Why Does My Summing Amplifier Clip Below 1 Volt?

[EngrHdr]

Hi everyone,
I am trying to make a Summing Amlifier. I have simulated on computer where it is working correctly, but on real circuit it is showing some mal functioning. I am using OP 27 Op Apmp. The circuit diagram is attached.
When I try to add DC voltage to AC sine wave, initially sine wave goes up as required but after even below 1 volt it starts clipping and hence I am not able to rise up the sine wave to about 5 to 8 volts.
I am also confused how to use dual supply for +15V and -15V. I think may be this problem is because of dual supply.
Please help me.
Thank you.

 

[meBigGuy]

Your dual supplies seem correct in the sim.

Since the circuit simulates, I expect it is a wiring error, sloppy grounds, or poor bypassing.
You need to look at the signals and voltages on all the pins 1 by 1 until something seems off.

pin 2 should be at ground, if not, that amp is not working correctly, etc

Check the first amp, see it is is clipping, then the second. You just have to proceed logically.

Do you have an oscilloscope? Do you see any oscillation? Can you source a square wave and see if there is ringing, etc etc etc

 

[EngrHdr]

Thanks for your reply. Yes I am using an oscilloscope. Even i tried with a square wave, but still faced the same problem. I can just see a clipped wave. As I raise DC input. The input sine wave rises a little and starts clipping just before 1 volt rise. In real it should start clipping near 12V as accoding to data sheet the output swing is around 12V. But the problem is it starts clipping at about 0.6 or 0.7 V.

 

[davenn]

try lifting the + input from ground with a, say, 2k2 resistor

 

[EngrHdr]

try lifting the + input from ground with a, say, 2k2 resistor

Sorry Sir, I didn't get you. Will u please explain it.
Thank you.

 

[davenn]

Well going by your circuit, at the moment the + input to the OP27 is going to ground
insert a resistor so its not directly going to ground

Many of the summing amp examples I have seen use that method

 

[EngrHdr]

try lifting the + input from ground with a, say, 2k2 resistor

I understood what you mean by lifting +input from ground. But will you explain me how with a 2K2 resistor. If possible with a ckt diagram please.

 

[EngrHdr]

Well going by your circuit, at the moment the + input to the OP27 is going to ground
insert a resistor so its not directly going to ground

Many of the summing amp examples I have seen use that method

Should I just insert a resistor in between + input and ground? or I have to make some connection with Vcc also?

 

[davenn]

 

[EngrHdr]

View attachment 89299

Thanks a lot Sir,
I'll try it.

 

[meBigGuy]

Just deal with 1 amp at a time. Again, if the sim works and the circuit doesn't, generally you have wiring errors. It could also be bypassing or grounding if you have "invisible" oscillation (too high freq for the scope)

Does the first amp clip? If so, ground the scope at pin 3. Set the input signal so the output is clamping by 1V. Look at the + supply, the - supply, and pin 2. + and minus supply should be +15 and -15 with no variation. When the output clamps, the - input might go above or below ground by the amount of clipping. That should not be possible when the amp is working correctly. If, on the other hand, the - input remains at ground when clipping is occurring, then for some reason the output stage is clipping. I would tend to suspect the power supply connections.

The easiest mistake to make is to number the pins wrong. Double check the data sheet.

It is also possible that the opamp is burned out because of a previous error.

Pull the opamps out and make sure the signals make sense. Then connect wires from 2 to 6 and make sure the signal levels still make sense.

If none of that produces a clue, then I'm stumped.

Post a picture of the layout.

 

[EngrHdr]

I would like to thank everyone for your sincere comments. You helped me a lot. As I had wasted a lot of time on this circuit but still it didn't work. So I decided to make another citcuit just by changing the resistors to 2.2KOhm. May be there was some wiring mistake or something else in previous circuit, but this circuit is working well.
Once again thanks to everyone for your help.

 

[davenn]

thanks for checking back in with an update

good to hear its now workingD