Hey guys, thanks for the replies!
Eric: Thats actually a very nice 1MHz opamp! Within 20mV of GND for 5mA output. Given the price I've added some to my farnell basket for next order
But, as I read over my question, as well as reading nigels post (and also refamiliarising myself with his 'analogue board on his tutorials) I guess for 'true' 0V output with accuracy for an ADC/DAC, a negative rail will be needed regardless of rail capabilities.
Nigel. Yep, as I said above, alas its true. A bipolar supply generally makes life a LOT easier for DC apps. Although for many apps a single supply is convenient and does the job, in my exerience, one must sacrifice certain parameters. As the ground reference is true ground, the power supply rejection means the power supply can be a little noisier.
Once again, I guess you've noticed I take things way too far, perfectionism is counter productive, and the '10mA draw' from the DAC was probably an extreme estimate - 5mA would be more than enough, which the DAC's themselves can cope with. However, I just realised that the opamps on my board used for ADC buffering would also benefit from a bipolar power supply, so even without a DAC buffer, looks like I'm going to need this anyway.
Given the stupidly tight specs, Vin: 3.3/5V, output: +6.0V min and -1V min @ 20mA max. Along with relatively high efficiency (for low power battery powered apps) its a bit of a bugger. But going through my bits box, I believe I've found a half decent solution, I've used them before for extra headroom for analogue circuits.
**broken link removed**
Using a couple of diodes, and a couple of caps, I can 'tap' the negative side of the pump cap (which is roughly a square wave, 0-VCC @ 11kHz) and make a crude charge pump for a negative voltage. Efficiency is still quite high, and it'll easily provide -(VCC-1) more than enough for my -1V target. The current output for the above chip is actually quite high, and should satisfy my requirements.
I know theres a sticky on dual opamp power supplies, thanks to Nigel, but I may post some schematics of 'cheap and simple' low power dual supplies for those who run things of batteries, or USB. Adding charge pumps is cheap and cheerful
If anyone is interested, I'll compile a list. Since downloading LTspice, I've wasted far too much time playing about with it, and making cheapo little solutions for power supplies.
Sorry if I've used this thread to 'think outloud', your advice is always welcomed and re-assuring, I hope I can provide the same for others.
Blueteeth