Data Acquisition

This chapter discusses several of the main hardware configurations used when integratingsignal conditioning products into a DAQ system, as well as the general signalconditioning functions that must be performed.3.2.1 AmplificationAmplification is one of the primary tasks carried out by signal conditioning equipment. Itperforms two important functions:• Increases the resolution of the signal measurement.• Increases the signal-to-noise ratio (SNR).3.2.2 IsolationAmplification is primarily used to increase the resolution of the signal measurement.Consider a low-level signal of the order of a fraction of an mV, fed directly to a 12-bitA/D converter with full-scale voltage of 10 V. There will be a resultant loss of precisionbecause the A/D converter has a resolution of only 2.44 mV (15.2 µV for 16 bit resolution).The highest possible resolution can be achieved by amplifying the input signal sothat the maximum input voltage swing equals the maximum input range of the ADC.Another important function of amplification is to increase the SNR. Where transducersare located a long way from the data acquisition board and the signal measurements aretransmitted through an electrically noisy environment, then low-level voltage signals canbe greatly affected by noise. Where the low-level signals are amplified at the data acquisitionboard after they have been transmitted through the noisy environment, then anynoise superimposed on the signal will also be amplified by the same amount as the signal.If the noise is of the same order of magnitude as the signal itself (i.e. the SNR is low),then the signal measurement may be lost in noise, leading to inaccurate and meaninglessmeasurements.Amplifying the low-level signals before they are transmitted through the noisy environmentincreases the level of the required signal before they are affected by noise, therebyincreasing the SNR of the signal for the same level of noise. Consider for example, a J-type thermocouple, which outputs a very low-level voltage signal that varies by about50 µV/ºC. If the thermocouple leads were to travel through a noisy electrical environmentfor say 10 m, then it is possible that the amount of noise coupled onto the thermocoupleleads could be of the order of 200 µV. This noise-induced error corresponds to 4ºC at themeasuring device. Amplifying the signal with an amplifier gain of 500, close to thethermocouple, produces a thermocouple signal that varies by approximately 25 mV/ºC.At this higher signal level, the 200 µV of induced noise coupled onto the 10m cablewould result in a much smaller error, adding only a fraction of a degree Celsius of noiseto the measured temperature.An isolated signal conditioner passes a signal from its source to the measurement devicewithout a galvanic or physical connection. The most common methods of circuit isolationinclude opto-isolation, magnetic or capacitive isolation. Opto-isolation is primarily usedfor digital signals. Magnetic and capacitive isolations are used for analog signals, modulatingthe signal to convert it from a voltage to a frequency and transmitting the frequencysignal across a transformer or capacitor without a direct physical connectionbefore being converted back to a voltage.