| Electronics > Metrology |
| [#3300] Wavetek 7000, the hidden gemstone. |
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| aronake:
Does any of all knowledgeable people here know if there is any disadvantage of reducing what is called R2 in the LTZ1000 data sheet to 30K as Datron/Wavetek have down to 30K? Analog may just not have thought of the benefit of a much lower R2, but for the ADR1000 datasheet they just lowered it to 61.9K, and it ought to be well known to them with the benefit of lower R2 by the time they wrote the ADR1000 data sheet. And for ADR1000, there is even more benefit than in LTZ1000 as no need for the extra around 10 ohm resistor to bring the non heated TC to zero. Their unheated schematic in the LTZ1000 data sheet show they were aware of the benefits of the 10 ohm compensator and the lower R2. Amazing reverse engineering here by Chekhov of the schematics! And the "secret" final TC adjustment mentioned in some places mentioned in relation to this design, what is that? |
| Kleinstein:
Lowering R2 gives less noise from the transistor - usually more the higher frequency part, not so much the 1/f noise. At 100 µA a small transistor (e.g. bc847) should be at some 4 nV/sqrt(Hz) of white noise, which is way less than the LTZ1000 noise. So the transistor noise should not be that relevant. However it also comes with a slightly higher TC of the ref. part if not temperature stabilzed. The difference is not much: some 22 mV higher BE voltage, which results in 70 µV/K less for the temperture effect, or 10 ppm/K more for the overall TC (e.g. 60 ppm/K instead of 50 ppm/K). This makes the temperature stability and setpoint divider a little more relevant. The adr1000 has a different TC to start with, here more current could even lower the TC. |
| aronake:
--- Quote from: Kleinstein on December 29, 2024, 04:02:41 pm ---Lowering R2 gives less noise from the transistor - usually more the higher frequency part, not so much the 1/f noise. At 100 µA a small transistor (e.g. bc847) should be at some 4 nV/sqrt(Hz) of white noise, which is way less than the LTZ1000 noise. So the transistor noise should not be that relevant. However it also comes with a slightly higher TC of the ref. part if not temperature stabilzed. The difference is not much: some 22 mV higher BE voltage, which results in 70 µV/K less for the temperture effect, or 10 ppm/K more for the overall TC (e.g. 60 ppm/K instead of 50 ppm/K). This makes the temperature stability and setpoint divider a little more relevant. The adr1000 has a different TC to start with, here more current could even lower the TC. --- End quote --- Thanks for this! My pretty basic tests with ADR1000 seems to indicate 0 TC unheated at around 25K ohm for R2 though. So benefit would be lower noice and lower TC. One abvious disadvantage thugh is slightly more current draw, but that ought to be compensated by the heater drwaing less current. So still very unclear why data sheet say 69K, but there seems to be benefit in most areas by setting this lower. |
| chekhov:
You may try different combinations of R2 and R1, can also decrease current back. Maybe question of time spent running thermal sweeps with different combinations in order to select desired temp set point. https://www.eevblog.com/forum/metrology/lowest-drift-lowest-noise-voltage-reference/msg5507800/#msg5507800 Unfortunately that all does not help with LTZ1000, and upper Rz may add much more long-term instability and its own TC as well. |
| aronake:
--- Quote from: chekhov on December 30, 2024, 04:46:53 pm ---You may try different combinations of R2 and R1, can also decrease current back. Maybe question of time spent running thermal sweeps with different combinations in order to select desired temp set point. https://www.eevblog.com/forum/metrology/lowest-drift-lowest-noise-voltage-reference/msg5507800/#msg5507800 Unfortunately that all does not help with LTZ1000, and upper Rz may add much more long-term instability and its own TC as well. --- End quote --- I am working on this now. I use an IET PRS300 for R2. https://www.ietlabs.com/prs-300-programmable-decade-resistor.html So I can automate change of R2 as temperature goes up and down. Then trying 70, 80, 90 and 100 ohm for R1 which I change on the board. Using an ADR1000. I still dont see any benefit in such high R2 as 70K as per the data sheet. |
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