LVDS Podlet Usage Hints

The Differential LVDS/TTL podlet is useful to convert differential signals to single-ended signals for a logic analyzer or scope. Input signals should not exceed 3.3 volts or go below ground unless the podlet is "modified" as described in the termination section below.

If the podlet is powered up, there is a faint green light visible through the front (where the leadset is connected). The podlet also has reverse power connection protection via a parallel reverse diode and polymer fuse (self-resetting). In systems where the power supply cannot supply more than 1.5 amps continuous current to the podlet, the reverse protection will pull the supply down.

Terminating the Podlet

The podlet provides for several different types of termination method. We invite the user of the podlet to open the unit and adjust the termination method as is appropriate for their needs. The resistor type is the common 0603 package. In addition, each signal input has the possibility of a series resistor and has a set of reverse bias diodes connected between power and ground to help protect the podlet from overshoots, undershoots and over and undervoltage. The podlet is shipped from the factory with 33 ohms connected in series with each input signal but without any differential termination. It is also shipped with some 100 ohm resistors used for LVDS differential termination as well as some 0 ohm resistors for shorting out the series 33 ohm protection resistors.
Schematics and board layouts are available on this website for download.
Some termination possibilities are outlined below.

The most common termination method is no termination on the podlet. This works well in low speed (read slow risetime) systems that can tolerate some stub. However, there are many applications where this in not acceptable.

Another method is to remove the differential termination in the system under test, attach the leadset, and add the differential termination in the podlet. In LVDS systems, this would be a 100 ohm resistor. Also add the 0 ohm resistors to short out the series protection resistors.

In differential TTL systems where the logic swing is large, 100 ohm termination may be too low and input voltages may be higher than 3.3 volts. In this case, we recommend that the 33 ohm series protection resistors be left in place and the appropriate additional differential termination resistor be installed. For example, a particular diff TTL system is designed for a differential termination of 250 ohms and each signal swings standard TTL of 4.3 volts. Since we already have 66 ohms in place , we need a differential resistor of about 180 ohms. Further, note that with both signals connected, the input voltage of the podlet is not exceeded since a voltage divider is now in place. However, with only one input connected, the input voltage is exceed and the diode protection kicks in dropping the voltage through the 33 ohm protection resistor.

Last updated January 30, 2001