The problem of amplitude decay in the output of noninverting summing amplifier

Thread Starter

btomekce

Joined Sep 23, 2019
5
Hello, I need to sum an AC and DC signal in the order of milivolts, at most 1 V. To do this, I built a non-inverting summing amplifier circuit as in this diagram.
summing_non_inverting.jpg
I tried various resistances with values 10k, 100k and 1000k, keeping all of them equal in the circuit. Although the output signal is correct, it remains for a few seconds; the amplitude of the wave decays quickly. What may be the cause of this? Could it be related to the op-amp? I tried a few types of op-amps. The result didn't change much. Thanks
 

danadak

Joined Mar 10, 2018
4,057
Sure sounds like a connection that is more capacitive than ohmic in your
breadboard. Some signals one row off in the breadboard ?


Regards, Dana.
 

OBW0549

Joined Mar 2, 2015
3,566
That's not a complete schematic.
What op amp and what supply voltage(s)?
Also, what is providing those supply voltages? A regulator? Are there power supply decoupling capacitors connected to the supply pins of the op amp? What is providing the two input voltages V1 and V2? Whichever one is the AC signal, what frequency is it? What's connected to Vout, and what resistance/impedance does it have? Have you checked, double-checked and triple-checked your wiring?

We need to know all these things, else you're only going to get wild-assed guesses as to what might be wrong.

As drawn, and with appropriate power supply voltages, supply decoupling capacitors and loading on Vout, the circuit should work as advertised since there's nothing odd about it-- it's just a run-of-the-mill non-inverting summing amplifier.
 

crutschow

Joined Mar 14, 2008
25,692
As drawn, and with appropriate power supply voltages, supply decoupling capacitors and loading on Vout, the circuit should work as advertised since there's nothing odd about it-- it's just a run-of-the-mill non-inverting summing amplifier.
As the saying goes "The devil is in the details". ;)
 

Thread Starter

btomekce

Joined Sep 23, 2019
5
Also, what is providing those supply voltages? A regulator? Are there power supply decoupling capacitors connected to the supply pins of the op amp? What is providing the two input voltages V1 and V2? Whichever one is the AC signal, what frequency is it? What's connected to Vout, and what resistance/impedance does it have? Have you checked, double-checked and triple-checked your wiring?

We need to know all these things, else you're only going to get wild-assed guesses as to what might be wrong.

As drawn, and with appropriate power supply voltages, supply decoupling capacitors and loading on Vout, the circuit should work as advertised since there's nothing odd about it-- it's just a run-of-the-mill non-inverting summing amplifier.
I tried UA741CN with supply voltages +-15 V, LF351N with +-15 V, LM358N +30 V (in the data sheet 3 to 30 V written for supply voltage), and LF347 with +-15 V. Supply voltages are provided by a DC power supply.
For AC input I'm using a function generator with Vpp: 100 mV to 1 V and f: from 100Hz up to 1 kHz
For DC input, I'm using a DC power supply with ranges 100 mV to 1 V.
I haven't connected any capacitors. I built the circuit exactly as in the image. I checked my wiring many times and simulated this circuit, it worked in the simulation perfectly.
I think I need to add capacitors to the inputs to increase the input empedance and probably to the output. I'm not an electronics student so I can't understand the details when I searched the forums and websites.
Thank you
 

Thread Starter

btomekce

Joined Sep 23, 2019
5
Except you don't show the supplies.
Are the power supply commons connected to the signal common?

If you posted the complete schematic, we wouldn't have to keep asking questions. :rolleyes:
Yes, I connected everything correctly. It worked properly when I changed the opamp. But now the setup has to work with a lock-in amplifier's oscillator instead of function generator. I think I'm having impedance mismatching problem. The lock-in amplifier has lower impedance compared to my circuit. The AC signal encounters 100k resistance. I tried making a buffer
 

Thread Starter

btomekce

Joined Sep 23, 2019
5
Voltage-follower.png The AC signal encounters 100k resistance and reflects back attenuating the signal. I tried adding a buffer amplifier as in the image. But it didn't improve. I know that for a buffer the input must have higher impedance and the output must have lower.
My case is the opposite. Is there any solution?
 
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