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Reverse-engineering the LM185 voltage reference chip and its bandgap reference

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Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference

#21

Post by exmadscientist »

londons_explore wrote: post #14

Lots of microcontrollers have a bandgap reference built in... And typically they have really rather terrible voltage tolerances - eg. on atmel devices the 1.1v reference doesn't even have guaranteed minimum and maximum voltages across the whole range of supply voltage and temperature, but you can only expect it to be somewhere between 0.9 volts and 1.3 volts.... So why are these circuits so bad? Do they use a differe…

In addition to what Adrian said, Atmel built-in references are just notoriously bad. Worst-in-class, you might say!

Other vendors' equivalents are usually (usually!) better.

Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference

#22

Post by wiml »

wanderer wrote: post #17

If the following is not well known to electrical engineers, I would appreciate a little explanation of how it is done: "Thus, analog ICs are designed so only the ratio of resistors matters, not the absolute values, since the ratios remain nearly constant" If it well known, a reference would be helpful. (K. Shirriff, your posts offer a lot, even to those of us who do not have a technical background. Thank you.)

It very much is a basic technique you learn in analog circuit design (or at least it was 30 years ago when I was in school). Similarly, there are circuits (like that current mirror) which rely on the nonlinear behaviors of diodes or transistors varying in sync — you can build them on the same chip, or you can carefully select matched sets from one manufacturing run.

An example with resistors is a voltage divider. The output voltage depends on the ratio of resistances not on the absolute values (within reason). The resistors in a long-tailed-pair also need to be matched but their precise value is less important. Or you might have a bias current that's temperature-dependent but in a way that matches the temperature-dependent needs of some other bit of circuitry on the output.

There's a famous early integrated opamp which has a four-way-symmetrical die layout to compensate not only for temperature variations across the die but also process variations.

Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference

#23

Post by timerol »

wanderer wrote: post #17

If the following is not well known to electrical engineers, I would appreciate a little explanation of how it is done: "Thus, analog ICs are designed so only the ratio of resistors matters, not the absolute values, since the ratios remain nearly constant" If it well known, a reference would be helpful. (K. Shirriff, your posts offer a lot, even to those of us who do not have a technical background. Thank you.)

It is well known. "Matched components" are common in analog design. (No two things are ever identical, but things fabricated next to each other are more likely to be much closer, and you can get better tolerances if you're going for a specific ratio.)

As a simple example, a voltage divider has the output voltage depend only on the input voltage and ratios of the resistance values. Vout = R1/(R1+R2)*Vin https://en.m.wikipedia.org/wiki/Voltage_divider

Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference

#25

Post by kens »

grishka wrote: post #24
kens wrote: post #2

Author here for all your voltage reference questions :-)

Why am I getting "access denied" when trying to open your website? I had to use a VPN to get past this.

Are you trying to access from Russia? Russia is currently blocked.

Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference

#26

Post by grishka »

kens wrote: post #25
grishka wrote: post #24

Earlier quoted context omitted.

Why am I getting "access denied" when trying to open your website? I had to use a VPN to get past this.

Are you trying to access from Russia? Russia is currently blocked.

Yes. Why?

Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference

#29

Post by beeforpork »

adrian_b wrote: post #15
londons_explore wrote: post #14

Lots of microcontrollers have a bandgap reference built in... And typically they have really rather terrible voltage tolerances - eg. on atmel devices the 1.1v reference doesn't even have guaranteed minimum and maximum voltages across the whole range of supply voltage and temperature, but you can only expect it to be somewhere between 0.9 volts and 1.3 volts.... So why are these circuits so bad? Do they use a differe…

The transistors available in modern digital CMOS processes are worse and worse for analog functions, the more recent that process is. The very poor device characteristics may be mitigated only using very complex schematics for the analog circuit, together with various auto-calibration methods. The additional cost may be deemed too much for a voltage reference in a cheap microcontroller. One can always use a good exte…

> The transistors available in modern digital CMOS processes are worse and worse for analog functions, the more recent that process is.

Does that mean that we have a future ahead of us when a 6581 can be produced again with an authentical 'bad' sound? :-)

Re: Reverse-engineering the LM185 voltage reference chip and its bandgap reference

#30

Post by blacklion »

kens wrote: post #27
grishka wrote: post #26

Earlier quoted context omitted.

Yes. Why?

Because I consider Russia's behavior in Ukraine to be unacceptable.

I think, you mostly banish people who are against war. Politics, bandits, marauders don't read about in-depth silicone analysis.
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