the importance of proper input network design when working with direct-sampling ADCs and differential amplifiers. A well-designed, symmetrical, and absorptive input network is key to achieving the best possible performance.
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