Hello
Unless you have something that produces a continuous air flow from the LM 399 number 1 to the 10, I don't think the temperature difference between all of them is that important.
Moreover we can see a gradient from 1 to 10, so I would think for a power supply / current problem.
Can you reverse your power supply and put it at the opposite side?
Even if the currents are weak, there can be interactions with this type of card
Yes, it is very easy to put the power supply at the opposite side. It is one more good test to do.
So you are almost there. 0.33 ppm of 6.9 V is only 2.3 uV. So another thing to watch out would be thermal voltages at the scanner input connection and in the link from scanner to voltmeter. To check that i would wire a short that i can measure with the voltmeter through the scanner so see how stable the residual voltage is. Or if you have two spare channels on each end of your "1" .. "10" lineup, you could try and see whether the "1" end is worse.
Another consideration would be EMI interference: using shielded cables, adding blocking capacitors on both heater and reference output of each LM399. Andreas wrote up lots of tests, including the LM399:
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684070/#msg2684070
Regards, Dieter
Yes, it is possible to check the scanner channels. The board I'm using has 20 channels and I'm using only 11 (10 LM399 and the thermistor). I connected the odd channels to the LM399s and the even channel to ground (except the last one, connected to the thermistor). Another test to include in the to do list...
And thanks for the link, I'll read it.
Some general rules for LM399 (since your results are still much too "noisy")
- use a well stabilized power supply (also for the heater).
- best PSRR is given with heater voltages >= 15V
- use bypass capacitors 100nF directly at the zener (+ heater) pins
- shield pins from air flow (not only on the top but also on the bottom side of the PCB).
- do not tilt the LM399 this may give several PPMs deviation. (see pictures here)
https://www.eevblog.com/forum/metrology/project-pimp-a-keithley-2000/msg1106829/#msg1106829
- for highest stability use LM399s with a zener voltage near 6875 mV @1 mA (gives lowest TC between heater off and on)
- keep EMI sources (LED lamps, SMPS, WIFI, Mobiles) away
- and of course you should give the LM399s minimum 14 days run in time before doing measurements.
with best regards
Andreas
The power supply is direct from the Agilent E3631A to the board green borne. See below the last 24h of the power supply voltage measurement, it's from 14.993-14.998:
I'll check the noise with the oscilloscope.
I'm using Vcc = 15V.
Yes, I'll add the capacitors. One more in the to do list.
I'll observe all your considerations to improve this test.
Regarding the run time before doing measurements, today is the second day that that references are running. The LM399s I'm using are new ones, never used before. So, it seems that I must wait some days before any conclusion.
Typically, the first thing, that you should be looking out for, when binning LM399 is popcorn noise. This is rather a rather frequent problem. For my university project I was binning LM399s and I had to throw our 10 out of 25. (All but 4 came straight from Mouser).
Attached I have a log of one the Zeners:
--PICTURE--
To add to the confusion, I tested each diode in an amplifier circuit, which amplified the Zener voltage by (20/17+1) to about 15V. Since my 34470A does not have a 20 V range, I was using another known good Reference at 15 V (tested against a Fluke 5440B) and measuring the differential voltage. So keep in mind, that the (white) noise is roughly 2.2*sqrt(2) of a single Diode and the popcorn noise of the zener has to be divided by the gain (~2.2). The data was recorded by a 34470A in a stable enviroment (< 0.5 K) using 10 PLC and Auto Zero = On on the 1 V range. The Zeners were burnt in for 100 h prior to testing.
So you will looking for popcorn noise on the order of 2 µV for your 6.95 V reference.
This is what a good example looked like:
---PICTURE----
Once you have weeded out the worst offenders, burn in the the rest for a few weeks, then start monitoring.
Another option is to use a low noise amplifier and have a look at the 1/f noise of each diode.
I'll pay attention in the noise. I'm using NPLC = 10, Auto zeron ON and filtering 10 samples (sequential, not moving average).
How many hours is necessary to know the better ones?
many: there may be some "popcorn noise" which shows up only from time to time
see comparison of 2 LM399s here (Ch7 has large initial ageing whereas CH6 has up to 2 ppm popcorn noise)
https://www.eevblog.com/forum/metrology/lm399-based-10-v-reference/msg796829/#msg796829
with best regards
Andreas
Interesting graphic, looking at the graphic scale (total of almost 2 years) the CH6 popcorn noise seems somehow sporadic.