I Have designed a circuit to acquire the EEG signal from electrode within 0-20 microvolt. My design consists of AD620 (gain = 2), -> LPF (4th order G=2.515, Fc= 0-40Hz),-> HPF (4th order G=2.515, Fc = 0.1 to 40 Hz), -> Two stage amplifier with G=180. But I am getting a constant output at AD620 of 1.28V without or with input connected. And the output after LPF filter HPF, and two stage amplifier is found to be 4.78V constantly. I am not getting why this error persists. I want to acquire EEG signal to make it in acceptible limit of ADC of 0-5V.I want to give the acquired EEG signal to 24 ADC and then to ARM Cortex M3 for further processing. Image 1 consists of Instrumentation amplifier , LPF ,HPF (4th order filter of 0-40Hz)
I can see a problematic schematic but I'm going to take a stab as to what is wrong further down the line.
Firstly the problematic schematic; there are no values on the power rails shown - you need to do this and, in addition, the negative supply to the AD620 is not connected. This is enough to make the circuit not work at all.
I'm assuming that the negative pin is connected but you are likely suffering from the inputs not staying within the common-mode range required by the AD620. What the data sheet tells you is this; the inputs MUST be within the range of -Vs+1.9 volts to +Vs-1.2 volts. The reference input is even more constrained.
This is your likely problem even if you were to connect power to the -Vs pin of the AD620.
Moving on to the next issue, the sallen key filter is connected-up incorrectly. For instance, C_2 should be connected to the right of R3 and C_1 should be connected to the right of R_3 and not directly on the output of the AD620 - this is asking for trouble.
Note that for +/-15V power, maximum output voltage swing for the TL074 is to +/-12V. The chip needs about 3V of headroom. With your 5V power supply, an input offset voltage of 10mV on the TL074, and the gain near 200 on one stage, you may well be saturating your TL074. This is aside from any of the input issues Andy was talking about.
In terms of debugging, I'd recommend boosting your power rails to +-15V to check for simple headroom problems. I would test each stage of amplification and filtering separately, or at least go from left to right, verifying that each stage is working, one at a time. To do this, I'd recommend building and testing a high pass filter, maybe using your last stage, and passing the output of each stage to that, so you can see signals more easily on a scope.
Also, you're unclear about what exactly your input is. Are you using electrodes and a person? If so, you need a third electrode to reference your subject. You should have about 500K of series resistance between this electrode and ground, for subject protection.
After all of this you still need to add 2.5V to your signal somewhere to bring the voltage into range for an ADC. I suggest using a precision voltage reference to generate a reference voltage, and use it EVERYWHERE in place of GND, instead of grounding -- and this includes the patient reference, but you REALLY need that 500K series resistor.