# Schematic notation for decoupling capacitor?

I'm wondering where this capacitor should be placed. Is it a decoupling capacitor? There are two supply rails on this particular op-amp, so I'm assuming that this indicates I need to put one between each rail and ground. Any insight would be appreciated!

Edit: the data sheet can be found here http://www.intersil.com/content/dam/Intersil/documents/ca31/ca3130.pdf

• It says 0.001 for 3 so that would indicate to me between pin 3 which is the positive side of the op amp and ground. Which makes some sense because you want the reference pin (3) to be non-moving usually. Commented Apr 10, 2014 at 2:13
• Why not just draw the cap on the non-inverting input? Usually power pins are drawn where the cap is drawn now. It looks like the (.001 FOR 3 is cut off. Whats the rest? There's no unit.. .001 uF? 1nF? Kinda small for a decoupling cap, no? Commented Apr 10, 2014 at 2:18
• @erythraios Could you provide more context? Where does the schematic come from? What's the model of the OpAmp? If you post a link to the datasheet, that would be ideal. To me, this doesn't look like a decoupling capacitor. The decoupling cap is usually between supply rail and ground. Welcome to EE.SE, by the way. Commented Apr 10, 2014 at 2:30
• I have a very vague recollection that some early op-amps had terminals for a frequency compensation capacitor (or maybe I'm confused with offset adjustment pots). Commented Apr 10, 2014 at 2:50
• For what it's worth, that's a very rare way to connect a compensation capacitor or network. More often it was a series R-C between pins 1 and 8. Kind of conflicts with the "ANY OP AMP" text. Commented Apr 10, 2014 at 3:57

## 1 Answer

Omitting the supply rails and decoupling capacitors is a common shorthand in circuit diagrams. It's admissible, when supply rails are not important for the discussion.

0.001 μF in the original post is not a decoupling capacitor. It's a compensation capacitor between pins 1 and 8 of the CA3130 OpAmp.

fig.8 on p.9 in the datasheet. Decoupling capacitors in this schematic are 0.01 μF, compensation capacitor in this schematic is 56 pF.

• Thanks! Is there any easy way to explain why this is connected between the strobe and null offset pins? I'm sort of new to hardware Commented Apr 10, 2014 at 3:59
• The consequence of using such a huge capacitor (~20x the minimum required value for unity-gain stability) is you'll end up with an extremely sluggish voltage follower. It might be justified if the amplifier was called upon to drive a cable or other capacitive load. Commented Apr 10, 2014 at 4:06