PMT Preamp Diagram

Signal scaling for both the PMTs and the photo diode are achieved through attenuation. This is easily accomplished using a potentiometer as a voltage divider. This approach has two advantages. First, the relative signal amplitude is directly proportional to the position of the panel indicator or knob. If one uses a multi-turn potentiometer and a c

ounting dial, this value is directly readable. Secondly, because the gain of each amplifier is fixed, and because the frequency characteristics of an amplifier are related to its gain, overall system linearity and reproducibility are improved. As in many optical measuring systems, there are several sources of DC errors in the output signals. The primary sources are unwanted light reaching the detectors, and leakage current from the detectors themselves. In the case of PMTs, this leakage current error can vary significantly with tube voltage. To compensate for these errors, a DC offset adjustment is included in the circuit - see diagram below. In this embryo sorting system, the signal from one of the PMTs is used to detect a very narrow band of light that represents fluorescence from a known source. However, light from other sources (autofluorescence) may also be present at these wavelengths, and may be detected by the PMT. To compensate for this, light is measured at wavelengths close to those of the known source, and is used to represent the relative intensity of the autofluorescence. A percentage of this autofloursence signal is then subtracted from the fluorescence signal. By proper scaling of these signals, a more accurate representation of fluorescence from the known source are obtained. The diagram below illustrates a simple circuit that accomplishes this. This circuit also AC couples the signals to eliminate any...

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