AD8039 Absolute Maximum Ratings - Differential Input Voltage

I have problem with Absolute Maximum Ratings of AD8039. The Differential Input Voltage is limited to +-4V.

May I apply a slightly higher voltage if I limit the current with the series resistor?

 

 

In my circuit I have ADA4897 Rail-to-Rail output amplifier driving AD8039.In+ configured as amplifier of Gain= +7, with Rg = 330R and Rf = 2K at In-.

Both Op-amps are supplied from the common: Vs+ = +5V, Vs- = -5V.

 

When ADA4897.Out is at 5V (RR out) the downstream AD8039 forces its output to max (about 4V) so we have about 0.6V on its In-.

Let's consider that AD8039.In- is connected to 0.6V via 280R (=330R || 2K) then.

 

But then AD8039.(In+ - In-) = 5.0V – 0.6V = 4.4 V and it is more than "Maximum Differential Input Voltage" = 4V.

So it is literally forbidden and literally leads to a complicated voltage clamping I wish to avoid.

 

It is often found that components has higher limits that indicated in AbsMaxRat. Maybe in this case too? I hope that AD8039.(In+ - In-) could withstand a little over-voltage when the current is limited? Does anybody know?

 

If I could assume that (In+ - In-) input is fragile but self-limiting to 4V than I could limit the current. Here we have 5.0V - 4.0V – 0.6V = 0.4V. So current is limited to 0.4V / 280R = 1.4mA.

 

I guess that this "4V" is mainly the max voltage of a revers biased Base/Emitter junction inside the AD8039. I do not know how fragile this junction is. It depends on the technology used.

 

 

 

I wonder whether AD8039.(In+-In-) at >4V may withstand:

- only microampers and I must put a complicated clamping,

- rather a few mA so a simple resistor is enough to limit the current?

 

 

In my circuit I expect that this over-voltage would be applied for very long periods and reliability is important to me.

 

Thank you for any help

 

Regards

Robert

  • 0
    •  Analog Employees 
    on Jul 11, 2017 7:41 PM

    Hi Robert,

    In applications where amplifiers were configured as normal Operational Amplifiers, Differential Input Voltage is not a consideration. If we go back to the principle of "virtual short", we knew that the inputs are perhaps at the same potential unless we add the effect of Input Offset Voltage. So to put it simply, the Differential Input Voltage is actually 0 V, Vos aside. This means that the Differential Input Voltage is within specifications and the Common-Mode Input Voltage should be the parameter you need to watch out. So yeah, you're safe.

    Regards.