I use a AD588 for my precision source. A few precision resistors, a couple of switches and one AD588 gives me 0V, 1V or 5V. Its probably more accurate than my overly expensive Tektronix DVM. http://www.analog.com/en/prod/0,2877,AD588,00.html I got mine as a sample many years ago. :-) Tony www.oakleysound.com