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LM431 (TL431) 0.1-10 Hz Vp-p noise.

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MasterT:
 I've been verifying noise level of the voltage references, and get puzzled by one part number from OnSemi. Here is the list:
AZ431 -1.2V/2.5V  Diodes Incorporated
TL431 - TI
TL1431 - ST
TL431 - ST
TLVH1431A - TI
TL431BCLPRMG
https://www.mouser.ca/ProductDetail/onsemi/TL431BCLPRMG?qs=xZq1yRCsb1eu65fxX1gBtQ%3D%3D

Others V.Ref compared with:
LM4040 - 2.5/3.0/4.096 - TI
REF6141 - TI

 All references shows more or less close data in the 0.1-10 Hz range, 5-10 uVp-p. Except TL431BCLPRMG, that have 3-5 times lower noise. Reading DS I notice  Figure 13. Spectral Noise Density, that is missing in DS from other makers. 

Does anyone else notice that not all 431 are the same?

Pulsepowerguy:
Unfortunately that Onsemi part is EOL. There are a few TI parts with noise data, and they appear to be way noisier than this one, however, their bias currents are 1mA whereas the Onsemi part's bias current is 10mA.
This is the best one I found:
https://www.ti.com/lit/ds/symlink/tlvh431b-ep.pdf?ts=1686444947389&ref_url=https%253A%252F%252Fwww.ti.com%252Fpower-management%252Fvoltage-reference%252Fshunt-voltage-reference%252Fproducts.html

magic:
TI is the original maker of TL431 and they have a noise density plot in their datasheet: 220nV/rtHz at 10Hz.
Indeed, onsemi's plot looks much better. It seems to apply to all packages and accuracy grades.

Not sure what the remark about 10mA bias current means; all onsemi's DC specs are similar to TI's. Minimum cathode current for operation is 1mA (edit: noise spec is given at 10mA, as are several other specs, but I doubt that cathode current makes a difference for noise).

The fact that onsemi's broadband noise is twice lower than TI's suggest that they may be using more bias on some internal transistors and possibly different topology altogether. What I do known is that many generic TL431 from China, such as the fairly common "WS TL431" in TO92 package use completely different circuit topologies; dunno about their noise performance.


There is a zillion chips functionally similar to TL431 with confusingly similar names, but  meaningful differences: other reference voltage (1.24V is common and surely requires a very different internal circuit), lower cathode maximum rating, lower minimum current for regulation (these are likely to have sky high broadband noise, physics has no mercy). The link above is an example of such part, it is not a 100% compatible substitute.

MasterT:
TL431's topologies from TI and OnSemi is exactly the same , both depicted in DS.  Seems like some improvement was made in technological processes by OnSemi, but it's not documented.

 I see a big problem with Voltage References IC documentation & standardization  in general, each manufacturer specified IC in different way - many omits spectrum charts.

 Having V.Ref with 10 uVp-p noise is only good up to 18-bits, what about 24 ? 
TL431 was developed in the 12-16 bits era, and it works well, but times changed. I was tested couple more IC REF5025 and MAX6164 - both show no differences with TL431 from 0.1-10 Hz noise perspective. 

There is some parts from AD in the 1-2 uVp-p range, that  insanely overpriced  and still at 20-bits level

 Is it a gap in the education, that nobody even understand that there is no V.ref. for 24-bits ADC on a market at all? 

 

magic:
Bandgap references are inherently noisy, as they amplify the noise of two transistors usually ~10x per each 1.2V of output voltage. And each halving this factor squares the area occupied by reference transistors, so it gets impractical fast. LT made some expensive and power hungry bandgap references with markedly lower noise, maybe these were the AD parts that you found.

Flicker noise can be improved by fab process, but wideband noise (which is also lower for onsemi) is often bottlenecked by collector current and can only be reduced by cranking up the bias. Not 100% sure what limits TL431 performance, although I feel like it's too noisy for base spreading resistance (the other limit on wideband noise) to be the dominant factor.

Resistor values in TI datasheet are known to be bogus, i.e. not representative of actual TL431 produced by TI. Resistor values in onsemi datasheet are the same as TI, so ::)

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