A data acquisition system measures and records voltages. In order to measure useful quantities like pressure, temperature, force, etc., a transducer, a device that produces a voltage proportional to the quantity to be measured, is needed.

Most transducers do not produce a voltage in a range suitable for direct connection to a data acquisition system and some form of amplifier is needed. This may be a stand-alone module or may be incorporated right into the transducer housing. Usually the output voltage is proportional to the quantity being measured, but some sensors (thermistors, for example) are inherently nonlinear. Nonlinear sensors used to be a real pain to deal with but now the necessary linearization can easily be performed in software.

There is a vast range of transducers available for measuring almost any quantity, and only some of the common ones that have relatively simple interfaces will be covered here. More complex devices such as pH meters, ion-selective probes and glass microelectrodes are frequently used in life science work but all of these come complete with their own special amplifier and connecting the amplifier output to a data acquisition system is not difficult.

Only a few simple circuits are given here since the emphasis of this book is on collecting and analysing digital data rather than electronics. Chapter 15 in Horowitz and Hill goes into more details of the circuits used with different types of transducers. However, if you have the money in your budget it is nearly always better to buy transducer amplifiers and let someone else work out the details.