Idea for a 1V reference (1.024V) - Page 1
Forum section: electronics / projects

author: SharpEars
author_rank: Unknown
timestamp: December 16, 2015, 06:34:19 pm
content: I had the following idea:Take a MAX6341CPA (4.096 V) voltage reference with very good temperature and long term drift characteristics, trim it to an accurate 4.09600 V in circuit (let's assume I can do this). Hand match four 5 ppm 0.1% 10k Vishay resistors for identical values and similar (identical?) tempco characteristics (or perhaps use LT5400-1A resistor divider with matching tempcos). Create a voltage divider (ladder) out of the resistors. Measure equivalent voltage at each of the four legs to confirm, in circuit, that resistor values and tempcos match.Tada, we have a 1.024 V reference anywhere along the ladder that can be used to calibrate the 1.000000V range of precision multimeters.Ideas, concerns, issues?

author: Fungus
author_rank: Super Contributor
timestamp: December 16, 2015, 07:25:12 pm
content: Why 10k resistors?I'd use something smaller to allow a bit more current to pass. It reduces noise.Also: Put a 0.1uF low leakage, low ESR capacitor from the reference point to ground. (eg. a nice multilayer ceramic).

author: SharpEars
author_rank: Unknown
timestamp: December 16, 2015, 07:30:50 pm
content: Quote from: Fungus on December 16, 2015, 07:25:12 pmBecause higher currents and voltage references do not mix well. Also, I want to avoid putting too much heat through the resistors and seeing larger tempco affects.

author: dadler
author_rank: Unknown
timestamp: December 16, 2015, 07:41:03 pm
content: The input impedance of the load (meter you are trying to calibrate) will load down your divider a bit, which you will need to account for.

author: SharpEars
author_rank: Unknown
timestamp: December 16, 2015, 08:48:23 pm
content: Quote from: dadler on December 16, 2015, 07:41:03 pm> 10GOhm like any good meter, but this was one of the points of consideration for the resistor values.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 16, 2015, 08:52:07 pm
content: I would start at 1.2xxx volts - lots of resonable band gap references around. This makes the resistors less critical - not much but about a factor of 2.

author: Fungus
author_rank: Super Contributor
timestamp: December 16, 2015, 09:34:23 pm
content: Quote from: SharpEars on December 16, 2015, 07:30:50 pmYour reference has 15mA output capability.Quote from: SharpEars on December 16, 2015, 07:30:50 pmPresumably all the resistors will have the same tempco so it doesn't matter.

author: IanJ
author_rank: Unknown
timestamp: December 16, 2015, 11:21:23 pm
content: Quote from: SharpEars on December 16, 2015, 06:34:19 pmLook at the datasheet at the thermal hysterysis figures and make sure you can accept them.My prototype Precision Digital Voltage source (seen on Dave's mailbag) used the MAX6350CPA (5v version of your ref) and 2ppm/degC resistors.........but I had too much trouble with it at the 5th dp and ended up changing my design to the good old LM399AH.........self biased with a good op-amp and it's miles better at those resolutions. Albeit I am upscaling to 10Vdc.Ian.

author: jwm_
author_rank: Frequent Contributor
timestamp: December 17, 2015, 12:09:41 am
content: Use the reference as the vcc of a uC and spit out a pwm signal at one quarter full. Low pass filter it and done. You can trim it in the cpu. Crystal frequencies are orders of magnitude more accurate than resistor values.You don't actually need a uC, a couple flip flops would do but trimming would be trickier.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: December 19, 2015, 07:16:14 pm
content: JWM....You are quite wrong about that, resistors can be much more accurate than crystals. If you go to the trouble of putting a crystal in an oven, still no advantage, you can do the same with a PWW resistor and get near zero tempco (even without an oven). Standard crystals are good to 0.005% in common grade, better than that is going to cost, just like better resistors.. There are no free lunches and PWM.....noisy and filtering out the much higher noise levels of square waves compared to a quiet linear reference...not even close.