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ADA4800 - Allowable Current and Noise Density for Internal Current Source

Question asked by flyerrap18 on Nov 21, 2013
Latest reply on Nov 27, 2013 by Zoltan

     Good morning.  afternoon.  I have a customer considering the use of the ADA4800 (Low Power, High Speed CCD Buffer Amplifier) with scientific grade CCDs and he has several questions:


          1)  The first concern is with the current noise of the current source load.  Current noise on the load will develop a noise voltage across the CCD's on-chip amplifier output impedance.  That noise will then be buffered by the ADA4800 internal op-amp and show up on the output.  Is there any information on the noise current as none appears to be included in the data sheet? The customer believes that it may be somewhat noisier than the "discrete" loads they have been using, but they don't have fully quantified, measured results to share (at least not yet).


          2)  The second issue is the allowable current that can safely be input to the IN pin during power-up.  The absolute maximum ratings only state Vcc is the maximum.  Are there protection doides built in?  How much current can they handle safely?  The customer uses CCDs that have output voltages > 20V, but can put a dropping resistor, with a bypass capacitor across it, to reduce the voltage at the input to < Vcc.  However, during the power-up cycle, the voltage may go above Vcc until the capacitor is charged.  Do you have any experience or guidelines for this situation?  The customer may be able to control the rise of the CCD Vdd so the capacitor gets to equilibrium, if they know how much current they had available.  we had available.


     Thank you.