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 will address the measurement and description of a differential signal so that the reader will be 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 noninverting 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 noninverting 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; it only exists in reference to its 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 above.

Figure 7-1 illustrates the two different definitions side-by-side for inputs and Figure 7-2 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 noninverting 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 noninverting signal voltage potential is now increasing and decreasing above and below the noninverting 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 7-1 Two Different Definitions for Differential Input Signals

Figure 7-2 Two Different Definitions for Differential Output Signals

Refer to AN-912 Common Data Transmission Parameters and their Definitions (SNLA036) for more information.