The Q(inj) introduces a step change in output voltage when switching. The change in output voltage is a function of the amount of Q(inj) and the load capacitance, or DeltaV = Q(inj)/Cload. This leads to DC errors in system, and varies with input signal/source voltage. Q(inj) generally varies from negative to positive as the input is swept from Vss or 0V to Vdd.