Advanced Practical Electronics – Circuits & Systems by Malindi Phumzile

Author:Malindi, Phumzile
Language: eng
Format: epub
Publisher: MLD Technologies
Published: 2022-01-20T00:00:00+00:00

7. CONDITIONING AND INTERFACING

7.1 INTRODUCTION Signal conditioners are ancillary circuits, which are intended to condition the raw signals that are produced by the sensors to usable signals for further processing. The signal conditioning may involve conversion form one form of electrical quantity to another, filtering, amplifying, attenuation (or scaling down the amplitude), linearization, common-mode rejection, and/or analogue to digital conversion. The conditioning circuit that are going to be covered in this chapter are those that are used for conversion, amplifying, scaling and filtering.

Interfacing is about putting two or more processes or circuits together. The emphasis here will be on devices and circuits that are used to drive the actuators and other loads. It will also include ways of providing isolation between different stages of an electronic circuit or system.

7.2 SIGNAL CONDITIONING It was mentioned in Chapter 1 that sensors are used in electronics to acquire data about their surroundings and convert that into an electrical quantity for further processing. This electrical quantity can be voltage, resistance, capacitance, inductance or current. Since processing is done using voltage, quantities such as resistance, capacitance, inductance and current need to be converted into voltage first before processing can take place. Signal conditioning circuits are responsible for this task and they are also used to improve the quality of the signals generated by the sensors.

7.2.1 Resistance-to-voltage Conversion Sensors such as RTDs, thermistors, LDR, strain gauges, load cells, carbon microphone, etc., exhibit a change resistance in response to quantity they are measuring or detecting. Some have their resistance increasing as the quantity being measured is increasing, and others have their resistance decreasing as the quantity being measured is increasing. For example, the resistance of an RTD increases as the temperature is increasing, whereas the resistance of an LDR decreases with the increase in the light intensity. In both cases, the change in resistance that is produced by the sensor must be converted to change in voltage before it can be processed further.

There are two most commonly used methods for converting resistance into voltage: the use of a voltage divider network and the use of a bridge. The use of a voltage divider network is the simplest method than using the bridge. However, it is not as efficient as the bridge method since it amplifies the entire voltage across the sensor, whereas the bridge only amplifies the voltage due to the change in the resistance of the sensor.

7.2.1.1 Voltage divider network

Voltage divider network comprises of a reference resistor Rr and the sensor Rs as shown in Figure 7.1a.

Vcc Vcc Vcc

Rr Rr Rr

Vs VsVo VsVo

Rs Rs RsRf

Ri

(a)(b) (c)

Figure 7.1 Resistance-to-voltage using divider network Voltage VS represents the voltage that is developed across the sensor. Depending on the application, this voltage can only need buffering as shown in Figure 7.1b above, or it can need further amplification, which can be accomplished by replacing the voltage-follower with an amplifier that is a having a gain that is greater unity as shown in Figure 7.1c above. The output of the divider network is given by

V

S  RS  Rr

RS VCC (7.

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