# CCD Gain - OfficinaTurini

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As we have already seen (A/D conversion), the acquisition system convertes the outgoing tension associated to the single pixel in digital and afterwards it treats it as number or ADUs (Analog-to-digital units). The necessary tension ( that is the number of received electrons or photons ) to produce one ADU is the gain of the device.

With a typical gain of 10 e/ADU are therefore necessary 10 electrons to give the outgoing value of 1. If a pixel captures 1000 electrons, the outgoing value will be 100 ADUs. With 17.234 electrons we will have 1723 ADUs and not 1723,4 as the exit can only assume integer values. As it is possible to guess, the device resolution is tight connected to the gain, whose value depends in his turn on the type of A/D converter.

Normally for the cameras it is possible to set up various gain values. In a camera that has an only one possible choise, one typically optimizes the maximum count of the A/D converter with the maximum FWC of the sensor, in practice if a CCD sensor has a FWC of 200000 e- and we use a 16 bit converter we will have:

This way it is easy finding the limit operating. For example if we use the binning, we can exploit the reduction capacity of exit node which is greater than the single pixel. Therefore we can have bigger outgoing signals, what will not however be able to exploit if we will not be able to decrease the gain of the system. There are different other cases where the control of gain is useful, it is certainly a further resource in the functional parameters of the camera.

Here below  a table reporting the data of all DTA's old and new CCD cameras:

Come abbiamo visto (Conversione A/D) la tensione in uscita associata al singolo pixel viene convertita in digitale e trattata in seguito come numero o ADU (Analog-to-digital units). La tensione necessaria (ovvero il numero di elettroni o fotoni ricevuti) per produrre una ADU e' il guadagno dello strumento.

Con un guadagno tipico di 10 e/ADU sono quindi necessari 10 elettroni per dare in uscita il valore 1. Se un pixel cattura 1000 elettroni, il valore in uscita sarà 100 ADUs. Con 17.234 elettroni avremo 1723 ADUs e non 1723,4 in quanto l’uscita puo' assumere solo valori interi. Come si puo' intuire, la risoluzione del dispositivo e' strettamente collegata al guadagno, il cui valore dipende a sua volta dal tipo di convertitore A/D.

Normalmente nelle camere si possono impostare diversi valori di guadagno. In una camera che ne ha uno solo tipicamente si ottimizza il massimo conteggio del convertitore A/D con la massima FWC del sensore, in pratica se un sensore CCD ha una FWC di 200000 e- e si utilizza un convertitore a 16 bit avremo: