Low voltage virtual ground

May 02, 2013 4 Replies

Rate this thread:

garyr 13 years ago

I need a half-rail virtual ground in a 3.3 volt circuit. I couldn't find anything like a TLE2426 that would work at that voltage so I thought I would use a low-voltage opamp configured as a follower with VCC/2 connected to the positive input. The TI LMV721 has a supply voltage range of 2.2 to

5.5 volts and a GBW of 10 MHz. That would be probably be acceptable but I would like to extend the range of low impedance output to as high a frequency as possible and so perhaps a cap on the output would be helpful. The data sheet implies that the '721 is stable with 2.1 nF on it's output with a 2.2 V supply and 21 nF at 5 volts, but doesn't state a maximum value of capacitive load. Linear extrapolation would imply that 9.5 nF would be OK at 3.3 volts. Any idea wha the actual maximum value might be?

Also, my circuit will not impose a DC load on the virtual ground; should there some DC load on the opamp?

Vote

John Larkin 13 years ago

I use LM8261 for things like this. It's stable with any capacitive load, RRIO, lots of current drive. But it's not cheap.

I've been meaning to find a cheap opamp that's stable with capacitive loads. Yet another spare-time project.

There's no reason to load the opamp.

If there's no DC load, why not two resistors and a big cap?

John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation

Vote

Vladimir Vassilevsky 13 years ago

Use passive RC circuit; with DC servo if high accuracy or high loading is required.

Keep in mind that TLE2426 is virtual mid rail rather then virtual ground. AC potential on the output is that of the center between the rails; not the lower rail.
LMV721 is no good choice as it is R-R output, terrible bias and fairly high noise.
A capacitor at the output would create a peak at the output impedance at some frequency.

Vladimir Vassilevsky DSP and Mixed Signal Designs

formatting link

Vote

Robert Baer 13 years ago

Well, i would put a resistor in series from opamp output to "load" /virtual ground and a capacitor from there to ground for the HF bypass needed.

Vote

whit3rd 13 years ago

or two resistors and two capacitors (if the power-on/off transient behavior matters).

Vote

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required