Gold

Looking for OpAmp Oscillation
« on: July 26, 2019, 12:44:22 PM »
I haven’t been successful in recognizing low level oscillation on an oscilloscope.

Yesterday I noticed a higher noise floor on one channel of a line amp. I touched the op amp and it was hot. I then realized I forgot to put rail bypass caps in that channel.

Before I fixed it I tried to look at the oscillation on the scope but it looked like broadband noise. I couldn’t tune in on a specific frequency.  Measured with the probe on the output pin.

Am I doing something wrong? Could someone post a snapshot of op amp oscillation on a scope?


JohnRoberts

Re: Looking for OpAmp Oscillation
« Reply #1 on: July 26, 2019, 01:50:45 PM »
The frequency of the oscillation may be above what the scope can easily sync/lock to.

When you hear oscillation as wide band noise, you are actually hearing rectification from the circuitry not being fast enough to keep up.

This is the same mechanism as radio station interference in input gain stages, but in the RF case the rectification decodes the radio signal modulating the RF carrier. For a relatively simple instability oscillation it decodes randomness in the oscillation (that may also make it harder for the scope to sync to).

JR

[edit if you see an unusually thick audio trace with the scope , that often suggests low level oscillation... [/edit]
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

benb

Re: Looking for OpAmp Oscillation
« Reply #2 on: July 26, 2019, 02:24:53 PM »
What's the model and bandwidth of the scope? Are you using a 1 to 1 or a 10 to 1 probe? A 10 to 1 is preferred due to its higher resistance and lower capacitance.

You could try putting the probe very close to  (like 1/4 inch) the output pin and see if that shows up on the scope.  The capacitance of the probe tip could change what the pin is doing, and a small "capacitive" connection might show it better.

moamps

Re: Looking for OpAmp Oscillation
« Reply #3 on: July 26, 2019, 02:37:20 PM »
...A 10 to 1 is preferred due to its higher resistance and lower capacitance....The capacitance of the probe tip could change what the pin is doing, and a small "capacitive" connection might show it better.

Agreed, just to add that sometimes probe's capacitance can stop temporary the oscillation at all.

Gold

Re: Looking for OpAmp Oscillation
« Reply #4 on: July 26, 2019, 03:28:16 PM »
What's the model and bandwidth of the scope? Are you using a 1 to 1 or a 10 to 1 probe? A 10 to 1 is preferred due to its higher resistance and lower capacitance.

You could try putting the probe very close to  (like 1/4 inch) the output pin and see if that shows up on the scope.  The capacitance of the probe tip could change what the pin is doing, and a small "capacitive" connection might show it better.

It's a Tek TAS 465 100MHz. I'm using a probe that has a a switch for 1x and 10x. I was using the 1x. I'll try the 10x next time.

Audio1Man

Re: Looking for OpAmp Oscillation
« Reply #5 on: July 26, 2019, 04:26:31 PM »
Hi Gold

I use my $3 Spectrum Analyzer, it is  AM battery powered radio with a loopstick antenna. Tune it to a blank station and move it around the PCB. If it goes silence the oscillation has no modulation, if it get louder and has a buzz the circuit has modulation of some type.

Checking for oscillation with a scope use a 10:1 probe and a 200 Ohm resistor is series with the tip (cut the leads short) this will reduce the extra stray C.
Duke

Gold

Re: Looking for OpAmp Oscillation
« Reply #6 on: July 26, 2019, 04:46:34 PM »
Hi Gold

I use my $3 Spectrum Analyzer, it is  AM battery powered radio with a loopstick antenna. Tune it to a blank station and move it around the PCB. If it goes silence the oscillation has no modulation, if it get louder and has a buzz the circuit has modulation of some type.

Checking for oscillation with a scope use a 10:1 probe and a 200 Ohm resistor is series with the tip (cut the leads short) this will reduce the extra stray C.
Duke

Thanks for the tips. I think a battery powered AM radio is in the budget. I'll pick one up.

JohnRoberts

Re: Looking for OpAmp Oscillation
« Reply #7 on: July 26, 2019, 05:00:17 PM »
Agreed, just to add that sometimes probe's capacitance can stop temporary the oscillation at all.
ding ding ding... I have heard stories of huge one-off military/government electronic system builds, shipped with a scope probe still attached, hidden inside.  ::)

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.


 

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