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List the different techniques used for signal conditioning in DAS. Explain in brief Any one signal conditioning technique.

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Types of Signal Conditioning techniques in DAS: 1) Attenuation 2) Amplification 3) Linearization 4) Filtering 5) Ratiometric conversion 6) Logarithmic conversion 

Attenuation: An attenuator is used to scale down the input gains to match the level of input signal to the converter’s full scale range. Most data-acquisition systems contain on-board instrumentation amplifiers as part of the signal conditioning stage with selectable gains ranging from less than to greater than unity. Gain is varied either by a resistor jumper or by logic switches set by software, which effectively reset resistor ratios across OPAMPs. Voltages can also be attenuated using a voltage divider circuit as shown below.


Amplification: Amplification increases a voltage signal to a level suitable for digitization by the DAQ equipment. Typically a data acquisition device is calibrated for input voltages in the 0 to 10 V range. A small voltage, such as that coming from a thermocouple or strain gauge bridge may need to be amplified 1000 times to make it between 0 and 10 V. 

Linearization: Linearization is required when the signals produced by a sensor don't have a linear relationship with the physical measurement, as is the case when using thermocouples to measure temperature. In this case linearization is performed by analog techniques using either linear approximation or smooth series approximation using IC amplifier. It can also be done digitally after A to D conversion. 

Filtering: Filtering reduces noise errors in the signal. For most applications a low-pass filter is used. This allows the lower frequency components but attenuates the higher frequencies. The cut-off frequency must be compatible with the frequencies present in the actual signal and the sampling rate used for the A-D conversion. 

Ratiometric conversion: It is a method used along with ADC to improve accuracy of a system having a bridge circuit connected with RTD or strain gauge sensors. The bridge output voltage is a function of each arm resistance and excitation supply. So to make the system accurate, the system sensitivity should be made independent of the supply fluctuations. This can be achieved using a ratiometric conversion circuit. Here the bridge excitation voltage is fed to the ADC as an external reference voltage. Now the conversion factor (Vo/Vi) of ADC is inversely proportional to the reference voltage, making the system sensitivity independent of excitation supply fluctuations.

Logarithmic conversion: Logarithmic signal compression is a method of signal conditioning for compressing wide dynamic range input signals to a range of an output device. A log amplifier compresses signals by offering equal output amplitude changes in response to a given ratio of input amplitude increase. Such conditioning can be used, where moderate accuracy is expected over a wide range of inputs like in medical investigatory circuits, photo detectors, ionizing radiation detectors and ultrasound receivers etc.

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