Taming Oscillations—the capacitive load problem

RE: Taming Oscillations—the capacitive load problem

Hao Zhang2 — Wed, 19 Feb 2014 03:14:02 GMT

Dear Bruce,

Thanks for your share on this topic, in fact, I have been confused by this problem for several days. Some other engineers from ADI and Microchip Inc have published some sheet on it, but I can't understand them accuratly. But your blog is very clear.

For the question raised by Randy Constan, i want to say that when you add a resistor between amplifier output pin and the capacitive load, a zero and a pole point will occur simutaneously, so it increase the magin in fact.

Hao Zhang, Beijing, China

RE: Taming Oscillations—the capacitive load problem

Peter Pan — Tue, 30 Apr 2013 04:55:56 GMT

I absolutely agree with Bubba here! I just had a near miss at a nightmare of having to ditch an otherwise successful 4 layer PC board of a recent project, using several LME49721 dual op amps to buffer the output of some digital potentiometers. These op-amps are set up as unity gain followers, so adding gain pretty much means scrapping the board. But, the outputs of my op amps were being routed some 802 sized 0 ohm jumper resistors to the on board output jacks. Those 0 ohm jumpers were really not necessary, but it just seemed a good idea at the time in case I wanted to add some future isolation resistors. Well I sure am glad I did that because during testing of the circuit, I found that if there was a long audio cable connected to the output jacks, the slightest board transient would trigger high frequency oscillation. When I tried shorting the outputs through 100nF caps, I got the same effect, proving the problem was the capacitance created by the cable. Well it turned out that replacing my zero ohm jumpers with just about any low value of resistor completely solved the problem. What a sigh of relief!!! II settled on 1000 ohms, at least for now, until I can better determine the minimum value to eliminate the problem. But since I indend to normally drive a fairly hi-Z load of around 100K, the 1000 ohm resistors are probably fine. I guess adding this external resitor lowered the "pole" frequency below a point where it could cause any instability?

RE: Taming Oscillations—the capacitive load problem

Bruce Trump — Wed, 06 Jun 2012 21:13:40 GMT

Thanks for the comments, Bubba. You've raised a couple of ways to improve cap load drive. Each would probably require another blog and I fear that I've worn out my welcome on this topic for a time. You will find at least some of this in Collin's presentation (attached). I may return to the topic later after hitting some other subjects. Thanks for reading.

Bruce

RE: Taming Oscillations—the capacitive load problem

Bubba — Wed, 06 Jun 2012 20:06:12 GMT

Bruce,

Thanks for the "intuitive" insight that you have provided in this series of articles. In driving cap load you have suggested increasing the closed loop gain as a method to improve stability... I have used a series output resistor in the past to "isolate" the cap load and improve stability when changing gain is not really an option. What are your thoughts regarding this or other methods oh sage one ;-)

Thanks again and keep em coming!

Bubba