But for my first steps here I decided to use the circuit from the 3330B. Reasons:
- I have two 10V modules of this kind (no plans to dissamble them but using as a comparing reference)
- less complex
Well this PCB can be used to test your LTFLUs, as this is THE intended circuit for this kind of Reference Amplifier.
That means, it always serves two purposes:
1) Provide constant current for the zener and correct collector current for the serial transistor, for zero T.C.
2) create temperature and timely stable reference voltage, which is always between ground and base of transistor (The LTZ1000 is identical in this sense, but has different topology)
3) Always provide a stable amplified reference voltage to 10V or 15V (inside Fluke 335D). Advantage: only one OpAmp is used for both purposes!
The 10V stability depends mainly on the amplification resistors used. The RefAmp voltage in comparison, is much more stable.
The 1N4148 are placeholders, I will not use them. I have not identified yet, what Fluke has used here, I hope I will and then I will use the proper diodes there.
Btw, the sticker on my modules mention a Zener current of about 31µA, resp. 40µA, so 3mA looks a bit high to me.
No, that's not the zener current. If you calculate all voltages in the 3330 circuit, you will see, that the zener current is also 3mA. Would be much too noisy, with 31uA.
Maybe, the LTFLU circuit in the 732B has even higher zener current.
Nope, these 31uA is the collector current for zero T.C.
I have a similar sticker on my 332B/AF reference, 'I =92', that's the collector current, 92uA, which fits to the collector resistor.
Yes, I had a look into these manuals. And to be honest: I have some diffieculties to understand how they are working.
Yeah, I also did not yet calculate the whole circuit, it's really tricky.
But if you do, I bet that the zener current will also be a smooth value of 3, 4 or 5mA. That would be the main
Erkenntnisgewinn
from these circuits, how to optimally use the LTFLU.
The collector current again will bring T.C. to zero, and has to be tested for each individual RefAmp (on the order of several ten uA) and that's all, what's critical about the RefAmp circuit.
The amplification to 10V or another random voltage, is present also in these circuits, as otherwise they won't work.
This amplified voltage is not used in these calibrators, therefore the calibrator outputs in their 10V range are principally much more stable , than the 732A/B, as the scaling is done by a much more stable PWM.
If you don't need the 10.000V, simply buffer the raw ~6.7 V RefAmp reference, like we've done for the LTZ1000 circuit, and you're done.
I had the same thought, too, but I was a bit unsure about the absolute values. Except for those two mentioned on the sticker.
Interestingly, they are measured at the base of the transistor, not at the cathode of the Zener / emitter of the transistor.
I did it and found a Uz=5.99034V (measured with a calibrated DMM 7510) for one of the modules.
Well, you really have to understand the RefAmp circuit. The 'raw' reference voltage is ALWAYS the sum of the zener and the base-emitter voltage. That's identical to the LTZ1000 circuit.
Time stability is achieved mainly by the buried zener. Temperature stability is achieved by this zener / Ube combination, as the zener has a positive, Ube a negative characteristic.
The LTFLU / SZA obviously is better in this aspect, as the LTZ1000, due to a smaller zener voltage, which matches better with the T.C. of a pn structure, which in the end allows for a really precise zero T.C. trimming.
The LTZ1000 sucks in this aspect, as it always has about +50ppm/°C w/o oven, and is not really trimmable to zero T.C., any further.
Too late, one piece is on it's way to branadic.
branadic should not open it.. I know him personally, and I think he's a cultivated person..
Frank