Digital-to-analog conversion

Dual-slope type A/D converter. For calibration, either V f or the buffer gain G can be adjusted to give the correct display for a known input voltage. 

The dual-slope method has a number of important features. Conversion accuracy is independent of the effect of temperature or aging on the clock frequency and integrating capacitor value, since they need to remain stable only during a conversion cycle. Accuracy 

For applications requiring high resolution and high speed, the successive-approximation A/D conversion method is used. This method is illustrated in Fig. 9 and consists of comparing the unknown input against a precisely generated internal voltage 

Successive-approximation-type A/D converter. A 5-bit converter is shown for simplicity of illustration, but commercial instruments typically use 10 to 16 bits. 

Note The dependence of the signal output on the sampling rate, i.e., on the frequency at which data points are recorded, presents a general concern for any (mass spectrometer) detector system (cf. Chap. 4.6). 

In case of 8-bit ADCs as typically used in MALDI-TOF instruments, the intensity of the detector output is converted into a numerical value of 0-255 [45]. The dynamic range is the ratio obtained by dividing the intensity of the most intense signal by that of the weakest while both are correctly detected in the same spectrum. To improve the small dynamic range delivered by 8-bit ADCs (0-255) several tens to a few hundred single spectra are usually summed up. 

Slower scanning instruments such as quadrupole, quadrupole ion trap, and magnetic sector instruments are typically equipp with 16-20-bit ADCs corresponding to intensity values of 0- 5535 (2 -l) and 0-1048575 (2 -1), respectively. This explains their superiority when a high linear dynamic range is required, e.g., for quantitation. 

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Channel AEBIL signal conditioning modules (variable amplifier, RMS measurement, floating threshold level, analog to digital conversion signal) ... 

Data system. Components used to record and process information during the analysis of a sample. The system includes a computer and an analog-to-digital conversion module as well as other control devices for data recording, storage, and manipulation. 

Pulse Inputs Where the sensor within the measurement device is digital in nature, analog-to-digital conversion can be avoided. For rotational devices, the rotational element can be outfitted with a shaft encoder that generates a known number of pulses per revolution. The digital system can process such inputs in either of the following ways ... 

Digitization rate The rate that represents the number of analog-to-digital conversions per second during data acquisition must be twice the highest signal frequency. 

Time-domain detection results in histograms of photon arrival times. In time-correlated SPC, the time-bins correspond to the analog-to-digital conversion levels, whereas in time-gated SPC, the time-bins are the time-windows during which the photoncounting gate is activated. 

Fig. 14.9. Schematic representation of the analog-to-digital conversion process. The continuous analog signal output by the detector is sampled, converted into digital data points and transferred to the data system.

The complete analytical system is under the control of a Motorola 6802 microprocessor. All switching of gas flows, timing, temperature control, error detection, analog to digital conversion, FID current measurement, signal Integration and manipulation, and data storage and transfer are controlled by this system. 

Each of the possible ways for analog-to-digital conversion requires some sort of reference, usually a reference voltage. The performance of the reference voltage source has a major influence on the overall performance of a data acquisition system. 

The techniques for analog-to-digital conversion can be classified into one of three fundamental principles they differ in conversion speed and sophistication of physical implementation. 

Results of on-line, real-time instrumental analyses are posted directly to the database, with or without processing, as soon as the instrument presents the analytical data. All results are available for review and validation immediately after being posted -a feature critical to effective laboratory management. Data is acquired from laboratory instrumentation of virtually any manufacturer or function. Instruments are interfaced to the computer via analog to digital conversion, RS-232C, current loop, IEEE-4888, binary coded decimal (BCD) or bit parallel techniques. 

Analog to digital conversion is performed by an interface operating at up to sixty readings per second in the +/-10 volt input range and is capable of resolving 0.3 microvolts. 

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