I am also surprised of how accurate the little DATEL DVC-8500A is.
May be you can open that one and show us what is inside.
Yes, a nice instrument. I have a pile of projects waiting, and not enough time, and I want to sell the thing since I got myself 6 digit Fluke 341A (see another repair video). I will keep this in mind. Perhaps one day...
Here is a description of this DVC-8500:
The DVC-8500 is a low cost 4 1/2 digit voltage reference source with a full-scale output range of +19.999 Volts to =19.999 Volts in 1 millivolt steps. The calibrator features high performance for such a small, low-cost instrument. An active buffered output amplifier provides very low output impedance and up to 25 milliamps output current. An oven-stabilized zener diode internal reference provides an overall output accuracy of ±500 uV and ±25 ppm of the setting with zero drift of ±5 uV/°C and full-scale drift of 4 ppm/°C max. Output is set by unique front-panel lever switches which provide rapid, positive contact adjustment. Voltage outputs may be continuously varied within ±1.5 millivolts of selected readings by using a front panel vernier control. The DVC-8500 output is available from both front panel banana jacks and rear panel gld-plated connector fitted with lug terminals. The rear connector has sense feedback inputs to reduce errors caused by cable resistance. A front-panel LED overload lamp lights if the output exceeds 25 mA and current limiting occurs at 70 mA output. The output circuit accepts up to ±25 mA source or sink current at rated accuracy. The rear connector also includes a low-Z output of the +10 Volt reference source with 5 mA maximum. Powered by a choice of 100, 115 or 230 VAC ±10% and 47 to 440 Hz at 10 watts, the DVC-8500 offers transformer isolation up to ±300V dc. Output line rejection is within ±50 microvolts of zero. The black-anodized extruded aluminum housing provides excellent shielding from electrical noise. The small size and light-weight design of the DVC-8500 makes it an ideal portable instrument for a technician's repair kit. When mounted on its bench-top tilt stand, the DVC-8500 uses very little space and can be positioned close to test circuits.