Split supply opamp decoupling?

[cupoftea]

Hi,
Doing a very busy board with slow comms and linear regulators on it driven by opamps.
Also have other opamps doing other things (buffering etc)
The opamps drive the low side PNP bases with -3v3, so that they can deffo
get down to zero volts out.
(The linear regs may also have to sink current from the output)
Opamp rails are 20v and -3v3.
Some of the opamp inputs are 0v referenced.
I would really like both caps from both rails to 0v, and caps from 20v to -3v3, but there is not room
Considering this, i will elect for cap from 0v to -3v3, and cap from 0v to 20v.
(no caps from 20v to -3v3)
Would you say this is a good compromise?

 

[danadakk]

Considering this, i will elect for cap from 0v to -3v3, and cap from 0v to 20v.
(no caps from 20v to -3v3)
Would you say this is a good compromise?

Yes.

Why do you have regs, that source, sink ? That has to be called out in datasheet.


Regards, Dana.

 

[crutschow]

Would you say this is a good compromise?

You don't need the caps from 20V ot -3.3V.
What caps are you using otherwise?

 

[D.A.(Tony)Stewart]

There never is a need to decouple Vcc to Vee in bipolar Op Amp supplies. which is already done by equivalence with 2 caps in series with 0V in between.

Why makes you think so otherwise?

 

[danadakk]

Decoupling Vcc to Vee, depends on OpAmp, read datasheets for supply
bypassing recommendations :

https://www.analog.com/media/en/technical-documentation/application-notes/AN-202.pdf

https://www.ti.com/lit/an/sloa069/sloa069.pdf

Regards, Dana.

 

[KlausST]

Hi,

I´ve worked with NE5532 and NE5534 in the past. They are not the same. (I also had to learn it the hard way)

The NE5523 is stable for gain >= +1
While the NE5534 is stable for gain >= +3

They are not considered to be high frequency, nor critical in decoupling. If the person encounters stability problems, then most probably it´s not because of the decoupling technique.

***

Also the statement: "... some sort of internal oscillation that degrades linearity without being visible on a normal oscilloscope"
is more than "non technical"
* some sort of
* internal oscillation
* not visible

****

Also the statement: "every capacitor between a supply rail and GND carries the risk of injecting rail noise to the GND".. is ridicoulos.

I want to ephasize this: GND IS THE reference. If you can´t rely on GND then you need to rectify this problem first.
GND has to be stable .. it has to be like a foundation of a house.

And then it´s not a "risk" to inject noise to GND, it is the "job" of the capacitor to conduct HF to the GND.
If the decoupling capacitor causes noise on his GND then he should improve his understanding on current loops and improve his PCB routing skills

****

The TI and Analg Devices documents are reliable.
And yes, the NE5532 datasheet is rather clear in how to decpule the power supplies:
"Connect low-ESR, 0.1-μF ceramic bypass capacitors between each supply pin and ground, placed as
close to the device as possible. A single bypass capacitor from V+ to ground is applicable for single
supply applications."

***

I recommend to simply draw the (HF) loop of the output current. Then you will automatically see where a capacitor is useful.

****

Klaus

 

[danadakk]

High Frequency definition :

High frequency - Wikipedia

en.wikipedia.org

Amateur radio frequency allocations - Wikipedia

en.wikipedia.org

For me uA709's were HF work.


More interesting dialog on bypassing and actual design experiences :

opamps and local decoupling of rails, some questions

maybe this has been discussed an awful lot of times, so just point me to the correct threads. Let's assume a somewhat generic scenario. We have a mixer, or some other audio device with a large number of opamps (+10). We have a somewhat generic dual rail PSU, designed around LM317/LM337 with...

groupdiy.com

Also the statement: "... some sort of internal oscillation that degrades linearity without being visible on a normal oscilloscope"
is more than "non technical"
* some sort of
* internal oscillation
* not visible

I have had the good fortune of having parts oscillate internally, heat up, but work scope unable to
see. Took a spectrum analyzer to confirm source. If memory serves me experienced this in both
hybrid and I think there was another, a LM318 that was at issue. 40 years or so ago.....


Regards, Dana.

 

[D.A.(Tony)Stewart]

Thanks Dana

I was assuming fig 3b