For the analog CH(A) & CH(B) calibration, I made a low cost constant DC voltage source that others may want to duplicate. I built this rig from mostly components I found on eBay.
The calibration uses a 2-point calibration curves for each voltage grid setting. The required voltage ranges are: 50mV (250-300mV), 0.1V (0.5-0.6V), 0.5V(2.5-3.0V), 1V (5.0-6.0V), 2V (10-12V), 5V (25-30V), 10V (50-60V).
I supplied these voltages from 3 banks consisting of four 9V alkaline batteries, one lithium 3V coin cell, and one AD584KH precision voltage reference which was powered by a 15VDC wall adapter. This provided fixed voltages of: 0.248V, 0.497V, 0.995V, 2.495V, 4.994V, 26.15V, 50.72V.
The AD584KH voltage reference supplies switchable voltages of 2.5V, 5.0V, 7.5V, and 10.0V. It’s claimed to be accurate to within 10mV @ 10.000V and 3.5mV at 2.500V. The more accurate AD584LH should be accurate to within 5mV @ 10.000V and 2.5mV at 2.500V. There are AD584KH voltage reference modules for sale on eBay for less than $9USD shipped. The AD584LH can be found for slightly more.
For the lower 250mV reference, I used a voltage divider consisting of two 0.1% tolerance 1/4W resistors (500 and 4.5K ohms) and the AD584 reference set at 2.50V and 5.00V. There’s an eBay seller in Taiwan that sells these 0.1% tolerance resistors in small quantities. DigiKey.com had a large minimum on these tight tolerance resistors when I last checked.
Additionally, I found a couple of low cost three wire digital voltage meters with LED display (One, a 5-digit 0.000-33.000V range, and the other, a 4-digit 00.00-100.0V range). Both meters were had on eBay for less than $6USD each with shipping. A multi-meter may have done the same job without adding cost.
A small bread board with header posts helped organize interconnecting wiring and allowed for quickly switching between voltage banks.
Hope this helps.