Provided are an analog-to-digital (AD) converter, a sensor system, and a test system capable of reducing the time for test processing. AD converter includes input part, AD conversion part, first output part, and second output part. The analog signal output from sensor is input to input part. AD conversion part digitally converts an analog signal to generate first digital data and second digital data. First output part outputs the first digital data to control circuit. Second output part outputs the second digital data to test controller before first output part outputs the first digital data. In the test mode, test controller determines whether or not sensor system including sensor is in an abnormal state on the basis of the second digital data.