Hello Alan, I'm investigating about the ACAL function of the Keysight 34465A / 34470A DMMs, and so collecting ideas, how they might have realized that in hardware. I had a long discussion with Keysight about an obvious 3ppm failure on the 100V and 1kV DCV ranges, (fixed now), and an additional odd gain shift caused by this ACAL feature, at last with Scott Stever, who also was the PM of the 3458A. They described the functionality of their ACAL very briefly only.. so I'm still guessing.. The similarity between the 1281 Selfcal and these Keysight DMMs ACAL is, that there is obviously a ratio circuitry inside, which gain factors are determined once during calibration, and then used later to improve the T.C., each time when ACAL or Selfcal is engaged. I think, I have to re-engineer the 34465A to find out. And no, I'm not designing any new Autocal device. I already have 3 different superior ratio devices, the HP 3458A, the Fluke 5442A, and another DIY Hamon divider, I think, that's enough! Frank > Frank, > Are you hatching up a new selfcal/autocal design for your 8.5 digit? I would be interested to hear your musings. > Alan