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| Position Sensing Detectors Theory Of Operations |
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Position Sensing Detectors
""PSD" are silicon photodiodes that provide an analog output directly proportional to the
position of a light spot on the detector active area. The PSD allows you to simultaneously monitor position and light intensity. The PSD is a continuous analog position sensor and compared to discrete element devices the PSD offers outstanding position linearity, high analog resolution, last response time and simple operating circuits. |
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The Position Sensing Detector consists of
n-type silicon substrate with two
resistive layers separated by a p-n junction. The front side has an ion implanted p-type resistive layer with two contacts at opposite ends. The back side has an ion implanted n-type resistive layer with two contacts at opposite ends placed orthogonally to the contacts on the front side. On a single axis PSD the electrodes are placed at opposite ends of the p-type resistive layer. A light spot within the spectral range of silicon will generate a photocurrent which flows from the incident point through the resistive layers to the electrodes. The resistivity of the ion implanted layer is extremely uniform so the photogenerated current at each electrode is inversely proportional to the distance between the incident spot of light and electrodes. The PSD outputs track the motion of the centroid of power density" to an extremely high resolution and ultra high linearity.
The ON-TRAK Position Sensing Amplifiers take the photocurrent from each electrode and process the signals to give X, Y outputs independent of light intensity. |
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Specification
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| PSD Type |
Spectural Range
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Responsitivity
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| Standard |
400-1100mm
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0.70 A/W
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| UV |
200-1100nm
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0.14 A/W @ 254 nm
0.19 A/W @ 340 nm
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| YAG |
400-1150 nm
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0.52 A/W @1064 nm
0.76 A/W @ 1000 nm
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| Nuclear |
<500 A
Dead Layer
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FWHM<20 KEV
(5.5 MEV ALPHA)
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| The position resolution of a PSD is the minimum detectable displacement of a spot of light on the detector surface. The position resolution of ON-TRAK PSD's have been proven better than one part in a million.
Resolution dependent on:
- Detector Size
- Detector Noise
- Light Input Intensity
- Bandwidth of the Electronic Signal -Processing Circuits
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 The position non-linearity is defined as the geometric position error divided by the detector length and is measured within 80% of the detector length. Position non-linearity is typically better than 0.05% for the single axis PSD and better than 0.3% for the duolateral. The ON-TRAK vs competitor two dimensional linearity plot shows the ultra linear characteristic of these PSD's. Pictured left: ON-TRAK Duolateral PSD Linearity; Pictured right: Competitor PSD Linearity (Typical) |
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The one-dimensional PSD is able to detect a light spot moving over its surface in one direction. The photoelectric current generated
by the incident light flows through the device and can be seen as an input bias current divided into two output currents. The distribution of the output currents show the light position on the detector.
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| Duoalateral Two Dimentional PSD |
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Duolateral Two-Dimensional PSD 
The duolateral two-dimensional PSD is able to detect an incident light spot position on its rectangular surface. It has four terminals. The photoelectric current generated by the incident light flows through the device and can be seen as two input currents and two output currents. The distribution of the output currents show the light position of one (Y) dimension, and the distribution of the input currents show the light position of the second (X) dimension. |
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