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Introduction to data acquisition systems

Interfacing resistive transducers to electronic circuits


Data acquisition systems

Analog Data Acquisition System

Digital Data Acquisition System

Display Devices in detail

Nixie Tube

Digital Display Methods

Signal Generators




Modern Signal Generator

AF Sine and Square Wave Generator


Interfacing resistive transducers to electronic circuits

The major problem with resistive transducers, which includes strain gages, temperature transducers is that the resistance change is very small. As an example, consider measuring the current through a resistance transducer such as an RTD. A simple panel meter is used as an Indicator to provide a remote reading of temperature. The change in the meter Indication is very small for small temperature changes. As an example, the change in resistance of a platinum resistance thermometer is 0.385 per cent per degree Celsius. In this case, a 1-
degree change in temperature will produce a 0.385 Per cent change in the indicating meter, which will be hardly visible.

A solution to this Problem is to connect the resistance transducer in a bridge circuit as shown in Fig.1

First, the zero output voltage Point can be set for a convenient point such as 0 degrees Celsius or 0 degrees Fahrenheit instead of an absolute zero, which would be the situation if only the transducer current were measured. The setting of the zero output point can be achieved by adjusting the values of R1, R2 , and R3 to provide bridge balance at the desired temperature. The change of output voltage in Fig 1. is

There is an improvement in the linearity of the output voltage as a function of the change in resistance, as the R term in the denominator is divided by a factor of 4R rather than 2R and thus the effects of the R term in the denominator are reduced (as compared to Eqn. 1)
If the change in resistance is small, which is often the case with a resistance thermal device, the error due to the lack of linearity is small. Absolute linearity can be achieved by using two transducers as shown in Fig. 3

This technique requires the use of two matched transducers in the environment to be measured. Most resistive transducers are not expensive items, and providing two matched transducers on a common header is not a difficult task. Often when a transducer is eventually connected to a digital system, a microprocessor can be employed to "linearize" the bridge output voltage.