Products > Test Equipment

LM399 Voltage Reference, Fluke 289, Gossen 29S, RM303, AN870

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Trader:

--- Quote from: bdunham7 on December 27, 2020, 12:29:21 am ---Has anyone popped the can on one of these to see what is actually in there?  Or how about a photo of the backside of the PCB board, which I notice none of the sellers post?

--- End quote ---

bdunham7:
Well, OK, looks legit (except maybe the soldering).  It must be quite a chore for them to bin all the resistors to get that close a match on the voltages.  And the tempco of the LM399 would seem insignificant compared to the divider resistors.  They can't possibly afford low-tempco precision parts in that application.

TiN:

--- Quote ---I also don't see any evidence to give thinking that reference was calibrated correctly.
--- End quote ---
I didn't say it was :) It's just my point was that you don't have equipment to properly verify LM399-based reference. There is no such thing as "correctly calibrated", because every device/reference/DMM is incorrect by design/operation (short of quantum standards, but that's another story). Question that matters - by how much is it incorrect?

If you'd buy two of these modules - you could use Gossen 29S to test one against the other using lowest mV range of the DMM.

Difference between two references is often little easier to measure to good accuracy than absolute large scale value. It's valid method even on commercial lab level metrology, using nanovoltmeters to test one zener reference vs other, getting better results than even 8.5-digit meter :)

masterx81:
My china lm399 reference after 1.30h of warmup measure 7.38621v on a in-cal 34401 while on the reference there is wrote 7.38599. 220uv of error.
While the 9.84664v measure 9.84713. The 2.54773 measure 2.46511.

mawyatt:
It appears these PCB references are biasing the LM399 Zener section with a 7.5K ohm resistor from the input +15 Volts which passes thru a series protection diode which also feeds the heater circuitry. If this is correct it seems like the heater current draw will affect the zener current as will temperature due to the diode drop temperature dependancy and the heater current temperature variation. Aren't most uses "bootstrapping" the LM399 from the 10.00 volt op-amp output to reduce the effects of input voltage and other factors?

After a few months continuous power, the couple LM399's we have from DigiKey read a stable ~7.04 volts with a new KS34465A and a couple 34401As. Earlier had an LM399 from eBay and it was not stable and read under 7 volts, sold as new but had solder on the leads :P

Best & Happy Holidays,

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