A wide range digitizer for direct coupled analogue signals
David N. MacLennan, F.H. Wells · Journal of Physics E Scientific Instruments · 1968
A serial analogue-to-digital converter is described which will accept currents in the range ±1 mA applied to a virtual earth input, or alternatively voltages between ±5 V applied to a high impedance input. An output pulse train is produced whose repetition frequency is proportional to the magnitude of the analogue input, the maximum frequency being 5 MHz corresponding to 1 mA or 5V input. The conversion is linear to within ±0·2%, and the accuracy using a calibration curve is to ±0·1% near full scale. The various factors which give rise to nonlinearity are discussed. The accuracy at a current input of 10 nA is to better than ±10%, giving a dynamic range of at least 10 5 : 1 without the need for range changing. The corresponding figure for voltage inputs is much inferior to this, having a value in the worst case of only 3300: 1.