AD623 unstable and not amplifying

Hello,

your observation could be explained, if the voltage source at the input has no defined GND-relation to the supply of the amplifier.

Your signal source delivers 200mV differential voltage, i.e. the voltage between Pin 2 and Pin 3 is 200mV. But what is the input common mode voltage, i.e. what do you measure between Pin 2 and Pin 5 or between Pin 3 and Pin 5?

In the diamond plot you state, that the input common mode voltage is between 0V and 2,4V. Is this ensured by the source of your signal, or is your signal source floating?

If it is floating: try to ground one end of your source (i.e. connect pin 2 to pin 5) to ensure the correct common mode voltage.

best regards

Achim

Hello,

your observation could be explained, if the voltage source at the input has no defined GND-relation to the supply of the amplifier.

Your signal source delivers 200mV differential voltage, i.e. the voltage between Pin 2 and Pin 3 is 200mV. But what is the input common mode voltage, i.e. what do you measure between Pin 2 and Pin 5 or between Pin 3 and Pin 5?

In the diamond plot you state, that the input common mode voltage is between 0V and 2,4V. Is this ensured by the source of your signal, or is your signal source floating?

If it is floating: try to ground one end of your source (i.e. connect pin 2 to pin 5) to ensure the correct common mode voltage.

best regards

Achim

Thanks a lot for the quick answer. I found 0.25V in the common mode. In order to avoid this kind of porblem I put two 1MOhm resistor beetween ground and both inputs, but the problem is still here. I'm very surprised..

Thanks again Achim.

Juan

Hello Juan,

EB3BNJ said:

I found 0.25V in the common mode

The input bias current of the AD623 can go up to 25nA. If your multimeter has an input resistance of 10MOhm (a typical value), then the 0,25V would have been created by 25nA*10MOhm. So the source was indeed floating and you "grounded" it with the 10MOhm of your multimeter. That fit's nicely.

EB3BNJ said:

I put two 1MOhm resistor beetween ground and both inputs, but the problem is still here.

Ok, that does not really fit. 1MOhm should be low impedant enough to provide the input bias current. If the problem persists, I have no idea based on the description you gave. I could only imagine, that something in the described setup and the real setup does not match perfectly. Maybe theres an open connection/a bad soldering.

In your case, I would once again measure the voltages at every single pin with reference to pin 5. I'd measure directly at the pin of the chip (not at some trace which "should be connected" to the pin). Maybe a table of the measured voltages could give a hint for further failure analysis.

best regards

Achim

Thanks Achim. I will measure all again because something, maybe a bad soldering or so it's giving me all my headaches.

Thanks again.