# AD8039 Absolute Maximum Ratings - Differential Input Voltage

Question asked by RnnKrk on Jul 10, 2017
Latest reply on Jul 11, 2017 by JinoL

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