Electronics > Projects, Designs, and Technical Stuff
Replacing Zener with vreg?
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ogden:

--- Quote from: TERRA Operative on November 09, 2018, 05:29:40 pm ---Yeah, I'm not sure why they went with such a precision part.

--- End quote ---

Could be so that they just had loads of that zener in stock and copy-pasted it. Anyway stability of test signal amplitude bias generator most likely will not impact measurements. I would just put TL431 here and see how it goes.


--- Quote from: SiliconWizard on November 09, 2018, 05:31:01 pm ---We can suppose they selected the opamps carefully and mounted them appropriately. Still, the "inverting position" poses even more questions here IMO.

--- End quote ---

Opamp is TL074CN, it's offset drift is 18 uV/C - way better than required. Also opamp inverter could be quite temperature-stable if low-TC resistor array used.
TERRA Operative:
Well I reckon the TL431B (0.5% version) sounds the way to go.  :-+

I'll just have to figure out the resistor values now and see how it goes.
GigaJoe:
so zener feed reference input for a  DAC that produce a voltage output.
means the long term precision for measurement defined by this particular zener diode.

I would suggest to replace it by LM399 + voltage divider, just that , R1-6 we disconnect and we can use to feed LM399, and voltage divider,
make sure about heater:
https://www.eevblog.com/forum/projects/negative-m399-voltage-reference/
resistors for voltage divider 5ppm PTF56, should work.

as switch U6.5 connecting positive and negative , producing a precision meander , i guess,  to feed input of opamp, so current are minimal and do not need buffer for resistor divider+LM399  ....

ogden:

--- Quote from: GigaJoe on November 09, 2018, 07:58:31 pm ---so zener feed reference input for a  DAC that produce a voltage output.
means the long term precision for measurement defined by this particular zener diode.

--- End quote ---

Well, it's not even measurement, but bias voltage applied to DUT. Most LCR/LCZ meters does not eve have bias option, yet they are fine.


--- Quote ---I would suggest to replace it by LM399 + voltage divider

--- End quote ---

I would not. LM399 is huge overkill for bias voltage gen.
GigaJoe:
"bias" - ?   
precision zener , 10bit DAC - ?   for bias ?
I'm skeptical,  I rather say  it a precision meander, generated by switch and then feed ADC to measure

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