A threshold detection circuit is provided for determining when a data input signal exhibits an amplitude which exceeds a first programmable threshold level, is less than a second programmable threshold level or between the first and second levels. Respective scaling circuits scale down the data input signal and a line voltage signal which is supplied to the circuit to generate a scaled data input signal and a scaled line voltage signal. The threshold detection circuit scales the first and second threshold levels by a predetermined factor. The scaled data input signal is compared with the scaled first and second threshold levels. In response to this comparison, the output of the circuit assumes a first logic state when the data input signal exceeds the first threshold level and assumes a second logic state when the data input signal exhibits an amplitude less than the first threshold level. The comparison is repeated for the second threshold level. A control processor correlates the threshold level with the appropriate logic states to identify the data signal magnitude with respect to the threshold levels.