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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)

 

Sim.png

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)

 

Sim2.png

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!!

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