SIGNAL CONDITIONING Signal conditioning is stage of instrumentation system used for modifying the transduced signal into a usable format for the final stage. Block diagram of instrumentation system. Depending on transducers output signal and required signal to drive output device, signal conditioning circuit is required. Input Transducer Signal conditioner Output device

NEED OF SIGNAL CONDITIONER:  To make the output of a system suitable to drive the output device connected at the output of system, signal conditioning is required.  Signal conditioner is required to perform following operations. 1. Amplification 2. Modulation/Demodulation 3. Attenuation 4. Sampling 5. Integration 6. Differentiation 7. Addition/ Subtraction

SIGNAL CONDITIONING TECHNIQUES: 1. D.C. Signal conditioning technique. 2. A.C. Signal conditioning technique.

D.C. SIGNAL CONDITIONING:  The D.C. system makes use of passive transducer which requires external power supply(D.C.) for their excitation.  Hence the signal conditioning required for such a D.C. system consisting of resistive transducer is called as D.C. signal conditioning.  Fig: Block diagram of D.C. signal conditioning system

 D.C. signal conditioning is used for resistance transducer such as potentiometer, strain gauge.  For D.C. signal conditioning D.C. voltage source is used for excitation.  The dc amplifier is used to amplify small signal at sufficient level.  The dc amplifier is followed by low pass filter, which is used to eliminate the high frequency components or noise from the data signals.  The main drawback of this system is the drift problem. This is overcome by A.C. Signal conditioning system. DISADVANTAGES: 1. System suffers from drift problem. 2. It require dc amplifier with high CMRR. 3. It is not easy to calibrate D.C. amplifier at low frequency. 4. D.C. amplifier requires high thermal and long term stability

AC SIGNAL CONDITIONING:  It is used for variable reactance transducers, for systems where signal have to be transmitted via long cables to connect the transducers to signal conditioning equipment is called as A.C. signal conditioning.  Fig: Block diagram of A.C. signal conditioning system.

 In order to overcome the problem of drift in the dc system, ac systems are used.  The transducers used are of the variable reactance type. They are employed between carrier frequencies of 50kHz to 200 kHz.  The carrier frequencies are much higher, at least 5 to 10 times the signal frequencies.  The output of transducer is applied to the bridge circuit, whose output is an amplitude modulated carrier signal. This waveform is amplified by an ac amplifier. This amplified modulated output is the applied to phase sensitive demodulator, the carrier signal. This produces the dc output that indicates the direction of the parameter change in the bridge output.  In carrier system amplifier, frequency drift and presence of spurious signals ate not much of a problem. Active filter can be used to reject this frequency and prevent overloading of the ac amplifier.  The function of phase sensitive detector is to filter out the carrier frequency components of the data signal.

 ADVANTAGES: 1. No frequency drift problem. 2. No spurious signal is present.  DISADVANTAGES: 1. It is difficult to obtain a stable carrier oscillator frequency. 2. It requires a A.C. amplifier of proper bandwidth to amplify the particular signal

DATA ACQUISITION SYSTEM:  Data acquisition system (DAS) is a process of collecting the input data in digital form as rapidly, accurately, completely and economically as necessary.  The data acquisition system (DAS) typically converts analog data into digital data for the purpose of processing, transmission display and storage.  The various configuration used in DAS are 1. Single channel DAS 2. Multichannel DAS APPLICATIONS: 1. DAS can be used for converting physical quantities into data. 2. DAS can be used for factory automation. 3. DAS can be used in weather observations. 4. In the supervisory and display systems.

SINGLE CHANNEL DATA ACQUISITION SYSTEM  It consist of transducer/sensor to convert a physical parameter into an electrical signal, signal conditioning circuit and A/D converter value.  Transducer converts the physical parameter into an analog electrical signal. It is processed by the signal conditioning system.

 The conditioned analog signal is applied to the sample and hold(S & H) circuit.  The S & H circuit samples this analog signal at regular intervals to convert it into a discrete signal.  Each sampled value is converted into an equivalent digital signal by an A to D converter.  As only one physical parameter is being converted into digital form, this circuit is known as a single channel DAS.

MULTICHANNEL DATA ACQUISITION SYSTEM  In multichannel DAS, there are n similar of different types of physical quantities. They are converted to electrical signals using their respective transducer and conditioned by their respective signal conditioners.  Fig:

 The analog signal at the output of signal conditioners is sampled using S and H circuits and then sampled signals are applied to an n:1 analog multiplexer.  The multiplexer connects these n sampled signals one by one to its output. The maximum output is converted to digital signal is an A/D converter.  Thus all the physical quantities are converted into their equivalent digital signals.

DATA LOGGER:  Data logger is a system which is used to make automatic record of readings of the instruments located at different parts of the plant.  Data logger measure and record data effortlessly as quickly, as often and as accurately desired.  Due to data logger measurement error are eliminated completely.  Additional facilities such as scanning and recording the data from a large number of inputs are provided in the modern loggers.

 Fig:  APPLICATIONS: 1. Power plant 2. Engine testing 3. R and D 4. Petrochemical plant 5. Cement plant 6. It is used in AVCS 7. It is used in railway for signaling.