AnsweredAssumed Answered

Input protection ADA4817

Question asked by TinoK on Nov 19, 2013
Latest reply on Jan 22, 2014 by AnnaFeB

Hello on and all!




Currently i am working on a CCD video amplifier circuit. The ADA4817 build up an important part as a programmable an as a instrumentational amplifier.


Now i have measured a behavior i can not explain myself...


For a simplified representation we look at the ADA4817 as a voltage follower. R_L is 500R0 and on the input we have a sample and hold stage. Supply voltage  +VS= 6.5V; -VS= -3.5V

The input voltage is from 0 to about 4V.

In OrCAD PSPICE 16.5 i have simulated a DC sweep from 0 to 5V. (blue:input; green: output; red: input curren ->1Ohm Shunt)



For me the result looks like an input protection diode that becomes conductive, due to the input current...more or less ?!

This result is the absolute horror, cause i have to reach the 4.096V... that is not possible with the current result!


Now with a 1kOhm resistor series to the input voltage. -> Current limited to Vin_max/1kOhm (4mA)



This time i can`t see an effect like in the first picture,here it looks like saturation or something else. Output current is limited to 8mA maximum and the output swing minimum is 6.5V-1.3V -> 5.2V, so what do i have here?


In conclusion:

The datasheet of the ada4817 doesn`t say anything about a limitation of the input current. In the maximum ratings, i can see that the input voltage can be +-Vs + -0.5V. So it is necessary to limit the input current to reach the maximum input voltage without an effect like in the first picture?

Do i have overlooked something in the datasheet?


Thanks a lot!!