I’ve been bothered about the choices for reference voltages lately. My new Keysight 34461A has voltage ranges of 0.1V, 1V, and 10V. My old HP972A (a freebie) has ranges of 40mV, 400mV, 4V, 40V, etc. If there is not another reference around 1.0V the 34461A I will not be able to test that range effectively. The 972A can use the 0.1V, 2.5V, and 10V references to cover most of its ranges. It’s interesting that neither the HP or Keysight DMMs need the 5V reference for calibration.

The easiest way to get 1V is to divide it down from another reference voltage. I chose to make a 1:5 divider from 5V to get the 1V reference. The divider uses the same 20kΩ resistors that went into the 0.1V reference. It will not use 50 resistors (there’s no room on the PCB) and will require the loading of the DMM to be higher than 10MegΩ — therefore only useful to the 34461A. 

The accuracy of the divider is expected to be only around 0.05% (0.1%/√5). But the 5V reference can be trimmed to yield the correct value at the 1V output if the 5V output is not expected to be used as a voltage reference for calibration. That’s the plan.

Either the 5V or 1V references can be brought out to terminals with the current PCB layout, but I expect to not use the 5V option, given the ranges of my current collection of DMMs.

Both the layout and schematic have been updated in the project details section to reflect this change.