Problem with using AD625 amplifiers

Hi, I have bought 10 AD625 instrument amplifier. In its datasheet it has been noted that differential resistance is about 1G Ohm and also common mode resistance has the same value. when I put one AD625 according to datasheet in a circuit for fixed gain configuration(page 9 of datasheet) I found out that input draws considerable current about 2.5 mA. supply voltage was +/-12V and input voltages were 0V and -3.5V gain is 50. I checked voltage of all pins for many times and all were according to datasheet. I replaced the opamp but result did not change. I measured resistance between +input and - input via a multimeter when they were not connected to any circuit or voltage and it was 700Kohm(it was about the same for all AD625s). Can someone please help me?
This is a link to data sheet
https://www.analog.com/UploadedFiles/Data_Sheets/AD625.pdf

Re: AD625 big problem

With +/-12V supply try to connect both inputs through (1kΩ) resistors to 0V and measure voltage on input pins ..

Also, do you have any input protection in place?

Regards,
IanP

AD625 big problem

Thanks a lot IanP, I did what you said this is the result:
All the measurements were made when inputs had no connection to anywhere except to ground via 1K resistors

V Input+ = 0.0 mV (Relative to ground)
V Input- = 0.0 mV (Relative to ground)
Diff Input Voltage = -0.4mV (V Input + Relative to V Input -)


About protection, input- directly is connected to a DAC that has a output of +/-10V so I did not provide protection for this pin. Input+ connects to an analog switch which may give +/-15V at rare occasions and it has been protected by two reverse biased diodes one connected to +12V and one connected to -12V.

During experiments there could be no over voltage condition because I removed the analog switch and I fed input+ via a power supply with -3.5V and I also removed DAC and connected input- directly to ground.
Can all 10 AD625s be broken or counterfeit?
Datasheet indicates that input maximum bias current is +/-50nA, how inputs can have about 700K ohm resistance? It should be Gohm range shouldn’t it?
Thanks a lot for your time.

Re: AD625 big problem

fala,
If I understand your description correctly, you have the amplifier set up for a gain of 50. With a 3.5 volt differential input, the input stage will saturate. Input resistance specs are valid only when the device is operating in its linear region. Also, input resistance measurements made with a multimeter are not valid, since th input R spec applies only with power applied, and the amplifier operating in its linear region. If you use a multimeter to measure input resistance with power applied to the device, the voltage that the multimeter (used as an Ohmmeter) will probably also cause the amplifier to saturate, so the reading will not be valid.
Regards,
Kral

AD625 big problem

Kral, Thank you very much indeed. Still one thing confuses me. I recall that last night when I measured the input current within linear range it was much lower but still considerable. If I recall correctly with 100 mV diff input I still had about 3uA input current but to confirm that I will measure the input current within linear range and I will report it in a few hours from now.
Thank you

Added after 1 hours 2 minutes:

I think you were right, first I did the measurement using an unregulated supply that had been lowered through a multi-turn volume to generate about 100mV to feed it as input+ to AD625 other input was grounded
This was the result
Diff Input ~150 mV
Output Voltage ~8V (Linear range)
Input current -5.0 – 5.2 uA
Then I repeated the experiment using a much more precise supply this was the result
Diff Input = 150 mV
Output = 7.6 V
Input Current = -0.4uA
I still don’t know why using a regulated input made such a significant difference but the result indicates that probably I made a mistake somewhere so probably input current is even lower if I investigate thoroughly
Thank you very much indeed for your time Kral and IanP.
I wish the best for you.