Methods and apparatus, including computer program products, to test analog to digital converters, are disclosed. In general, data is received that characterizes a first digital code from a device under test at a first analog voltage of an analog signal generator and a second digital code being a digital code threshold, and a step size is generated for another test of the device by performing a calculation by a processor. The calculation may include multiplying a least significant bit size of the device with a difference of the first and second digital codes to generate a product, and dividing the product by a least significant bit size of the analog signal generator. The first digital code may be calculated from results from multiple subtests in the test, where each of the subtests includes multiple analog to digital conversions by the device at the first analog voltage.