Circuit - oscilloscoop result

[Freehawk]

Hi,

I have made an audio analog 'simple' circuit, it has onee audio input and one audio output (via opamp)
The gain is 1

When i probe the incoming and outgoing signals with oscilloscope i see a much fatter noisier line in the out than in the in.

What could be the cause of that? And is it a big problem or not?

when i listen to the sound i hear no difference between orignal sound and outgoing sound

In attachment screenshot of oscilloscope probes

 

[KlausST]

Hi,

I have made an audio analog 'simple' circuit, it has onee audio input and one audio output (via opamp)
The gain is 1

When i probe the incoming and outgoing signals with oscilloscope i see a much fatter noisier line in the out than in the in.

A vague description of your circuit. Nothing we could verify. Why don´t you post the circuit?
Maybe you see just some class D output.

***

What could be the cause of that?

* Either there IS higher noise
* or the scope wrongly shows higher noise. --> just interchange the channels.

And is it a big problem or not?

To whom?
To me not, for the OPAMP not, for the scope not....
If it´s not a problem for you ....

BTW: you say gain = 1, but I see gain > 1.

Klaus

 

[Freehawk]

Hi,


A vague description of your circuit. Nothing we could verify. Why don´t you post the circuit?
Maybe you see just some class D output.

***

* Either there IS higher noise
* or the scope wrongly shows higher noise. --> just interchange the channels.


To whom?
To me not, for the OPAMP not, for the scope not....
If it´s not a problem for you ....

BTW: you say gain = 1, but I see gain > 1.

Klaus

Well



This is the circuit

 

[KlausST]

Hi,

The circuit and the OPAMP is not the reason for the highly increased noise.

If the noise really exists (no measurement error) then maybe it´s
* a wiring problem
* ground loop problem
* power supply problem (switch mode? ... maybe in hickup mode?)
* something else...

Klaus

 

[Freehawk]

Hi,

The circuit and the OPAMP is not the reason for the highly increased noise.

If the noise really exists (no measurement error) then maybe it´s
* a wiring problem
* ground loop problem
* power supply problem (switch mode? ... maybe in hickup mode?)
* something else...

Klaus

Yes but how do one solve this, how to check wiring, groud loop, something else?
Because it's al soldered down well and i cant find any misconnections (it's a simple circuit so not so much to check)

 

[wwfeldman]

things to try:

swap probes at measuring point.
swap probes at connection to scope
put both probes on the same signal
check ground for both probes - ground them at the same location

most 'scopes have a scope calibration signal - use it to ensure both scope probes are calibrated

if the "loop" formed by the scope probe and its ground wire is "large" it could act like an antenna and pick up radio signals
wrap the ground wire around the probe to reduce the size of the loop.

the bypass capacitors should be as close to the power pins of the op amp and possible - that is, as close to +V / GND as possible and as close to _V/GND as possible.

the two gain resistors also should be close to the op amp.
make sure R3 really is grounded. the circuit will act as a follower if R3 is not grounded, essentially as it does with gain of 1
in fact, since the circuit is a non-inverting amplifier, the gain should be 1 + r2/ r3, or a gain of 2. - so check that r3 connections.

the behavior of your circuit , gain =1 indicates r3 is not connected to ground.
if it was, the gain would be 2.

 

[Audioguru]

I suspect that it is built on a solderless breadboard and the wires that are all over the place are antennas that pickup all kinds of interference.

 

[Freehawk]

It's indeed a soldeerless breadboard, i wouldn't really say the wires ar all over the place.
But there are wires indeed. but if that's the problem then i would be solved making a pcb?

I hope so in fact, because then i can more or less neglect the "prob"

 

[wwfeldman]

I take it you still have the noise on one channel
try stretching the time base in the microsecond region where you can see the frequency on top of your signal that appears as noise
if both signals show the same noise, the op amp is amplifying the noise
put a low pass filter on output - it looks like your input signal is about 250 Hz (period is about 4 ms)
1000 ohms and 1 uF cuts off at about 1600 Hz

that should take out the noise.

note this only works well if the cutoff is beyond (x2 minimum) the maximum frequency your circuit is designed for and less than the noise frequency
it also introduces a phase shift

this may help
https://www.electronics-tutorials.ws/filter/filter_2.html

 

[KlausST]

Hi,

Yes but how do one solve this, how to check wiring, groud loop, something else?
Because it's al soldered down well and i cant find any misconnections (it's a simple circuit so not so much to check)

You might say it is a simple circuit. But we professionals see it with different eyes.
We look for wire length, signal paths and their return path, loops and their size, capacitors at power supply...and so on.

Thus if you want us to check it (btw: including your scope connections) you just post a couple of photos.

Klaus

 

[Audioguru]

The AD8656 opamp you show has very low noise and a very wide 28MHz bandwidth. Your circuit should have a gain of 2 but your oscilloSCOOP shows less than 2. I think the long wires and many rows of contacts on a solderless breadboard is causing the opamp to oscillate at a frequency higher than the 'SCOOP can show.

Is your opamp a genuine Analog Devices one or is it a fake one sold "over there" on ebay?

 

[Freehawk]

Hi,

It's a genuine Analog devices AD8656ARZ
https://benl.rs-online.com/web/p/op...34313233323126&searchHistory={"enabled":true}

 

[Audioguru]

The problem is :
1) One defective channel on the 'SCOOP.
2) Defective opamp.
3) Solderless breadboard.

 

[Freehawk]

Dont think defective channel on the scoop (i interchanged them, same result)
defective op-amp? maybe

But i rather go for the solderless breadboard in first place, try to solve that and then i'll see from there

 

[xjackal]

are you sure this is the circuit?
why not to prepare this one

 

[barry]

are you sure this is the circuit?
why not to prepare this one
View attachment 152327

Why not prepare that one? Maybe because that's a totally different circuit.

 

[KlausST]

Hi,

It seems it is a randomly chosen circuit from the datasheet: THD + N test circuit

Klaus

 

[Freehawk]

Ok next 'problem'.

I now have a clean signal when nothing is connected to the output.
I just added 4.7uF decoupling caps on +/- 2.5V VCC

But from thee moment i connect the output to the next stage (input of an audio interface/mixer) (rme fireface)
There is noise again (less then beforee) on the output signal.

As soon as i disconnect, the signal is clear.. so it's not really in my circuit?

Just asking

 

[xjackal]

can you post the scheme?