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PI1024HSN Datasheet(PDF) 3 Page - AMI SEMICONDUCTOR |
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PI1024HSN Datasheet(HTML) 3 Page - AMI SEMICONDUCTOR |
3 / 8 page PI0256HSN, PI0512HSN, PI1024HSN Engineering Data Sheet Page 3 of 8 March 13, 2002 0.0E+00 5.0E-05 1.0E-04 1.5E-04 2.0E-04 2.5E-04 100 300 500 700 900 Wavelength (nm) QE=20% QE=40% QE=60% QE=80% Figure 4. Predicted spectral response. The Quantum Efficiency (QE) can be calculated by dividing the responsivity by the area of the sensor's element and multiplying the resulting ratio by the energy per photon in electron volts (eV). The dark current is typically 0.2 pA at 25°C and varies as function of temperature. The dark current will con- tribute dark-signal charges and these charges will increase linearly with integration time. The dark sig- nal and the photo-generated signal combined result in the total signal charge. Selectable Charge Capacity The HSN devices have the unique feature of having a selectable charge capacity. There is a bank of ca- pacitors with one capacitor for each photodiode pixel. When the capacitors are connected to the photodi- odes, they give the photodiodes a charge capacity of typically 65 pC. This large charge capacity is useful in applications that demand high dynamic range and high signal-to-noise ratios. With the capacitors discon- nected, the photodiodes have an intrinsic charge capacity of typically 25 pC. With a reduced capaci- tance, the photodiode array can operate with a lower reset (kTC) noise. The ADDCAP pin is provided to control the connection of the capacitors. When ADDCAP is high, all the capacitors are connected. When ADDCAP is low, all the capacitors are disconnected. It is advised that all the photodiodes are reset after each toggle of ADDCAP. This is simply done by clocking one lines- can of the photodiode array. Anti-Blooming Circuit Each photodiode pixel has a built-in anti-blooming circuit structure. Without an anti-blooming circuit, it is possible that a fraction of the excess charge from one pixel will flow into neighboring pixels. The anti- blooming circuit prevents this by redirecting the excess current into the anti-blooming drain before the photodi- ode is too full. A self-biased anti-blooming gate sets the level at which the charge begins to flow into the drain. Think of it this way. If the photodiode were your bathroom sink, then the anti-blooming circuit would be your sink’s overflow drain. The anti-blooming circuit may be disabled by ground- ing the anti-blooming gate. This would in effect raise the drain level. Self-Scanning Circuit Figure 5 shows a simplified electrically equivalent circuit diagram of the photodiode array. An MOS read switch connects every photodiode in the array to a common output video line. Incident photons generate electron charge that is collected on each imaging photodiode while the switch is open. The shift register is activated by the start pulse. A pulse propagates through each shift register stage and activates the MOS read switches sequentially. As the shift register sequentially closes each read switch, the negative stored charge, which is proportional in amount to the light exposure, from the corresponding photodiode is readout onto the video line, QOUT. Typically, an external charge-integrating amplifier senses the nega- tive output charge on the video line from each photodi- ode pixel. The shift register continues scanning the photodiodes in sequence, until the last shift register stage is reach, at which time the fourth and last dummy pixel is read out and end-of-scan (EOS) output is held high for one clock cycle. The next start pulse can then restart the shift register. |
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