March 2008

Signal conditioning for one

By C.S. Chen, Francis Chen, Cynthia F. Tielsch

In a conventional temperature measurement system, a signal conditioner often sees use to filter, linearize, and amplify corrupted, weak analog sensor signals. The computer then digitizes and processes the signal conditioner output for monitoring or control applications. Because of the weak sensor signal, sensors must connect in the proximity of the signal conditioner. A multipoint measurement en- vironment requires an excessive number of wires.

In the past few years, a semiconductor integrated circuit (IC) sensor has come on the scene, featuring not only on-board signal conditioners, a 12-bit digitizer, and unique address identifier, but also a One-Wire net interface. One-Wire is a device communications bus system that provides low-speed data, signaling, and power over a single signal, albeit using two wires, one for ground, one for power and data. The user, through a properly designed controller, can easily interface to One-Wire network channels. If you must monitor multiple points along the network line, you could arrange a network of One-Wire compatible sensors in a multi-drop network connection.

In a conventional temperature measurement system, a temperature transducer converts thermo variables (temperatures) into electrical signals. Since this electrical signal is very weak, in most cases corrupted by noise, and generally not linearly proportional to the physical variable, the signal conditioning unit processes it for noise filtering, linear-ization, and ampli- fication before digitizing it and converting it through an analog-to-digital (A/D) converter. The computer then pro-cesses the information for readout, display, and/or control.

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Temperature sensors

Thermocouples and resistance temperature detectors (RTDs) are the most commonly used temperature sensors. How- ever, the IC based temperature sensor, also known as a digital thermometer, has slowly gained in popularity due to its technology. Since the output of the IC sensor is digital in nature, a temperature measurement system configuration is very different from the system using thermo- couples or RTDs. Their outputs are analog and require signal conditioning and A/D converter units.
 
T.J. Seebeck introduced the commonly used temperature transducers, such as thermocouples, in 1821. The principle of thermocouples is based on the Seebeck effect, which states when two different metals connect together, a voltage generates at the junction, which is a function of the temperature at the junction. This voltage, though not necessarily a linear function of the temperature, can calibrate to measure the temperature. A common problem with thermocouple temperature measurement is the thermocouple lead wires and the terminal block form another thermocouple. It must compensate to account for the voltage generated at this lead wire-terminal block thermocouple.

RTDs

H.L. Callendar first introduced the RTD in 1885. It works on the principle that the resistance of a metal is a function of temperature. The temperature coefficient is very small. The most common metals used include platinum, nickel, copper, and nickel/iron. The temperature RTDs can measure ranges from -400°F to 1200°F. When measuring temperature with RTDs, resistive heating in the RTDs and the current-carrying wires are major considerations in measurement setup. Thus, there are 2-wire, 3-wire, and 4-wire measurement schemes. Furthermore, the resistance versus temperature relation is not a linear one over the entire temperature range of interest, and you need calibration for accurate measurement.

Dramatic changes

Little has changed with these transducers. On the other hand, advances in data acquisition, signal conditioning, and computer processing capability have been dramatic. Recent developments in IC temp- erature sensors greatly alter the conventional temperature measurement scheme. Since the functions of signal conditioning and data acquisition are all built into the semiconductor sensor, the user no longer needs to provide separate external signal conditioning and data acquisition units. It not only greatly simplifies the temperature measurement system design, but also lowers the system cost.

One advanced method of measuring temperature is using a semiconductor temperature sensor that directly communicates digitally with a PC master over a one-wire network. Each sensor has a unique, permanently lasered 64-bit address that permits the master controller to identify and select. Because of this address, multiple sensors can connect to the same network, and the software can automatically identify and process the data from any given sensor.

In a multi-drop topology, the controller, also called the master, digitally communicates to the one-wire devices, or the slaves, over the one-wire net. The network bus transfers conventional TTL/CMOS data over two wires, signal and ground. The one-wire net incorporates a third line, power, to supply power to all slave devices attached to the network. A one-wire net based system consists of three basic hardware components:

  1. A master controller embedded with software such as TMEX, Windows' drivers to develop an API viewer
  2. Wirings, splitters, and connectors
  3. One-wire compatible slave devices such as temperature sensors. In this system, the master initiates conversation, which occurs only between a master and a slave. The master and the slave are transceivers, which allow them to read from and write to the same bus. Although bi-directional, the network bus is half-duplex, which implies the bus may be in write mode or read mode but not both.

Data is transferred in time slots on the one-wire net. To write a logical 1 to a slave device, the master controller pulls the bus low for 15µs or less. To write a logical 0, the master controller pulls the bus low for at least 60 µs. To initiate communication, the master resets the network by pulling the bus low for at least 480 µs, releasing it and searching for a responding presence pulse from each slave device connected to the network. Once it detects the presence pulse, the master uses the device's identification address to access the device, issue device-specific commands, and perform necessary data transfers between the master controller and the slave devices. Other key functions such as READ BIT/BYTE and WRITE BIT/BYTE also use the time slots to transmit the commands and receive the data on the bus. Reading a bit on the data bus requires a 15µs delay before the bit value on the bus returns. In a WRITE BYTE command, a byte writes to the bus sequentially by bit. After all eight bits have written to the bus, the WRITE BYTE command retains the last bit value on the bus for 75 µs. These basic commands build the essential functions for operation, such as reading the device temperature, performing redundancy cyclical checks, or searching for the devices attached to the data bus.