Hi everyone, I built an ac coupled non inverting amplifier for amplifing 2 mV input signal. Problem is when I connect output of opamp to the oscilloscope and connect supply opamp with +-9 V, I see signal with 50 Hz as shown in Figure below.When I off -+ 9V, the 50 hz is gone. Circuit schema is attached also. What should I do for get rid of this signal and what is the reason of this signal? regards
1) check if you have correctly applied the decoupling capacitor (between the +9 and -9 of the opamp)
2)check if the ground of the osciloscope probe is close enough to the ground of your circuit, ideally they should be making contact.
please upload a picture of how you connecting the probe for more help.
I hope you get rid of the problem soon!
Namig,
what is the output voltage when you short circuit the input. One of the strong possible causes for the coupling with 50 Hz supply may be the power supply, in case of amplifying very small signals is to use very good regulated and filtered power supply.
One of the quick tests is to use battery operated amplifier to locate the origin of the 50 Hz at the output.
The other strong reason circuit the high input impedance which makes a high impedance coupling circuit to the nearby or stary 50 Hz source. The resistance R1 in your circuit is very high and you have to reduce it drastically.
If you still have 50 HZ then you have to eliminate it at the output by suing a notch filter.
There are other trick to mitigate the interference of the 50 Hz such as using differential input amplifier rather than single ended input. The last trick is to screen the whole circuit inside an iron box.
wish you success
Congratulations! The amplifier is working. What you see is most likely the omnipresent coupling from the 50 Hz grid.
Shielding the input (preferably driving the shield with the voltage from pin 2 - eventually through a buffer amp) should be able to cope with this noise pickup. A high input impedance always results in picking up noise...
As Prof. Zekry recommended, shielding the whole amplifier will add to signal quality, but shielding the input is mandatory in your case.
BTW: I haven't run your circuit through the simulation, but C5 is quite unfamiliar. Is it really necessary or could you do without it?
As U Dreher points out polarised (electrolytic) capacitors are not usually designed to operate with close to zero bias voltage.
How important is the 16 Hz high pass response? If you need a second order high pass response, consider using a Sallen and Keys configuration, as described in the second half of this page: http://sim.okawa-denshi.jp/en/OPseikiHikeisan.htm
please check the followings:
Check the ground connection..
Check the voltage between neutral and ground, of the input AC power supply, connected to DC power supply and scope. It must be zero..
keep all leads very short to avoid supply line pick up, preferably use twisted pair of wires.
C5 appears to be too large. It may be 10 times C7.
best wishes
@V.T.Ingole: reducing C5 to 150 pF will reduce the amplifier gain from x100 to +x10, and raise the low frequency high pass corner frequency from 25 Hz to around 100 kHz, which is unlikely to be acceptable.
Replacing C5 by a short circuit will preserve approximately the same gain and 95 kHz bandwidth, but degrades the rejection of frequencies below 25 Hz. As stated previously, a Sallen and Keys configuration can be used if a second order high pass response is essential. Alternatively, use a low leakage non-polar capacitor for C5.
desired frequency amplitude response should have been mentioned.
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