Can a low-cost, low-resolution RDAC and a precision voltage reference by used to make a coarsely adjustable high-precision DC voltage reference? Consider for example the AD5121 7-bit RDAC with ±5ppm/°C drift and 8% resistor accuracy driven by the ADR4525 reference with 1.25μVpp from 0.1 to 10Hz and 4pmm/°C drift. Assume that the output will be buffered with a zero-drift amplifier.
The initial resistor accuracy aside, will this circuit give me an adjustable voltage with DC performance nearly as good as the reference on its own? The reference will be used to periodically null the input of a 24-bit ADC for precision DC voltage measurements at the 5ppm level.
I ask this because only high resolution DACs and RDACs provide DC (1/f) noise specifications like the voltage references do. If the low-resolution DAC can really act as a resistor with 5ppm/°C TC then I would expect it to work fine.
On a related note, there are no SPICE models for these RDACs so do they really behave enough like a discrete resistor/pot to simulate as such?