Data Acquisition Amplifier

The apparatus consists of a tip-position controller, an electrochemical cell with tip, substrate, counter and reference electrodes, a bipotentiostat and a data-acquisition system. The microelectrode tip is held on a piezoelectric pusher, which is mounted on an inchwomi-translator-driven x-y-z tliree-axis stage. This assembly enables the positioning of the tip electrode above the substrate by movement of the inchwomi translator or by application of a high voltage to the pusher via an amplifier. The substrate is attached to the bottom of the electrochemical cell, which is mounted on a vibration-free table [, and ]. A number... 

Splicing into the existing series of thermocouples was not attempted in order to avoid possible interference with the normal control functions of the ARC. Pressure readings were obtained by tapping directly into the pressure transducer of the ARC. In this case, an amplifier had to be interposed between the ARC and the data acquisition board in order to boost the signal. Finally, the thermocouple and the amplified pressure transducer wires were... 

The signals from the detectors are amplified to create a voltage pulse with amplitude proportional to the energy of the charged particle. Data acquisition, storage and display is effected by an MCA providing pulse-height analysis. 

The instrumentation for voltammetry is relatively simple. With the advent of analog operational amplifiers, personal computers, and inexpensive data acquisition-control system, many computer-controlled electrochemical systems are commercially available or custom made. Programming complex excitation waveforms and fast data acquisition have become a matter of software writing. 

FIGURE 7.10 Block diagram of a basic electrochemical detector. All three electrodes are controlled by a regulated power supply coupled to a potentiometer and a series of amplifiers. The output from the electrodes is fed into a data acquisition system. 

Figure 6. Layout of a sensor data acquisition CMOS chip consisting of (a) eight buffer amplifiers (b) a multiplexer (c) a pulse edge modulator and (d) a RF transmitter.

The prism at the outlet of the laser serves to separate the laser emission of the gas fluorescence and allows for a clean excitation of the sample. For excitation using solid-state lasers, this element is dispensable. The lens (element 5) collects the fluorescent signal and focuses on the aperture of the monochromator. The filter is used to eliminate excitation that is spread over the surface of the sample. The optical chopper serves to modulate the light at a defined frequency, which serves as reference for the lock-in amplifier. A data acquisition system controls the pace of the monochromator and reads the signal of the lock-in, generating the sample s emission spectrum. 

The deformation of the sample and the electrical potential difference over the sample were measured. The electrical potential difference between both electrodes were amplified by a Unicam 9460 amplifier (Unicam, USA). During the experiments, the displacement of the piston was recorded via a linear variable displacement transducer (LVDT, Schaevitz, USA). The data acquisition sampling was 0.5 Hz. A vibrator was attached to the set-up in order to overcome the sticking of the piston to the wall. This device vibrated intermittently at 50 Hz during 1 second. The vibration started 0.5 second after the data-acquisition. Lateral forces on the piston were minimised by allowing free lateral motion of the measuring chamber floating on a silicon oil film. Further-... 

A test system, controlled by personal computer (PC), was developed to evaluate the performance of the sensors. A schematic of this system is shown in Figure 3. The signals from the sensors were amplified by a multi-channel electrometer and acquired by a 16 bit analog to digital data acquisition board at a resolution of 0.0145 mV/bit. The test fixture provided the electrical and fluid interface to the sensor substrate. It contained channels which directed the sample, reference and calibrator solutions over the sensors. These channels combined down stream of the sensors to form the liquid junction as shown in Figure 1. Contact probes were used to make electrical connection to the substrate. Fluids were drawn through the test fixture by a peristaltic pump driven by a stepper motor and flow of the different fluids was controlled by the pinch valves. 

Theoretically, CCD s offered the most attractive features of the remaining choices, photodiodes and CCD s. However, at the time when the decision had to be made, CCD technology was, and still is, too much in flux, for their use in a mass spectrometer system and too higji in cost to be a reasonable choice. The cost factor would be amplified even further when one considers the increased requirements on the data acquisition system due to the 60-fold increase in data rate ( 860,00 vs 14,300 ) data points per spectrum. These and other considerations led to the decision to implement the second generation detector with a photodiode (Reticon) based camera. This system is now in operation producing excellent data and will be described in detail in the following section. 

Fig. 8.1 Typical configuration of a data acquisition system (Ti... T3 are transducers, At... A3 are amplifiers).

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