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| RC Time Constant |
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The concept of an RC time constant is the single most important aspect of circuit analysis for applications in Physical Chemistry. The RC time constant determines the time response of circuit. This has application to the measurement of the time course of any process, whether optical or using other types of detection. The RC time constant is determined by the magnitude of the largest resistor and capacitor in a ciruit. That product (in units of ohms and farads, respectively) gives the time response of the circuit in seconds.
Analog-to-Digital Conversion
Signal digitization is a crucial step for the acquisition of useful data. The conversion an analog voltage to digital form is accomplished using a device known as an analog-to-digital converter (ADC). This device converts a range of voltage (e.g. 10 Volts) to bits. The resolution of the ADC can be measured in bits. And 8-bit ADC measures the analog signal in 256 different bins. For a 10-Volt signal this means that the voltage resolution is 40 mV. This is not particularly good resolution and 8-bit ADCs are not used in practice in modern instrumentation. The ADCs used to today are either 12-bit (4096 bins) or 16-bit (65,536 bins) ADCs. The resolution of a 16-bit ADC is sufficient that one can afford to use smaller signals that do not fill the full range of the ADC. This is quite useful in practice. Filling the ADC is both difficult in practice and risks having the signal cut off at the top or bottom of the range.
Operational amplifiers and signal gain
Gain is a crucial aspect of electronic signal processing. The basic signal in every measurement can be represented as a small voltage. In many cases the signal is generated as a current or photocurrent, which is then converted into a voltage by passage through a resistor. This is the first step in converting the experimental signal into usable data. Examples are Si photodiodes in spectroscopy, the inductive coils in NMR, X-ray detectors etc. However, the voltage the results from such conversions is usually too small to be useful for measurement using an ADC. For this reason we need to use a circuit that permits the voltage to be boosted. This segment discusses the concept of gain that arises from a central circuit component known as an operational amplifier.
Detectors
The most common optical detectors are the Si photodiode and photomultiplier tubes. The following segment introduces you the principles of each of these. Si photodiodes are not extremely sensitive and therefore require amplification. The steps involved in current-to-voltage conversion and analog-to-digital conversion are described in the previous segments. Photomultiplier tubes have the advantage of built-in amplification in the dynode chain. The disadvantage of photomultiplier tubes is that they are expensive and fragile. However, for many years they have been the the most useful detectors of fluorescence and time-resolved experiments on the nanosecond time scale (using time-correlated single photon counting). By way of comparison, we have discussed the intrinsic time limitations of Si photodiodes that arise from the RC time constant of the amplifier circuit.
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