Floating-gate based trimmable current sources

One of the most crucial building blocks of an electronic system is the digital-to-analogue converter (DAC) which forms the interface between the digital and analogue component parts. In order to achieve the high performance required by many systems, particularly communications systems, the DAC requires careful, time consuming and expensive design and layout which increases the cost of the entire system. This design process is required to minimise the mismatches between the current sources in the current steering DAC. Trimmable current sources, based on floating-gate devices, are therefore proposed in order to simplify the design of a DAC. A floating-gate device is a form of an analogue non-volatile memory which can be programmed to control the threshold voltage of a MOSFET. Four different current sources are designed to use a floating-gate device in three distinct ways: as a trimmable current source, as a trimmable voltage source and as a trimmable resistor. All the current sources have a sufficient trimming range to correct for any process and parameter variations. However, a major concern about floating-gate devices is their ability to retain a programmed voltage. It is shown that these current sources can be biased so that they retain a programmed floating-gate voltage over several days of normal operation. Even with this characteristic, a variation in the operating temperature of the current sources will cause a significant variation in the output current. Therefore, the temperature characteristics of the current sources are investigated to determine the impact of temperature variations. A distinct biasing condition is identified in all the current sources which causes the output current to be insensitive to temperature variations. Furthermore, one design is identified which is temperature insensitive over a significant trimming range.