Thursday, July 29, 2010

Dual Difference Amplifier with On-Chip Resistors Implements Precision ADC Driver

Discrete difference amplifiers exhibit mediocre accuracy, significant drift over temperature, poor CMR, and a small input voltage range. Monolithic diff amps have better CMR, but still suffer from gain drift due to the mismatch between on-chip devices and the external gain resistor.The AD8270 dual diff amp overcomes these limitations, providing a complete, inexpensive, high-performance solution.

Power Cycling 101: Optimizing Energy Use in Advanced Sensor Products

Highly integrated sensor systems allow system developers to readily use sensors that embrace technologies with which they may have little experience—with minimal investment and risk. Although accuracy is specified at a given power level, power cycling provides an opportunity to reduce average power consumption in applications where energy use must be tightly managed.

Tuesday, July 06, 2010

How does one measure temperature outside the silicon range?

If only King Alfred had had better oven temperature measurement he need not have allowed the peasant woman’s cakes to burn. And even in 880 A.D. it would have been possible to make a thermocouple had he known how.

Thermocouples are very simple devices which can measure temperature over a very wide range.

If we have two conductors of different material joined at two points which are at different temperatures, there will be an emf (electromotive force) in the loop so formed–this emf is a function1 of the temperature difference and can be used to measure temperatures as high as 2300°C and as low as a few Kelvin. One junction is at the temperature to be measured and the other (often known as the "cold" junction, even if it is at a higher temperature than the one being measured) is at a reference temperature (in the past a mixture of ice and water was often used to provide a 0°C reference).

Two pieces of wire of different materials are simple and, usually, inexpensive2, so thermocouples are widely used. Where the reference will normally be within ±10°C of room temperature and accurate measurement is unnecessary, it is quite usual to use an uncompensated thermocouple for measurements. An example is the flame sensor in a boiler.

If reasonable accuracy is required, the cold junction must be held at a known reference temperature–which is inconvenient, as ice may be unavailable. But the cold junction need not be at a constant temperature as long as it is always known. The cold junction temperature, which is unlikely to be outside –55°C to +155°C, may be measured with a silicon sensor; we then use the emf and the cold junction temperature to calculate the "hot" junction temperature.

This "cold junction compensation" may be done by a digital controller, but it is quite possible to integrate the cold junction sensor, the thermocouple amplifier, and the compensation in a simple analog chip. The AD594/AD595 and AD8494/AD8495/AD8496/AD8497 are examples of such circuits; if King Alfred had had one the cakes would have been perfect.

1 The function is (approximately) a polynomial of 5th - 13th order, but for some thermocouples simple proportionality is adequate over some ranges.
2 Platinum (B, R & S) and chromel/gold-iron thermocouples are among the expensive exceptions to this.

High-Speed, Current-Feedback Amplifier Drives and Equalizes up to 100-m VGA Cables

In classrooms, lecture halls, and conference rooms, PCs transmit RGB video signals to projectors through VGA cables. The cable length depends on the room size and ceiling height, but most cables are shorter than 100 m. This article shows how the ADA4858-3 triple high-speed current-feedback op amp with integrated charge pump can drive and equalize up to 100 m of VGA cable.

Quad 16-Bit Voltage-/Current-Output DACs Save Space, Cost, and Power in Multichannel PLCs

Programmable logic controllers use deterministic functions to control machines and processes. As the number of sensor and control nodes continues to increase, some distributed control systems can handle thousands of nodes. A major challenge is to provide greater efficiency and reduced power consumption. The AD5755 4-channel, 16-bit DAC offers an integrated solution to resolve these issues.