A method of background calibration of aperture center errors in a data communication system is provided. In an implementation, in response to detection of a low sampler output (“0”) in between two high sampler outputs (“1”), the method includes: determining a direction of an ADC output signal at the time of the detected low output; and adjusting timing at a selected sampler based on the determined signal direction. In an example implementation, the method includes watching for bubbles in the thermometer code output, and estimating the first derivative of the signal at the time of the bubble, then estimating the sign of the errors. In an example implementation, the errors are used in a control loop to reduce the aperture center error.