Unwanted signal in an op-amp circuit (inductive pickup?)

In summary, the speaker is still receiving a signal even though the input resistor is grounded out and there is no signal coming from the pot. The 1/2 Vcc pin is flat relative to ground and increasing the electrolytic cap did not have any effect. There are two of these circuits with the same values, but one has higher amplitude crosstalk. The crosstalk increases with frequency and there are long traces going an inch or so to the pot. Shielded cable and checking the power pins relative to ground are suggested solutions. It is suspected that there is inductive pickup in the pot and the circuit may be oscillating at a high frequency. It is advised to check for miswiring, replace the IC, and add
  • #1
Omegatron
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2
I've got a simple audio circuit that I wouldn't expect to have any signal coming out of it (the resistor at the input is a pot and is being grounded out).

But the previous stage's signal is still coming through:

http://img399.imageshack.us/img399/5804/opampinterferenceaz8.png

* It's not the contact resistance of the pot; I've soldered the terminals directly together and it's still there.
* 1/2 Vcc pin is perfectly flat relative to ground.
* Increasing the electrolytic cap didn't seem to have any effect
* There are two of these, and the crosstalk on one is higher amplitude than the other, though all the values are the same
* The crosstalk increases as frequency increases

So I'm suspecting some kind of inductive pickup in the pot (20k resistance)? Does that sound likely?
 
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  • #2
Do you have long wires going to the pot?
Could be inductive or capacitive coupling.
Use sheilded cable.

Or feed back through to power supply.
Did you check the power pins relative to ground?
 
  • #3
NoTime said:
Do you have long wires going to the pot?

Just traces going an inch or so.

Could be inductive or capacitive coupling.
Use sheilded cable.

That wouldn't work here; they're on the same board pretty close to each other.

Or feed back through to power supply.
Did you check the power pins relative to ground?

I didn't actually check it, but I was assuming it was fine, especially because there are two channels on the same dual op-amp and they have different output levels.
 
  • #4
If the 1/2vcc is flat and you shorted the c1 - to ground then power supply seems probable with 1" traces.
I'd check it.

Is it an audio amp?
 
  • #5
Smells like a miswire. Double-check your pinouts and wiring. Also replace the IC, in case it's a bad chip (preferably from a different source -- I once wasted a day with multiple bad opamp chips from the same vendor/lot). Definitely check the supply as NoTime says, but the PSRR of most (non-bad) opamps would generally preclude that path.
 
  • #6
dollars to donuts, you've built yourself a very nice high frequency oscillator.

op amps have HUGE gain, they will oscillate at the drop of a hat.. you probably need to put a low pass filter in the feedback loop, and possibly reduce the gain of the circuit.

it could well be oscillating at quite a high frequency as well, we'll 1 or 2Mhz, we'll above audio range.
(thats my guess).
 
  • #7
NoTime said:
If the 1/2vcc is flat and you shorted the c1 - to ground then power supply seems probable with 1" traces.
I'd check it.

I will.

Is it an audio amp?

Yep.

berkeman said:
Smells like a miswire. Double-check your pinouts and wiring.

Definitely a possibility.

Also replace the IC, in case it's a bad chip (preferably from a different source -- I once wasted a day with multiple bad opamp chips from the same vendor/lot). Definitely check the supply as NoTime says, but the PSRR of most (non-bad) opamps would generally preclude that path.

That's what I was thinking.

Darryl said:
dollars to donuts, you've built yourself a very nice high frequency oscillator.

op amps have HUGE gain, they will oscillate at the drop of a hat.. you probably need to put a low pass filter in the feedback loop, and possibly reduce the gain of the circuit.

Well, there's the standard frequency compensation cap in the feedback loop, limiting it to a 3 dB point around 30 kHz probably.

it could well be oscillating at quite a high frequency as well, we'll 1 or 2Mhz, we'll above audio range.
(thats my guess).

The signal I'm seeing is in the audio range, though, and the same as the previous stage, so I think it's just crosstalk of some type. That's why I suspect it's coupling into the pot. There are a lot of things in the vicinity; I just lack knowledge of how to model interference sources like that.

If I were getting capacitive coupling into my pot, for instance, it would appear like a capacitor connected from some component in the previous stage to somewhere on the pot?
 
  • #8
if its the same signal as the previous stage, and you have truly shorted out the input singnal, at the pot, ie you've tied it to zeor volts.

but the signal is still at the output, and the same as the previous stage, what happens when you remove the input signal to the previous stage.

but that original signal is not coming from the input pot from the previous stage, and if its not self oscillation. its has to be getting into your circuit somehow. then i would guess, its because you run a common power supply, and you don't have enough supply decoupling in your circuit.

what happens if you
1, remove the input signal to the previous stage. ?
2. vary the amplitude or frequency of the signal into the previous stage.
3. does your signal follow your changes, ie is it really the input sigal or its own oscillations.

how are you deriving your 1/2 VCC supply, ?

good luck, I am sure you'll get it worked out, divide and conqure.
 

Related to Unwanted signal in an op-amp circuit (inductive pickup?)

1. What is an op-amp circuit and how does it work?

An op-amp (operational amplifier) is an electronic device that amplifies the difference between two input signals. It has a high gain and can be used to perform various mathematical operations, making it a versatile component in electronic circuits. It typically has two inputs, an inverting and a non-inverting, and one output.

2. What is an unwanted signal in an op-amp circuit?

An unwanted signal in an op-amp circuit refers to any signal that is present at the output of the op-amp but is not the intended output signal. This can be caused by various factors such as noise, interference, or external signals being picked up by the circuit.

3. What is inductive pickup and how does it affect an op-amp circuit?

Inductive pickup is a type of interference that occurs when a varying magnetic field induces a current in a nearby conductor. This can occur in op-amp circuits when there are nearby power lines, motors, or other sources of magnetic fields. The induced current can interfere with the operation of the op-amp and result in unwanted signals at the output.

4. How can I prevent unwanted signals in an op-amp circuit?

There are several ways to prevent unwanted signals in an op-amp circuit, including using shielded cables, adding bypass capacitors, and placing the circuit away from potential sources of interference. Additionally, using a low-pass filter can help to filter out high-frequency signals that may be causing interference.

5. What are some troubleshooting steps for unwanted signals in an op-amp circuit?

If you are experiencing unwanted signals in your op-amp circuit, there are a few steps you can take to troubleshoot the issue. First, check all connections and make sure they are secure. Next, try adding bypass capacitors or using a shielded cable. You can also try moving the circuit away from potential sources of interference. If the problem persists, it may indicate a faulty component or a need for a more complex filtering solution.

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