DPECL White Paper
The Benefits of PECL Technology
PECL differential logic output technology provides many advantages over more traditional logic systems, such as CMOS devices, in high speed systems.
Compared to CMOS, PECL offers the following benefits:
- Lower system jitter is produced by the lower slew rate of PECL. This technology has a smaller characteristic transition region compared to CMOS. PECL produces fast rise and fall times, which are important for accurate clocking. Since capacitively coupled noise currents are I=C dV/dt, CMOS (2V/ns) will produce six times the noise of PECL (0.34V/ns) if compared single ended. If PECL is used differentially, the advantage is even greater.
- PECL technology eliminates common mode noise (emissions) by offering differential inputs and outputs, not available in CMOS.
- Unlike CMOS, PECL further minimizes noise by using an emitter follower output stage which does not generate a large current spike when switching states. The power source and ground stay relatively noise free.
- PECL output devices deliver greater stability and reduced skew between outputs because they inherently have very little difference between Tplh and Tphl delays. (Clock driver signals need to be output simultaneously; delays produce skew and cause signal integrity problems).
- PECL technology is a better choice for driving transmission lines, due to its low impedance outputs (typically around 6 to 25 ohms) and high impedance inputs (typically 75 kohms). Its low impedance outputs are structured as open emitter followers, allowing for the maximum flexibility to terminate the interconnecting scheme (coax, twisted pair, PCB traces) appropriately to minimize reflections.
- PECL can drive 50 ohm transmission lines directly.
- PECL offers low power supply consumption at high frequencies (see below). The power consumption stays constant with frequency. By contrast, CMOS power consumption starts low at low frequencies, but steadily rises as the frequency rises. CMOS power consumption is equal to PECL power consumption at 65 MHz, after which it continues to rise sharply.
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