Patent ID: 8837573

Claim:
An interface circuit for compensating for gain changes of an N-level pulse amplitude modulation (PAM-N) modulated signal, N being an integer, comprising: a number of N−1 comparators for comparing the PAM-N modulated signal to N−1 configurable thresholds, wherein the PAM-N modulated signal was also equalized and the N−1 configurable thresholds are N−1 different voltage levels; a compensation comparator for tracking gain changes in the input PAM-N modulated signal by comparing the input PAM-N modulated signal to a compensation threshold; and a compensation circuit for changing at least one of the N−1 configurable thresholds of the N−1 comparators based on an output of the compensation comparator, thereby offsetting a crossing point of at least one comparator of the N−1 comparators respective of the at least one of the N−1 configurable thresholds to compensate for gain changes in the input PAM-N modulated signal, wherein the compensation circuit further comprises: an accumulator for accumulating a number of crossings and non-crossings of the compensation threshold over time as output by the compensation comparator; a controller for performing at least a process for changing the at least one of the N−1 configurable thresholds of the N−1 comparators based on the output of the compensation comparator; and a voltage setting unit for setting the compensation comparator and the at least one of the N−1 configurable thresholds based on a control signal generated by the controller, wherein the controller, when performing the at least process, is configured to: repetitively change the at least one of the N−1 configurable thresholds of the N−1 comparators based on crossings of a single PAM-N level of the compensation threshold and a fixed factor; and wherein the at least process further comprises: set an initial compensation factor (ICF); set the compensation threshold to an initial value; detect, by the compensation comparator, crossings around a first PAM-N level out of N PAM-N modulation levels; iteratively change a voltage level of the compensation threshold until an accumulated number of crossings and non-crossings of the compensation threshold respective of the first PAM-N level over time is equal; save a voltage level of the compensation threshold that achieves an equal accumulated number of crossings and non-crossings as a current compensation threshold value; and set the at least one of the N−1 configurable thresholds to a difference of the current compensation threshold and a previous value of the compensation threshold, multiplied by the ICF.