This is a detailed review, noise measurements and teardown of Stanford Research Systems DC205 voltage source. The source can generate DC voltages on three ranges +/-1V, +/-10V and +/-100V with 6 digit resolution and 4-wire sensing. The current is limited to 20-50 mA. It is based on LM399 voltage reference and has 1 ppm short term stability and linearity. Overall the voltage source works quite well and may be the best commercial DC voltage source short of a calibrator. However, it would be nice to improve it even further by switching to an LTZ1000 reference as well as changing a few chips. I have also explored using the 4-wire sensing capability to increase its output current and to convert it to a current source.
This picture is of course carefully timed during initial warm-up, but the absolute accuracy of the 3 sources I checked is quite good. The voltages are measured by a HP3458 recently calibrated to TiN voltage standard from USA cal club.
Unit 1: 1V: +2.5 ppm, 10V:-3.3ppm, 100V: 0.5 ppm
Unit 2: 10V: -1.6ppm, -10V: -1.1ppm
Unit 3: 10V: +0.9ppm
The linearity is about +/- 1 ppm. The DC source uses a lower grade AD5791A and applies a correction. Since the digital resolution is 1 ppm, there are sharp steps of 1 ppm in the residuals.
The noise spectrum of the output is quite good, but clearly can be improved at low frequency with a better voltage reference and op-amps. One can see they contribute about equally to the drift because low-frequency noise decreases when the output voltage is set to 3V, but it doesn't go down further for lower voltages or for 1V output range.
The broadband noise of the voltage source is pretty good at 50nV/Hz^{1/2}, comparable to LTZ1000 reference.
Below are the pictures of the insides, with some of the chips labeled.
It uses a number of ADA4522 zero-drift amplifiers and LT5400 resistor networks for voltage scaling. However, there are also OPA192 FET input amplifiers which maybe responsible for some of 1/f noise. Perhaps replacing them with zero-drift OPA189 would help. But I have not yet tried to trace the circuit and would welcome people's observations and suggestions.