The differential voltage of a differential signal can be described by two different definitions causing confusion when reading data sheets or communicating with other engineers. This section addresses the measurement and description of a differential signal so that the reader is able to understand and discern between the two different definitions when used.

The first definition used to describe a differential signal is the absolute value of the voltage potential between the inverting and non-inverting signal. The symbol for this first measurement is typically V_ID or V_OD depending on if an input or output voltage is being described.

The second definition used to describe a differential signal is to measure the potential of the non-inverting signal with respect to the inverting signal. The symbol for this second measurement is V_SS and is a calculated parameter. Nowhere in the IC does this signal exist with respect to ground, this signal only exists in reference to the differential pair. V_SS can be measured directly by oscilloscopes with floating references, otherwise this value can be calculated as twice the value of V_OD as described in the first description.

Figure 7-8 illustrates the two different definitions side-by-side for inputs and Figure 7-9 illustrates the two different definitions side-by-side for outputs. The V_ID (or V_OD) definition show the DC levels, V_IH and V_OL (or V_OH and V_OL), that the non-inverting and inverting signals toggle between with respect to ground. V_SS input and output definitions show that if the inverting signal is considered the voltage potential reference, the non-inverting signal voltage potential is now increasing and decreasing above and below the non-inverting reference. Thus the peak-to-peak voltage of the differential signal can be measured.

V_ID and V_OD are often defined as volts (V) and V_SS is often defined as volts peak-to-peak (V_PP).

Figure 7-8 Two Different Definitions for Differential Input Signals

Figure 7-9 Two Different Definitions for Differential Output Signals

See also the AN-912 Common Data Transmission Parameters and their Definitions application note.