A Σ Δ modulator with extended supply voltages in 0.8 µm SOI CMOS for direct ground referred instrumentation interfacing

Until recently, SOI technology has been seen as only useful for its survivability in harsh environments such as radiation and high temperatures. However, the total device isolation has other benefits. In this paper we present a ΣΔ modulator aimed at a modest instrumentation specification which uses SOI technology to allow bipolar input signals above and below ground to be applied directly to the input pin, thereby eliminating level shifting circuitry from the transducer interface. At the same time, the device gives a conventional 0-5V or 0-3.3V logic output from the same die. Potential issues with anomalous SOI device behaviour are identified at the design stage with specific device models, and electro-thermal simulation of key cells is employed to highlight any significant problems. The design is realised in a 0.8mm radiation-hard 5V CMOS -SOI process, and runs from supplies of +/-2.5V (analogue) and +5V (digital) down to +/-1.5V and +3V.