4 Receiver Characteristics

The key receiver specifications are input voltage threshold, input common mode range, and input leakage current. The input threshold levels differentiate the two types of M-LVDS receivers. Type 1 receivers have threshold levels centered at 0V differential and provide higher noise margin than Type 2 receivers. Type 1 receivers are used in clock or data transmission applications that either require application specific external failsafe networks or don’t require failsafe provisions at all. Type 2 receivers have threshold levels shifted by +100 mV differential. The shift lowers noise margin but provides a known, low output state when a bus or a transmission line is undriven and having 0V differential bias. In addition to the failsafe, Wired-OR function is another possible application of Type 2 receivers. More information about the Wired-OR implementation using M-LVDS devices is given later in the note.

When compared to RS-485 and LVDS receivers, M-LVDS receivers have the tightest threshold levels. Figure 4-1 illustrates threshold levels and maximum recommended differential input amplitude levels of RS-485, LVDS, and both M-LVDS receivers.

Figure 4-1 M-LVDS Standard Defines Two Receiver Types

The M-LVDS receiver input common mode range of –1.4V to 3.8V makes M-LVDS a robust interface for connecting sub-systems that may have a potential difference between their ground references of ±1V. Given the fact that many M-LVDS drivers have a much tighter VOS specification than what the standard specifies, the unwanted potential difference between network nodes may be greater than ±1V. RS-485 receivers are suitable for even harsher environments. Their common mode range of −7V to 12V allows for ±7V of unwanted potential difference between the nodes. The LVDS receivers pose as the least robust receivers given their input common mode range of 0V to 2.4V (0 to V_DD is also very common), however, tight VOS specification of LVDS drivers allows a potential difference between driver and receiver circuit commons of ±1V.

As all M-LVDS, LVDS and RS-485 devices may be used in multipoint networks, the receivers have to pose as light loads to the active drivers so that many of them can be connected on a single bus. Based on this requirement, all three standards specify maximum input leakage current which allows up to 32 loads (receivers or inactive drivers) on a bus. A load of a single device or one unit load for M-LVDS and LVDS receivers is equivalent to a 120 kΩ resistor into a 0V to 2.4V voltage source. One unit load for an RS-485 receiver is equivalent to a 12 kΩ resistor into a 0V to 5V source. In addition, the RS-485 devices are available in 1/2, 1/4 and 1/8 unit loads allowing up to 256 devices on a bus. Table 4-1 shows key RS-485, M-LVDS and LVDS receiver characteristics.