HOTLink DX PECL Output to LVTTL Input

Question: - How do I connect the differential PECL outputs of the CY7C924ADX or CY7C9689A to a single-ended LVTTL input?- Is it possible to use a LVTTL optical module?- How do I make a PECL to LVTTL translation?

 

Answer:

An LVTTL input requires a larger voltage swing than what is provided by a single PECL output of the CY7C924ADX or CY7C9689A.  To make the differential to single-ended transition, we typically use a transformer.  By providing both outputs to the input of a transformer, we create a single output that has twice the swing of a single PECL output.

The CY7C924ADX PECL signals have a 600-1100mV swing with voltage levels between 3 and 4 volts. LVTTL inputs usually require VIH = 2V and VIL = 0.8V.  This translates to a swing of at least 1.2V centered on 1.4V.

In order to meet that requirement, we can use the transformer to create the 1.2V swing.  Then we need to bias the single output to 1.4V. Please see attached diagram for an example.

Useful Links:
For more information regarding transformers and how they're used, please refer to the following Knowledge Base articles:

Transformer recommendations for HOTLink II
Do I need a transformer if I want to use a single-ended serial interface?