Need some help/advice with select-able and stable voltage divider in the mV/uV

[rwgast_lowlevellogicdesin]

Ive been working on a dummy load for quite some time now, im trying to make a piece of quality gear. My issue is this, I need to select between different voltage dividers on the output of my DAC. The first issue im having is stability, when I stick a voltage divider on the out put of my DAC the voltage doesn't seem to stay stable, im working in the nv/mv range. The 1 mv from the DAC stays stable, but the 100nv out the divider drifts around. I thought this may just be an isssue with noise/ripple in the uv range, but Im still experiencing drift well up to 1 volt in 100mv out.

I would also like to select different ranges by changing the resistor divider, my fist thought was to use a digital pot on the output of the dac, this could also help me trim out any INL/DNL issues. The problem is most digital pots dont have enough resolution to set up perfect divide by 1, divide by 10, divide by 100, ranges.. let alone trim the DACs INL/DNL.

I found this article about using a DAC itself as a resistor divider but it seems a bit over my head im not sure how to select the resistors, http://edn.com/design/analog/4326864/CMOS-DACs-act-as-digitally-controlled-voltage-dividers.Im hopping someone can give me a bit of advice here.

[David Hess]

Which DAC are you using and how is it configured?

[rwgast_lowlevellogicdesin]

Right now I am using an mcp4922 12 bit DAC and a 4.096 voltage reference. Although I plan on switching to a 16 bit dac (ltc2602) in the future. Right now I'm just expermenting with what I have on hand.

[David Hess]

It is not real clear if you are talking about nanovolts or microvolts but in either case, grounding and decoupling of the DAC will be critical to getting good performance. 

The measurement itself can be non-trivial and some voltmeters are going to have problems.  I would try connecting the divider to the reference instead of the DAC output as a test.