Hi,
I want to show the oven that I'm going to use for this LM399 reference, this because I already have quite a few test hours in it and that the "gain" is above 150x.
So 15C temperature increase gives less than 0.1C change in the oven.
How good it really gets depends on how well the insulation is done for the oven.
By placing the furnace in a simple polystyrene box, I got more than 150x gain.
I showed this oven before on this forum, and the controller is equipped with a LM723 IC.
And the NTC is located in the 7mm thick bottom under one of the power MOSFets that heat the box.
The extra TO220 IC is a LM35DT, I thought it would be easy to measure the temperature of the oven, but its a bit of a drama temperaturesensor, noise as hell.
The oven is made from an already used HF box from which I removed the connectors.
It is a beautiful box for an oven because, of the very thick walls, the bottom plate on which the MOSFets are mounted is 7.5mm thick.
The battery is for the impression of the size of this oven.
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Here you can see the extra 5K NTC measuring the temperature in the oven, this NTC was connected to a 34461A DMM.
It is clearly visible that I made sure that the wiring is mounted against the oven body.
This has been done to prevent you from making wrong measurements by leaking energy from the connection wiring.
During measuring, the lid was closed and the holes covered with tape.
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This picture is for the impression how much space there is left for the Reference circuit, the parts around the LT1010 buffer do not come in this box.
That leaves some extra space to place the parts in an optimal way.
This because I'm still thinking about the remarks to leave out the bootstrap capacitor filter and only use the noise reduction of the 4x LM399 parallel.
I can then add more insulation to the LM399 IC in the oven, because no filter capacitors are needed.
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I also thought about not connecting the oven in the LM399 IC's, but this oven is not good enough for that.
It requires an oven with a gain well above 1000x.
That's why this oven should be placed in a flask, or you could opt for a double oven.
As far as I'm concerned, both fall outside my project objective.
My first tests with the two types of Nichion VR(M) 470uF capacitors are good.
Eventually the 35V versions look good after first forming the capacitors at 15V and after that I started measuring leakage current around the 7V voltage which is the reference level.
One of these 35V types is at less than 1uV at 7V across the 10K resistor series.
So this is not the bootstrap filter but 1 single capacitor!
I've been thinking about "maat" his remarks about the strange drift of the bootstrap filter he tested.
My experience is that it works well but I also came across some weird effects today.
That wasn't the filter, but the cause was the power sources I used for the tests.
Those were two different power supplies, an old Harrison and the Rigol DP832.
Both can't be used properly because of the 1/F noise of these power supplies.
Today I picked up some different types of batteries I ordered from the wholesaler.
I first had to get these batteries to LAB temperature otherwise the drift of these batteries also caused too many errors...
Now the 4x penlite are quieter than my 10V reference PCB with the LT1021 on it.
This is a picture of the measurement on the 16V 470uF bootstrap filter with my old Philips PM3424 microvolt meter.
The DC value over the 10K resistor is less than 2uV.
Oh yeah, of course I regularly check if the "null" level of this meter hasn't changed.
And this is for the impression of my 10V Quad LT1021 LAB using a capacitor bootstrap filter.
The PostIt note is from June 2018 and the measured value on the 3458A is from this evening with a well warmed up 3458A and after an Autocal.
And below a link to the website of Rubicon with clear information about the properties of capacitors.
If you look at the leakage current between 22C and 42C, that difference is only small.
You have much more advantage in properly forming the capacitor and selecting the one with the lowest leakage currents than to worry about maybe 20% extra leakage current at 42C.
But, uh... still a lot to think about.
For the sake of completeness, I'm not looking for the perfect 10V reference, but try to do the best I can with the stuff I've got.
Kind regards,
Bram