Analog-to-Digital Converters ADC

As discussed in Sec. 9.9, the backbone of an MCA is the analog-to-digital converter (ADC), the unit that digitizes the input pulse height and assigns it to a specific channel. Many types of ADCs have been developed, but the most frequently used is the Wilkinson type.  

The Wilkinson-type ADC operates as shown in Fig. 10.39. When a pulse enters the MCA, two events are initiated  

An input gate prevents the acceptance of another pulse until the previous one is fully processed and registered. 

The capacitor keeps charging until the peak of the pulse is reached. At that point in time, two new events are initiated  

The voltage on the charged capacitor is discharged by a constant current. 

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This microsystem also comprises three 10-bit successive approximation analog-to-digital converters (ADCs) that are used for reading out the microhotplate temperature, the bulk chip temperature, and the sensor resistance, three programmable offset... 

The attenuation of the optical signal by a monolayer is small and the upper limit on S/N is usually determined by the dynamic range of the analog-to-digital converters (ADC) (9). A simple method to diminish the ADC limitation, and improve the S/N ratio would be to increase the amount of sample probed by the beam. Since the ADC peak-to-peak noise level for a single scan is -10-3 absorbance it may be necessary to average several... 

Modern HPLC systems equipped with analog-to-digital converters (ADCs) and initial data acquisition and signal analysis are performed by either built-in microprocessor or attached computer [38,39]. Depending on the data acquisition rate and initial analysis criterion (peak height, peak area, or sensitivity thresholds, etc.), the integration limits or area calculation could be erroneous, and in some cases the whole peaks could be missed. 

Figure 12.4. Block diagram of a modem NMR spectrometer. These systems use superconducting magnets that are based on a solenoid of a suitable alloy (e.g., niobium/titanium or niobium/tin) immersed in a dewar of liquid helium. The extremely low temperature of the magnet itself (4.2 K) is well insulated from the sample chamber in the center of the magnet bore. The probe in which the sample is housed usually incorporates accurate temperature control over the range typically of 4 to 40°C for biological samples. The rf coil in the probe is connected in turn to a preamplifier, receiver circuitry, analog-to-digital converter (ADC), and a computer for data collection.

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