Wide-Dynamic-Range and High-Sensitivity Current-to-Voltage Converters

A wide dynamic range and a high sensitivity are often incompatible with each other in analog circuits, such as signal converters or amplifiers. In this paper, we present a method of developing current-to-voltage converters featuring both. It comprises two current-to-voltage conversions. One is to convert the DC component, i.e. the average level of the current input, into a voltage with a nonlinear compression. The other is to apply a " linear " conversion to the signal component of the input current. This conversion is considered to be " linear " as the gain is made to be almost constant if the varying signal is rippling at a given current level, and to increase if the level is changed to be lower. Hence, the gain is adaptive to the input current, i.e. getting stronger if the current signal is weaker and vice versa. This adaptability is implemented by (a) an adaptive bias by means of the voltage converted from the DC component of the input current, and (b) the current-dependent finite drain-source resistances of MOS transistors. A low-pass current filter is used, in the proposed conversion operation, to separate the DC component of the input current from the signal one. We propose, in this paper, a basic structure of the filter and techniques to improve the filtering quality over a wide current range, also an approach to an effective reduction of the effect of the device mismatch as well. A design example of the proposed conversion is presented in this paper. The simulation results have shown its dynamic range of 5-decades and its sensitivity high enough to detect sub-nA current variations.