a test and measurement system including a device under test , an accessory , a controller and a test and measurement instrument . the accessory is connected to the device under test and includes a signal input to receive an input signal from the device under test , a compensation unit configured to apply a compensation signal internal to the accessory , and a signal output to output an output signal read from the device under test . the controller is connected to the accessory and includes one or more receivers to receive the input signal and the output signal from the accessory , and a microcontroller or correction circuit configured to compare the input signal and the output signal and in response to the comparison provide a compensation signal to the compensation unit .