Picking the right op-amp.

Thread Starter

Bod

Joined Sep 18, 2016
301
If you don't use any of the outputs from the microcontroller, then what in the world is it doing?? Surely it must be doing something...
Ok well slight lie. There's one output and it controls some LED strips but its so far from anything I dont think it's too big a deal.
I wouldn't use those converters as power sources for analog circuits if I were you. Those are switch mode power supplies and are extremely noisy and will make it difficult to get clean signals from your analog components. It's best to use a linear type power supply for analog purposes. Even an old fashioned 7805 (properly filtered) is a better choice.
Be very, VERY wary of those things, especially the cheap ones. They can produce huge amounts of noise on both the output and input pins, and if you don't use them correctly they can easily make things worse instead of better.
Ah ok interesting. So what's actually going on inside them? I thought it was just some coils - sort of like a transformer - that allows for power to flow through but means both sides are isolated.
 

OBW0549

Joined Mar 2, 2015
3,515
Ok well slight lie. There's one output and it controls some LED strips but its so far from anything I dont think it's too big a deal.
As I thought. @cmartinez's advice on separating analog and digital grounds still applies, absolutely.

Ah ok interesting. So what's actually going on inside them? I thought it was just some coils - sort of like a transformer - that allows for power to flow through but means both sides are isolated.
The innards are a high-frequency switching regulator, probably a flyback type with voltage regulation feedback from one of the secondary windings. They produce lots of noise (high frequency triangle waves/sawtooth waves on the outputs) plus BIG spikes at the switching frequency. I would never power any sensitive analog circuits from one of these without a ton of external filtering, like this:

temp.png
 

Thread Starter

Bod

Joined Sep 18, 2016
301
The innards are a high-frequency switching regulator, probably a flyback type with voltage regulation feedback from one of the secondary windings. They produce lots of noise (high frequency triangle waves/sawtooth waves on the outputs) plus BIG spikes at the switching frequency.
That is a hell of a lot filtering. But I mean it's not like they're being advertised as low noise so it's something I probably should have thought of originally. It actually is a bit more helpful not having one since I burnt out the last one I had and they're about 5 quid from RS Components which just means it makes the board a bit cheaper.
 

OBW0549

Joined Mar 2, 2015
3,515
That is a hell of a lot filtering. But I mean it's not like they're being advertised as low noise so it's something I probably should have thought of originally.
Yes, that was a lot of filtering. For that project, though, it was necessary. To determine whether filtering is necessary and if so, how much, you have to consider what kind of analog signals and circuitry are involved and how susceptible they are to interference.
 

Thread Starter

Bod

Joined Sep 18, 2016
301
Yes, that was a lot of filtering. For that project, though, it was necessary. To determine whether filtering is necessary and if so, how much, you have to consider what kind of analog signals and circuitry are involved and how susceptible they are to interference.
To be honest, I think the everything I'm going to do now is probably enough. It's not like I have a super important device on the other end that is very susceptible to noise.
I don't think it would be possible to just eliminate all noise (or not without going to many extremes) but the lower it is the easier my code becomes so I think what I'm doing now is going to be plenty better than before.
 

MrChips

Joined Oct 2, 2009
21,687
I would follow the advice and suggestions given to you so far.

1) 0.1μF ceramic SMT chip capacitors between Vcc and GND pins at every IC is mandatory.

2) Use single op-amp package instead of dual op-amp package

3) Use linear voltage regulators instead of switch-mode power supply regulators.

4) Use separate voltage regulators for analog circuitry.

5) Keep analog components and signal lines away from MCU and digital components and signals.

6) Keep analog grounds and digital grounds separate.

7) Create two ground planes on the bottom layer of the PCB, one for analog and one for digital

8) Join the two grounds at the power supply.

9) On the top layer, flood around the analog traces with analog ground plane.

10) Keep signal traces as short as possible.

1593953279850.png

Reference: http://embedded-computing.com/article-id/?2573=
 

MrChips

Joined Oct 2, 2009
21,687
You're gonna have to explain that one to me... is that because the op-amps have a tendency to interfere with one another on dual chips?
Simple.

1) We put decoupling capacitors across each active element, correct?
How are you going to decouple op-amp1 power supply rails from op-amp2 power supply rails?

2) In order to prevent cross-talk (contamination) between two circuits we want to keep them physically and electrically away from each other.

It is called social distancing for electronics.
 

Delta prime

Joined Nov 15, 2019
433
Hey, how are you .:)
I hope my response is not considered
a thread highjacking, but...off the top of my head.The thermal effects of having multiple op-amps in a single package may become a concern, as the op-amps output voltage changes so does it's thermal dissipation.
Crosstalk, in two forms, both electrical and thermal. Thermal from the wave propagating across the chip will look like low frequency feedback across the input. Electrically as supply pins are shared so a large load in a single section will cause a drop across the bond wires.
 
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