Sensor Signal Conditioning : bridge circuit part1

Typically a sensor cannot be directly connected to the instruments that record, monitor, or process its signal, because the signal may be incompatible or may be too weak and/or noisy. The signal must be conditioned—i.e., cleaned up, amplified, and put into a compatible format. The following sections discuss the important aspects of sensor signal conditioning.

Measuring resistance indirectly using a constant current source

Conditioning Bridge Circuits

This section discusses the fundamental concepts of bridge circuits.
Resistive elements are some of the most common sensors. They are inexpensive to manufacture and relatively easy to interface with signal conditioning circuits. Resistive elements can be made sensitive to temperature, strain (by pressure or by flex), and light. Using these basic elements, many complex physical phenomena can be measured, such as fluid or mass flow (by sensing the temperature difference between two calibrated resistances) and dew-point humidity (by measuring two different temperature points), etc. Bridge circuits are often incorporated into force, pressure and acceleration sensors.
Sensor elements’ resistances can range from less than 100 ? to several hundred k?, depending on the sensor design and the physical environment to be measured (See Figure 4.1.1). For example, RTDs (resistance temperature devices) are typically 100 ? or 1000 ?. Thermistors are typically 3500 ? or higher.

Bridge Circuits

The Wheatstone bridge

Resistive sensors such as RTDs and strain gages produce small percentage changes in resistance in response to a change in a physical variable such as temperature or force. Platinum RTDs have a temperature coefficient of about 0.385%/°C. Thus, in order to
accurately resolve temperature to 1°C, the measurement accuracy must be much better than 0.385 ?, for a 100 ? RTD. Strain gages present a significant measurement challenge because the typical change in resistance over the entire operating range of a strain gage may be less than 1% of the nominal resistance value. Accurately measuring small resistance changes is therefore critical when applying resistive sensors. One technique for measuring resistance (shown in Figure) is to force a constant current through the resistive sensor and measure the voltage output. This requires both an accurate current source and an accurate means of measuring the voltage. Any change in the current will be interpreted as a resistance change. In addition, the power dissipation in the resistive sensor must be small, in accordance with the manufacturer’s recommendations, so that self-heating does not produce errors, therefore the drive current must be small.

Bridges offer an attractive alterna tive for measuring small resistance changes accurately. The basic Wheatstone bridge (actually developed by S. H. Christie in 1833) is shown in Figure. It consists of four
resistors connected to form a quadrilateral, a source of excitation (voltage or current) connected across one of the diagonals, and a voltage detector connected across the other diagonal. The detector measures the difference between the outputs of two voltage dividers connected across the excitation.

A bridge measures resistance indirectly by comparison with a similar resistance. The two principal ways of operating a bridge are as a null detector or as a device that reads a difference directly as voltage.
When R1/R4 = R2/R3, the resistance bridge is at a null, regardless of the mode of excitation (current or voltage, AC or DC), the magnitude of excitation, the mode of readout (current or voltage), or the impedance of the detector. Therefore, if the ratio of R2/R3 is fixed at K, a null is achieved when R1 = K·R4. If R1 is unknown and R4
is an accurately determined variable resistance, the magnitude of R1 can be found by adjusting R4 until null is achieved. Conversely, in sensor-type measurements, R4 may be a fixed reference, and a null occurs when the magnitude of the external variable (strain, temperature, etc.) is such that R1 = K·R4. Null measurements are principally used in feedback systems involving electromechanical and/or human elements. Such systems seek to force the active element (straingage, RTD, thermistor, etc.) to balance the bridge by influencing the parameter being measured.

Output voltage and linearity error

For the majority of sensor applications employing bridges, however, the deviation of one or more resistors in a bridge from an initial value is measured as an indication of the magnitude (or a change) in the measured variable. In this case, the output voltage change is an indication of the resistance change. Because very small resistance changes are common, the output voltage change may be as small as tens of millivolts, even with VB = 10 V (a typical excitation voltage for a load cell application). In many bridge applications, there may be two, or even four, elements that vary. Figure shows the four commonly used bridges suitable for sensor applications and the corresponding equations which relate the bridge output voltage to the excitation voltage and the bridge resistance values. In this case, we assume a constant voltage drive, VB. Note that since the bridge output is directly proportional to VB, the measurement accuracy can be no better than that of the accuracy of the excitation voltage.

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