A/D Convertor

The bias can be implemented from the computer through a D/A converter from the computer. The typical output from a typical D/A converter, 10 V in range and 4.88 mV per step, is ideal for bias control and for local tunneling spectroscopy. The speed of output from a D/A convertor and the speed of reading by an A/D convertor are typically 30 kHz, which matches the speed of the current amplifier. With an additional A/D conversion for the tunneling current (the output of the current amplifier), the local tunneling spectroscopy can be implemented by the computer without additional analog electronics. 

Scattered light collected by the FI.5 lens (f = 30 cm) is relayed to the photomultiplier tube via 1 mm slits, a 1 nm bandwidth interference filter and a polarization filter, to reduce background from flame luminescence. The PDP-11/34 computer instructs the A/D convertor to make a conversion every 100 vsec. The resulting digital data are stored sequentially in core memory. The memory is saturated at 16,000 temperature measurements, at which time the data are transferred to a hard disk memory. The data in this transfer constitute one time... 

MINC Preamp (4 channel) and A/D Convertor (16 channel) Modules. 

During the measurement or calibration, these photodiodes measure the light reflected from the reagent strips or the standard reflectance plates previously moved to the measuring position. The output of each photodiode is taken out as short-circuited current, which is then converted to voltage by an A/D convertor. 

This voltage is digitized in an A/D convertor. The dynamic range of measurement is then finally limited at the lower side by the dark current of the photomultiplier and at the high side by saturation at the last dynode. The linear dynamic range of a photomultiplier is usually more than 5 decades of intensity. 

With an ICP atomic emission spectrometer, for the spectral background intensity at 250 nm a signal of 104 digits is obtained with a 10 V A/D convertor and 12 bit resolution (4096), thus representing a voltage of 0.254 V. When a capacitor with 1.11 x 10 8 F and a measurement time of 10 s are used, this corresponds to a... 

Acknowledgement We thanks to Dr. Z. Vedral for construction of A/D convertor and writing the main data handling programm. 

The parallel data transfer mode is simple to use and allows for greater flexibility than the serial mode. However, it requires numerous connections between the PIA and the A/D convertor, and is, therefore, limited to those cases where the convertor and computer are very close. This type of transmission is very popular in instruments which have a built-in dedicated microprocessor. 

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