EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: ivonenand on December 12, 2016, 10:12:02 pm
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Hi Guys,
I'm looking for a good stable reference for my 24-bit percision ADC. I need a reference voltage of 4.096V, or at least something in the 4 - 4.2V range. I see that LM399 is used often with 6 or even 7 digit DMMs so It seems like a good choice. The problem I see is that the LM399 puts out a voltage of 7V, so I would need to divide that down to 4V. I'm worried that that divider (and extra buffer) would introduce more ppm/C than I would like. So my question is, is the LM399 a good choice for a 4V-ish reference, or am I better of using one of the SOIC type reference voltages from Maxim or AD that have 1 - 2 ppm/C and an already made 4.096V output?
On a side note, I saw allot of reference designs with LM399 (and LTZ1000) that use 15V for power supply. I only have an (unstable) 12V available. I would put a LDO on that to 10V, for which I don't know if it's enough for s LM399 and it's heater.
Thanks for any advice in advance,
Regards,
Ivo
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you can use the LTC1043 to divide the reference by 2 . so if you make your lm399 reference output 8.2V (e.g. the standard 10V reference circuit scaled down to 8.2V) and then divide it by 2 and you'll end up with a fairly good reference voltage coming from a overly complicated circuit ;)
i would just use a 4.096V reference with a low(-ish) tempco instead.
btw... 4.096V is not the best reference for 24bit ;) 4.194304V would make more sense for 24bit... but the point is you can use whatever voltage you want or use a ref voltage at which your ADC gives the best results... because you'll have to do calculations in software anyway ;)
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If the ADC is a sampling instead of integrating type, then beware of broadband noise from the reference. Filtering reference noise can be difficult if low DC drift is still required.
I like the idea of dividing an LM399 reference output by 2 using an LTC1043.
Matched resistors networks with good TC tracking are available which may make the LM399 more favorable.