Design Ideas: March 2, 1995

The circuit in Fig 1 is one way to link ECL to positive ECL (PECL). So far, no dedicated level-shifting ICs are available for these purposes.

Because the circuits in Fig 1 are ac-coupled, you cannot use them for discontinuous data. If the inputs go quiet or are pulled high or low, the devices' differential inputs float, possibly resulting in self-oscillation.

Fig 2a shows an alternative to ac coupling for ECL-to-PECL-level translation: here, an ECL MNC10116 or an MC10H116 line receiver with +5V applied to its V_CC pin and -5.2V applied to its V_EE pin. Although these devices are not specified for these voltages, testing shows the devices to be reliable when operated at these voltages.

The V_BB output of the 116 is referred to V_CC; therefore, the output is at a PECL level (V_CC1.3V), which is not usable for standard ECL inputs. Consequently, you can use the 116 only for applications in which the ECL inputs are differential and not single-ended. This configuration allows an ECL-to-PECL translation at the same speed as a standard 116, although the power consumption is slightly higher. Successful translations can occur at frequencies up to 250 MHz.

Here, the V_CC and V_EE rails are at +5V and -5.2V, respectively. The PECL differential levels connect to the differential comparator's inputs, and the latch-control is high. The Q and Q outputs provide a differential ECL translation of the PECL inputs.