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| 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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