I`m designing an picoammeter on AD549 and I want I want to add input protection.
Answer me, please, AD549 has protective diodes at the input?
Hi,AD549 safely handles any input voltage within the supply voltage range. Subjecting the input terminals beyond to voltages beyond the power supply can destroy the device. As a general rule, the absolute maximum current the input pin should be limited to 5mA. If over-voltage is expected at the input, external components for over voltage protection may be needed.May I know how high the voltage you are expecting at the input and how long will the device be exposed to such extents?You may want to read this tutorial for in-depth infromation about OV protection and its effects and how to design one: Op Amp Output Phase-Reversal and Input Over-Voltage Protection.
This is only a short-term overload at the input. I believe that the protection against accidental supply to the input of 100 volts to be quite sufficient
I would like to apply a typical protection scheme from measuring equipment. For example
But I fear that JFETs in the diode connection will cause serious leakage currents
or a diagram from the brand-name picoammeter service manual
No one in ADI Corporation knows how the AD549 works! Ok, I found the internal diagram of AD549 myself. This IC does not have protective diodes on inputs.
In input stage uses a JFETs with two gates. I think that when the input voltage exceeds supply voltage, p-n junction of gate will be forward biased. If I added high value resistor, which will limit forward current through gate, in series with input, this will be adequate method of protecting?
Hi Kirill,The failure of the device will not be due to the overvoltage itself but the current that the overvoltage produces that flows to the input pin. Adding a series resistor and limiting the input current to 5mA(rule of thumb) will do the work. But be wary that continuously exposing the device to such extents may cause a change in parameters over time. Also, you should limit the series resistor's value as low as possible (expected overvoltage/5mA) to minimize offset and noise errors.Apologies for replying late as I have been looking for a very low leakage diode that suits your application. What I've seen so far are in the range of nA which is way higher than what you are trying to sense.Regards,Goz
Thank you, Goz!
For these purposes are used FETs the role of the diodes. For example, 2N4117 this is classical solution.
Offset voltage is applied to these diodes (for AD549 no more 300μV) and no processes in p-n junction occur. Flows leakage currents only.
This is my idea:
What do you say about it ?