how can I deal with a current spike?

As long as the opamp has current limiting(a lot of them), there is no problem, the current wont rise above the stated current capability. If there is no inherent current limiting, use a resistor to ensure that the current does not damage the opamp. You might use a reed relay to short the resistor, after the worst initial surge has abated.

Anotehr limit might be to current-limit the supply rails if it's a single opamp package. But remember, Opamp do not like to drive capacitive loads as that can drive them unstable, the usual tricks are needed.

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Regards, Joerg

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Oh? I didn't know about the op-amps not liking the capacitive loads... is this something that is an issue even for a buffer passing a dc voltage... or is it more for passing ac signals... or frequencies.... I'm googling this and most of the hits are talking about op-amps with some gain and for some ac signal at the input... I wonder though if the issue still applies to my dc buffer?

much thanks!

Yes, it still applies to buffers. And the fact that you may *intend* to only pass DC is not recognized by the op-amp. If the capacitive load causes the amp to be unstable, you'll have an oscillator instead of a DC signal.

The fix is to add a small resistor between the op-amp output and the load. Then, instead of taking the feedback from the op-amp output, you take it from the junction of the resistor and load. You also need a small cap from the op-amp output to the inverting input. I've forgotten the rule for calculating the R and C (anybody?) but for normal op-amps driving long cables and such you can use R = 47-100 ohms and C = 22-47 pF.

In addition, you can also use (for example) a 10k resistor instead of the normal solid-wire feedback that is typically used for buffers. (Note that per the above it will now go between the inverting input and the load, not the op-amp output.) That won't change the DC gain, but may slow down the transients in conjuction with the above C.

Best regards,

Bob Masta DAQARTA v4.51 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card!

Yep, well said. There is an app note from National about it but I didn't find it back. Probably tossed it but maybe it can be found on their server und "opamps" plus "capacitive load".

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I don't know how you're applying your op-amps to the circuit how ever, if you're using some kind of feed back to the inputs like (-) for example, you can use a low value resistor on the output that will be calculated for the max load, from there, that network point is your output and also the point where you feed the remaining r value back to the input. Many op-amps already have a short term limiting designed into them, but you don't know this until you check the specs.

If you are not employing a feed back circuit where this idea will be a solution for you, you could then use an inductor on the output to help reduce the inrush currents which is a common practice or fix a permanent series resistor there if the small drop isn't going to effect you much.

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Both my opamps are set up with their outputs being fedback into their inverting inputs.... and on one I am controlling the input voltage at the non-inverting input (and varying this between 0-10V) and on the other I have a regulated 5V going into the non inverting input... my load then goes across the two op-amp buffer's outputs.... and when I vary my voltage between 0-10V I see -5 to 5V across my load.... I'm still trying to figure out why I could have a potential issue here with a capacitive load... it seems most the stuff I've found online, talk about the capacitive load adding an extra pole to the transfer funciton... but it looks like this is at a frequency above 1Hz... so I don't know if this applies to me at a steady DC output...

I'm thinking about using the inductor at my opamp outputs to ease up the current...

thanks for all the comments

As Bob wrote, the opamp doesn't have a crystal ball, it doesn't know that you only want DC. Worst case it barely works, you turn around and walk away, then it begins to oscillate and all hell breaks loose.

Here is how it's done when you have a capacitive load, and they also explain the theory quite well:

You could, for example, dimension the resistor so that the desired maximum current results when the opamp does a full swing. Its output will peg close to the rails. How close would be mentioned in the sink/source capabilities in the datasheet.

[...]

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Regards, Joerg

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Thanks for the link Joerg. I never thought of diddling with the noise gain of the opamp to help fix the oscillation problem.

George Herold

But brace yourself for design reviews. Some of those tricks are met with disgust there ;-)

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ok.... the potential to oscillate really took me off on a tangent, but while my original question and problem was about what to do about the current surge from the capacitive load... it seems that adding those resistors at the outputs of my opamps may get rid of my current surge? by limiting the current to the voltage out of the opamp divded by my resistor... thus allowing me to control the amount of current by selecting the right resistor?

Yes. The opamp tries to maintain regulation but it would swing farther than your desired output voltage. The worst case current would occur when the capacitor is at its most negative voltage and the opamp swings full positive (close to the rail) and vice versa. The maximum voltage differential divided by the resistance would be the maximum current.

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