Michael,

There is a Analog Devices chip available (called AD420) that is a serial D/A converter with 16-bit resolution, capable of producing either a 0-5V (from the back of my head, but maybe 0-10V as well) output or 0/4..20/24 mA directly, even without additional output stages. We still use it for a 4..20 mA output board. It's not that cheap, but does the job nicely and without much additional hardware (everything on board including voltage reference). If you are going to use this one, let me know because I found a particular strange thing with the max. output current it provides in 20 mA mode (doesn't go any further than 19.98 mA !!).

In a newer design of a similar circuit, we use a simple MAX 14-bit D/A with a single transistor. It's a cheaper solution, but in order to get to the same possibilities we had with the AD420 quite some additional hardware had to be added.

Menno