TRANSDUCERS Page 43
C
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380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.
TRANSDUCERS
Many different types of transducers are used in science and engineering. They have various
requirements for excitation voltages, amplification, recording techniques, and mechanical
connections. In the past this variety has required a different amplifier for each transducer type. The
CyberAmp 380 introduces a convenient new approach to interfacing transducers. The 15-pin D
connector input on each channel of the CyberAmp 380 provides the excitation voltage, differential
inputs, offset correction, and filtering required by a wide variety of transducers. The versatility of the
CyberAmp 380 eliminates the need for an amplifier dedicated to each transducer type.
SmartProbes contain both the analog circuitry required by their target transducer and an EEPROM
with calibration data for the transducer. A SmartProbe can easily be constructed using readily
available components. Information in the chapter Making Your Own Adapter or Active Probe
describes the electrical connections and presents sample circuits to help in this task. Making a custom
adapter is not a daunting task. Nor is it unusual. Because there is a large variety of transducers and
transducer manufacturers, including custom made transducers, many CyberAmp users make their own
adapters.
Common Transducers and Their Applications
A transducer is a device that converts variations of one quantity into variations of another quantity.
For simplicity, the discussion will be restricted to physiological measurements. In physiological
applications a transducer usually converts variations of a physical quantity such as temperature or
pressure into variations in a voltage that can then be processed and recorded.
Temperature transducers for physiological temperature measurement
Three transducer types are suitable for physiological temperature measurement.
1) Thermistors
Thermistors have a resistance that drops significantly as the temperature increases. They are
composed of various metallic oxides and exhibit temperature coefficients many times greater than
those of pure metals. Commonly their resistance falls by 4-6%/°C with increasing temperature.
A typical resistance vs. temperature curve is exponential, but can be considered to be linear over a
range of a few tens of degrees Celsius. By combining multiple thermistors and some resistors, the
useful linear range can be extended to span more than 100 degrees Celsius. The Yellow Springs
Instrument Co. Series 700 Thermilinear Probes contain a matched pair of thermistors. When
connected by precision resistors to an accurate DC excitation voltage, these probes are
interchangeable, with an accuracy within ±0.15°C over the recommended range of -30 to +100°C.
YSI probes are available in a wide range of shapes and sizes for various applications such as
esophageal, rectal and surface body temperature measurements, or for immersion in hostile chemical
environments. Each temperature probe can be calibrated in the laboratory and this data stored in the
SmartProbe. For convenience, accuracy, and price, the YSI Series 700 Thermilinear Probes are often
the best choice. The Yellow Springs Instrument Co. and other manufacturers produce many other
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