Virtual Ground Distortion And Capacitor Values - Page 1
Forum section: electronics / projects

author: Plecto
author_rank: Regular Contributor
timestamp: January 14, 2014, 02:43:04 pm
content: Hi. I have two questions regarding virtual grounds, more specifically I'm talking about the simple resistor voltage divider with paralleled reservoir caps. I'm currently messing around with a simple NJM4556 circuit and I started wondering how big these reservoir caps and resistors actually had to be. If I'm making a battery powered application I would want the resistor values to be as big as possible, and if my thinking is correct, these could be as big as I want them to be as long as I manage to get the offset voltage to a low enough value, is my thinking correct? It will of course heavily depend upon the load impedance as the virtual ground point will shift heavily as the offset voltage increases and the load impedance decreases.I also started wondering about those reservoir caps and if it's perhaps a point where there is no use in having bigger caps. As the load current increases so will the voltage (or ripple) across the reservoir caps so I'm thinking that if the output capacity of the op-amp yields before the ripple across the reservoir gets big enough to actually limit the output voltage of the op-amp, there should be no point in having caps with a bigger value. In other words, if the output starts clipping because the op-amp can't output any more current rather than it starts clipping because the op-amp can't output any more voltage (further caused by reservoir caps ripple), I should be in the clear. I did some testing to see if this was the case. I had a 16Ohm load (worst case scenario in this application) and sent a 20hz sinewave through it and scoped the output as well as the reservoir cap voltage. Increasing the cap value above 100uF didn't seem to do much, the output still started clipping at the same voltage. Is there a reason to increase the capacitor value above this limit? If not, there's a reason to face palm when looking at Cmoy designs that use a OPA2227 with 1000uF capacitors isn't there?I'm also reading that these virtual grounds should be used when there aren't any other options, that these grounds will cause distortions. I'm struggling to see how distortions can be introduced by this design. The op-amp will have this virtual ground as it's reference point and it will try to output what it needs to. Unless something is preventing the op-amp from doing so (I can't see how), how can the op-amp care whether the electrons is flowing to a "real" ground or a virtual ground? It's just doing it's job regardless. Could someone help me shed some light on this matter?

author: nickm
author_rank: Regular Contributor
timestamp: January 14, 2014, 04:27:08 pm
content: The best way to generate the virtual ground is a single divider with a single cap buffered through a unity gain buffer. This creates a very low impedance virtual ground. The issue with the series caps with balancing resistors is the virtual ground impedance looks like the parallel combination of the caps. So worst case is 20Hz where the impedance will about 4 ohms with 1000uF caps. You will see the problem if you run both channels in phase and as low a load as you can. There will be a little bit of crosstalk because as the virtual ground is shifted around by one channel it directly affects the operating point of the other channel and vise versa. For a low parts count low cost headphone amplifier this is fine. The headphones run by the amp will be relatively high impedance and won't have much low frequency response so the issue won't be heard. The amount of ripple on the virtual down will go down 20dB/decade so it probably won't be heard and will only show up on the distortion analyzer. Sorry I don't think I directly answered any of your questions.