The input requirement for a differential interface is different from the requirement for a single ended interface. In a single ended interface such as LVCMOS, LVTTL, single-ended HSTL or single-ended PECL/ECL, the VOH and VOL must meet the VIH and VIL requirements of the receivers. In the differential interface, such as LVPECL, HSTL, HCSL, CML, LVDS, or SSTL, the VIH and VIL are no longer referenced. The input parameters VPP and VCMR are now the requirements. Only VSWING and the VOH of the incoming signal are required to meet both VPP and VCMR.
VPP < VSWING < VPP_MAX
VCMR_MIN < VOH < VCMR_MAX
VPP = Input peak-to-peak voltage requirement
VCMR = Input Common Mode Range Voltage requirement
VSWING = Driver output swing
VOH = Driver output logic high
The figure below shows the relationship between VSWING, VOH and VPP, VCMR. The following conditions must be met for a valid input signal: VSWING must be within the required range of the specified VPP and the VOH must be within the VCMR range. Refer to application note AN-835 for more details. For other questions not addressed by the Knowledge Base, please submit a technical support request.