I have a question for the group. I was looking at an article for building an ultra-low noise voltage reference by Walt Jung, published in Electronic Design June 24, 1993 and a URL to the article is below. I want to filter the output of an LTZ1000 based 10V reference I am building and this circuit has a very low freq corner of 1.6 Hz. I was concerned about the leakage through R1-C1. If C1 had as little as 1ua leakage, it would drop the voltage through R1 by 1 mV. The spec on 100 uF electrolytic and tantalum capacitors show a leakage of 20 ua at rated voltage so this could be of great concern. However, at the low few tenths of a volt that should be across C1, the capacitor should have a much lower leakage amount, which is the theme of the article.
To get a better appreciation of the issue, I connected a precision 0 to 10 V source (100uV resolution steps) to a series combination of a 1 Kohm resistor and a 100 uF electrolytic and, later, another 47uF tantalum and a 47 uF electrolytic capacitor. In all cases the leakage, as measured with a Keithley 414 picoammmeter, showed a leakage or around 0.08 uA at 10V and varying 0.04 to 0.12 uA, around .1uA at 1V and varying , and around 1 pA at 0.1V, but with widely varying leakage current of 0.5 to 1.5 pA, with occasional peaks of -0.5 to 2 pA. This would equate to about +/- 2 uV voltage variation across R1, making a 10 V 0.1ppm stable voltage reference of questionable value. I also tried a 0.68 uF polystyrene capacitor and also saw leakage current variations, although much less than the electrolytic and tantalum capacitors, as one would expect. Thinking the problem might be the the picoammeter, I put a 100 megohm 0.1% precision resistor in place of the capacitor across the precision voltage source set for 0.1 V and measured the current through the resistor at a very stable 0.9 pA on the Keithley 414 (sb 1pA but accurate enough for my measurements - the resistor shielded box likely has some sub pA leakage also). Note that I used shielded cables for all measurements, and the resistor and capacitor were in a shielded box, as well as the 100 Mohm calibration resistor. Touching the cables or boxes did not change the picoammeter reading at all, indicating to me that the shielding was reasonable. I suppose the best approach is to build it and characterize it, but it's not fruitful if someone has already done this. So my question is: has anyone built this circuit and characterized it, particularly over temperature for stability at the sub ppm level? Thanks, Randall Evans http://waltjung.org/PDFs/Build_Ultra_Low_Noise_Voltage_Reference.pdf _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.