Author Topic: Bypass capacitor(s) on opamp rails, two or one?  (Read 205 times)

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Max999

Bypass capacitor(s) on opamp rails, two or one?
« on: February 12, 2018, 08:31:10 PM »
I am working on a circuit that uses an opa1641 opamp. In the layout guidelines I read " Connect low-ESR, 0.1-F 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 singlesupply
applications".

The thing that confuses me is this: when I am using a "virtual ground", is the circuit still considered to be singlesupply? Don't we effectively make a bipolar supply by using a virtual ground?

I have made two quick schematics to illustrate the two versions of decoupling. Which one is correct? The U1 version or the U2 version?




thermionix

Re: Bypass capacitor(s) on opamp rails, two or one?
« Reply #1 on: February 12, 2018, 08:43:58 PM »
I read it to mean like your first diagram.  If the .1 connects to Vref, it sees ground through the 47uF electro, which may not be low enough ESR for the purpose.

antonis

Re: Bypass capacitor(s) on opamp rails, two or one?
« Reply #2 on: February 13, 2018, 04:45:44 AM »
The thing that confuses me is this: when I am using a "virtual ground", is the circuit still considered to be singlesupply? Don't we effectively make a bipolar supply by using a virtual ground?
Virtual ground concerns Signal, not Supply.. :icon_wink:

We may have a +18/-18 op-amp with a Vref of different than 0 Volts level..
Our opamp is still considered as bi-polar fed.. :icon_wink:
Or a +36/0 with Vref of +18V (or any other desirable level..)..
Our opamp is still considered as single supply fed.. :icon_wink:

For supply decoupling caps:
If single supply is used there is only one cap needed (+Vcc -> GND)
If bi-polar //       //       //  are two options: either 2 caps (Vcc -> GND & Vee -> GND) or one cap (Vcc <-> Vee)
(the second prefered in cases of signal directly refered to GND to prevent supply noise coupling..)
« Last Edit: February 13, 2018, 08:39:45 AM by antonis »
Can't follow your signature changes, Stephen..!!

iainpunk

Re: Bypass capacitor(s) on opamp rails, two or one?
« Reply #3 on: February 13, 2018, 11:12:59 AM »
Antonis already said most of what i would say but there are a few little things that I want to add.

1) Personally, I always use only one capacitor, due to noise isolation between power ground and signal ground. Especially with dual supplies, since the caps could complete a "ground loop" which picks up extra noise and hum.

2) if you ever decide to use two caps, make shure they are not of the same value. Having caps of the same value makes it easier for a system to cause oscillation (often far above our hearing range), which could potentially kill your opamp. I had this happen to me last year in school (electrical Engineering), one IC just stoped working, after checking what the problem was, we tried it on again, after about 3 seconds, the signal went all weird, clipping a lof of un natural harmonic content appeared. My mate went to get a teacher and while he was gone, the IC started smoking and it exploded.
To be honest, I don't suppose this will ever happen to a normal OpAmp, but still, a thimg to look out for.
If we don't study the mistakes of the future, we're bound to repeat them for the first time.
-Ken M

EBK

Re: Bypass capacitor(s) on opamp rails, two or one?
« Reply #4 on: February 13, 2018, 02:05:02 PM »
My take: When one of your supply pins is connected to power supply ground, that is single supply, whether you use ground or some other voltage as your signal reference.  Use a single cap, as the spec says, in this situation.

I suppose I didn't really add anything to what everyone else has said....
« Last Edit: February 13, 2018, 02:13:45 PM by EBK »
No affiliations. If I glowingly mention specific merchants or products, it is because I like them without having to be paid to like them.

Max999

Re: Bypass capacitor(s) on opamp rails, two or one?
« Reply #5 on: February 13, 2018, 10:13:08 PM »
It is very nice to get the same answer from everybody. So a virtual ground circuit is still a single supply circuit.
I found another answer from somebody online that I wanted to add, for completeness of the topic. He said it depends on to whih ground the load resistor goes. He stated that if the output load goes to ground you use two capacitors, and when it goes to either supply rail you use one. He made this diagram.



Thoughts?

@iainpunk My killed opamps all fizzled out and gave magic smoke. I am kinda jealous you made one explode.