Hi there,
You will not be able to connect in series or otherwise be able to output 20V using the AO-210 withouta switch or relay in addition to the module. The internal ADC gets the input voltage down to 10V and there's no way around getting more than that out of it.
Check out some of our relay Compact FieldPoin modules, which could be helpful depending on your application: cFP-RLY-421, 423, and 425.
Also, do you need to step between 10V and 20V? Or do you simply need 20V even? Our Digital Output modules such as the cFP-DO-400 all have logic levels of 24V or 30V, if that might be helpful.
Best,
Ryan C.
Applications Engineer
National Instruments