A Model for Temperature Insensitive Trimmable MOSFET Current Sources

High-precision current-steering digital-analog converters (DACs) can only be created by limiting variations between the outputs of individual current sources within each DAC. One possible approach to limit these variations is to create a trimmable circuit which includes a floating-gate device acting as an analogue nonvolatile memory. Results are presented which show that one type of trimmable current source has a bias condition at which its output current is both trimmable and robust to temperature variations. A model of this potentially useful behavior is presented which suggests that it arises from a combination of the temperature dependence of a MOSFET and the modulation of its source voltage caused by a second MOSFET acting as a voltage controlled resistor.