One thing I was thinking about A/D noise of 2001, perhaps one can try using lower noise zener on ADC board. For example chinese 2DW232?
Hi my friend...
If you have one why not?
Just replace the VR801 and ready to go... may be fine tuning on the bias current (R854 value) but yeah piece of cake.
What I don' t like in the K2001 design is that has a separated ADC VREF (not using the LM399 one) and moreover this one is setting also the bias current on the LM399 --> Why this is not the case in the K2000? And also in the K2002 (I could be wrong here.. just did 5 min investigation...)? ... so why only the K2001 is ADC noisy and there other two look good? mmmm
Just to confirm I have the right understanding (hope to be right):
VR801, 2DW232 are short time references: they don't jump but they drifts slowly in the long period ("worst" TC).
LM399 or LTZ are long time references: they could have a pop corn noise (ppm jumping) but in average they are solid constant in the long period (better TC).
On the side, not ADC related, U328 AD707 (OPAMP for current bias on the LM399) why not replace it with something new? Didn`t check to much but it seems there are better OPAMPs out there pin to pin compatible. Are there other pin to pin compatible better devices replacement which could improve the references circuits?
Just writing straight off my brain.. sorry to all if I dropped something very stupid here, I have so much respect for all of you dragons thanks for reading this.