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OPA2134 Stability
OPA2134 Stability
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Old 25th February 2021, 08:05 PM   #51
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by RussellKinder View Post
With rail-to-rail bypass, the "junk" goes to the other rail. If the op amp has no ground references internally or in the feedback, that is OK. The delta between rails is maintained, and that is enough to prevent oscillation, in most cases.

However, power supply spikes become common-mode spikes, so supply disturbances are more likely to show up on the output with rail-to-rail decoupling. I prefer coupling from rails to ground, but if that's not feasible, or the local ground is sensitive to noise, then rail-to-rail coupling is generally OK.
Good points, yes. I often wonder if the main benefit from rail-to-rail decoupling is just providing a local 'short' to improve stability as you mention, rather than actually smoothing the rails, what with high performance regulators and opamp huge PSRR.

I normally put the decoupling cap at the back side of the PCB, between the opamp pins. Keep leads short, that's important too.

Jan
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Old 25th February 2021, 09:20 PM   #52
MarcelvdG is offline MarcelvdG  Netherlands
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Originally Posted by Bonsai View Post
I would feed a realistic rise time signal into any sims. 10 ns rise times are great to stress circuit for overshoot etc, but you will never encounter these on any music signal from any source. 2-5us is practical. You will often see overshoot in a sim (or with a fast rise/fall time generator in a practical circuit) that you will never get in practice).
That's irrelevant, as the simulation is just meant to check whether there are any pole pairs close to the imaginary axis - the step not meant to be a model of a musical transient. In my opinion, 10 ns is rather long, I would rather use 100 ps or so (or any other value << 1/(2 pi GBP)).
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Old 26th February 2021, 04:34 AM   #53
RussellKinder is offline RussellKinder  United States
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Quote:
Originally Posted by jan.didden View Post
I normally put the decoupling cap at the back side of the PCB, between the opamp pins. Keep leads short, that's important too.

Jan
If you don't have a ground plane layer, it's probably the best way. With sockets, what else can you do?
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Old 26th February 2021, 05:40 AM   #54
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by jan.didden View Post
Good points, yes. I often wonder if the main benefit from rail-to-rail decoupling is just providing a local 'short' to improve stability as you mention, rather than actually smoothing the rails, what with high performance regulators and opamp huge PSRR.
You know, the rail to rail chips are popular for low voltage applications. The inputs can typically go to the rails too on these chips.

A side benefit is that these chips are much more immune to latching up. And it seems like the good old 5532 is resistant to latch up. I've had zero problems using it as a unity gain buffer at line level.

Remember, in a typical inverting configuration, both inputs are at ground (inverting at virtual ground) and so the voltage (theoretically) always stays at zero. And also, the beta of a unity gain inverting buffer is 2; the beta of a noninverting buffer is 1.

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I normally put the decoupling cap at the back side of the PCB, between the opamp pins. Keep leads short, that's important too.

Jan
That's how I always do it.
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Old 26th February 2021, 05:45 AM   #55
MarcelvdG is offline MarcelvdG  Netherlands
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Originally Posted by MarcelvdG View Post
I would rather use 100 ps or so (or any other value << 1/(2 pi GBP)).
For example, suppose one of the circuits were at the verge of oscillating at 30 MHz. When you apply a small step with 100 ps rise time, you will see it ring severely at 30 MHz and know there is trouble.

When you apply a small step with 2 us rise time, you just see the 2 us-rise-time ramp and have to zoom in to the start or end of it to notice that there is anything ringing. If you use the wrong integration method and insufficient accuracy settings, the simulator may even step over it without showing any ringing at all.
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Old 26th February 2021, 06:30 AM   #56
jean-paul is offline jean-paul  Netherlands
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Originally Posted by RussellKinder View Post
If you don't have a ground plane layer, it's probably the best way. With sockets, what else can you do?
When using a SMD-to-DIP adapter the PCB is layed out for DIP. So, in 99% of cases there are already 2 decoupling caps (often 2 x 10 F electrolytic caps) that now are a bit too far away from the SMD IC power supply pins. One could use that GND point for the new decoupling caps and remove the old ones that are too far away. If the device originally does not have 2 decoupling caps per opamp it probably is not even worth it to try out better opamps.
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Last edited by jean-paul; 26th February 2021 at 06:36 AM.
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Old 26th February 2021, 02:36 PM   #57
Mark Tillotson is offline Mark Tillotson
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Fast opamps often need 100nF ceramic decoupling from rail-to-rail typically to stabilize them at HF. This is different to bulk decoupling of the rails to ground which is about removing sources of noise in the audio band.
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Old 26th February 2021, 03:04 PM   #58
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by RussellKinder View Post
If you don't have a ground plane layer, it's probably the best way. With sockets, what else can you do?
Here's my approach:
https://www.mouser.com/ProductDetail...xiZEABVA%3D%3D
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Old 26th February 2021, 03:40 PM   #59
RussellKinder is offline RussellKinder  United States
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I was not clear. I meant adapter PCBs to adapt SOIC to DIP sockets.

In my head, I knew what I meant, but I didn't write it. The SOIC PCB would have a V+ to V- bypass cap. There is no ground available on the adapter PCB.
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Old 26th February 2021, 05:09 PM   #60
Bonsai is offline Bonsai  Europe
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OPA2134 Stability
Quote:
Originally Posted by MarcelvdG View Post
That's irrelevant, as the simulation is just meant to check whether there are any pole pairs close to the imaginary axis - the step not meant to be a model of a musical transient. In my opinion, 10 ns is rather long, I would rather use 100 ps or so (or any other value << 1/(2 pi GBP)).
But if you feed these ultra fast rise time signal into any amp, you will provoke severe overshoot unless you have an input filter. So ideally you would need to do it without the filter. How do you know that a real world audio type opamp would even behave properly with a signal like that? What about stray capacitances? Does the sim consider these effects?

Surely a simple loop gain/phase plot would tell you if you were going to have stability problems? If yes, what is the point of looking for pole pairs near the imaginary axis by feeding in a 10 ps rise time signal that does not take into account board parasitics etc?
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