I'm an electronic engineering undergrad and your discussions helped me a lot. As a project, i was instructed to design the front end for a data acquisition system. The specs as below.
- Supply +/- 5V DC
- AD 8065 (This was specifically given)
- Two attenuation modes 1/10 and 1/100 (I properly tuned and tested it)
- Input impedance is 1 MOhm and 15 pF (Same as an oscilloscope)
- Amplifier gain is 2 non-inverting
So, I built the example circuit given in the datasheet and connected it to the attenuator. The problem is i'm getting a huge offset around 200mV when the attenuator input is zero. It is also changing when i change between two attenuations. But the offset indicated in the datasheet is around 1-2 mV. I tried simulation with LTspice but it didn't indicate any offset like this.
After playing around a bit (I just connected a pot to the inverting input and turned it until the offset is zero) , I've found a solution as this.
For the 1/10 attenuation
For the 1/100 attenuation
After adding the resistor, the offset is almost zero volts but i think this is not a standard topology. To be sure, i have tested the circuit with 2 ICs but the results are same. I also tried adding a series resistor to the non inverting input but it is increasing the offset.
I ordered the chips from the Aliexpress so i'm not sure about their authenticity. Can anyone help me with this? I just want to decrease the offset without affecting the amplifier operation. I'm not sure where the problem is.