LVDS Comparison: APEX 20KE vs. Virtex-E Devices
APEX 20KE devices offer a True-LVDS solution with data transfer rates up to 840 Mbps. Embedded dedicated circuitry enables the True-LVDS solution to deliver high performance and high-quality signals. Virtex-E devices have emulated the LVDS I/O interface using device resources such as configurable logic blocks (CLBs), block RAM, and on-board components such as external resistor packs. LVDS emulation in Virtex-E devices leads to degraded performance and poor signal quality.
Comparing APEX 20KE & Virtex-E LVDS Driver Quality
The quality of output drivers is typically measured using an "eye diagram," which is a superimposition of signal transitions over a period of time. Figures 1 and 2 below compare the outputs of APEX 20KE and Virtex-E LVDS drivers. The results were obtained in the lab using EP20K400EFC672-1X and XCV2000E-8BG560C devices, respectively. For details of how an eye diagram is formed, see the Formation of an Eye Diagram page.
| Figure 1. APEX 20KE LVDS Eye Diagram |
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| Figure 2. Virtex-E LVDS Eye Diagram |
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The APEX Advantage
The APEX True-LVDS solution provides the following advantages over the Virtex-E emulated LVDS solution:
- 5x longer data valid time
- 67% less jitter
A longer data-valid time and less jitter means APEX 20KE devices present a more robust and easy-to-use solution.
| Table 1. APEX 20KE True-LVDS vs. Virtex-E LVDS features |
| Feature |
APEX 20KE |
Virtex-E |
APEX Advantage |
| Maximum LVDS data rate |
840 Mbps |
622 Mbps |
Higher data transfer rate |
| Dedicated transmitter and receiver circuitry |
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High performance |
| On-chip (phase-locked loops) PLLs for 8x multiplication |
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Easy board design |
| True differential drivers |
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Superior LVDS interface |
| Dedicated power and ground pins |
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Limited ground bounce |
Conclusion
Altera APEX 20KE devices provide a superior LVDS solution compared to the Virtex-E devices. The eye diagrams show that the APEX 20KE LVDS drivers generate higher quality signals than the Virtex-E emulation. Dedicated serializer and deserializer circuitry, on-chip 8x PLL, and dedicated LVDS drivers make the APEX 20KE True-LVDS solution the most robust LVDS solution in programmable logic devices.
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