Abstract: Testing the transfer function linearity of analogue input circuits. The transfer function linearity of an analogue input circuit, such as an analogue-to-digital converter (ADC) 25 is tested by generating two consistently reproducible digital test signals (5), converting the digital test signals to analogue form (7) and forming a combination test signal as the average of the two `simple` test signals (16). On three successive occasions the digital test signals are generated and on each occasion a different one of the analogue signals is applied to the ADC under test and the digitized signals stored. A digital combination signal is formed by sampling the digitized `simple` signals and taking the average of the signals, the digital combination and digitized combination signals being subtracted for each sample and any discrepancies summed. The summed total after all the sample points comprising the test signal have been taken gives a measure of any non-linearity of the ADC.