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How to convert LM385 voltage reference to 5V system?

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niz_se

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Hi!

I´m in need to design a voltage reference to be used with an A/D converter (10bit). I need the reference to be 2.048V
I´m designing a 5V system.

So, I was looking at the LM385 because the referensce design looked simple but I do not know how to convert it from 9V to 5V system (if possible).

Later, because of the need of greater Temp. range I wanted to use the TS431 but the reference design for that one made a "brain fuse" burn...

Can anyone please help me out here?

Documentations for these devises can be found here:
LM385:
http://www.elfa.se/pdf/73/733/07338502.pdf
TS431:
http://www.elfa.se/pdf/73/730/07305345.pdf

Thanx! :?
 

House_Cat

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Analog Devices makes several 2.048v reference chips that have a wide input voltage range. National Semiconductor also makes the LM4120 that is adjustable to 2.048V, and can take input voltages from 2.5 to 14V. Maxim makes the MAX6062 that outputs 2.048V with input voltages between 2.5 and 12.6V

For example - take a look at the AD380, AD381, and the AD370. You can view the data sheets at:

http://www.analog.com/UploadedFiles/Datasheets/27775266ADR380_1_0.pdf

http://www.analog.com/UploadedFiles/Datasheets/362343661ADR370_0.pdf
 

niz_se

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Thanx for the replies!
I´m checking those chips out right now...

Anyway, because I don´t like unsolved puzzles, can anyone help me out on how to calculate the resistors, that sets the output voltage at the TS431?

And tell me if it´s possible to redesign the reference design for LM385, to work with 5V instead of 9V.

Thanx again!
 

silvio

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Niz_se, Sorry no ofence, but if you're not able to calculate the values of resistors how could you understand a good voltage reference design. I'm curious how you'll manage with PCB, noise, temperature drift and so on. I don't even try to imagine how will manage with the ADC or DAC.

Take a look here, seat and think what are the requirements in your system. Even if you're angry now on my advice, one day will succeed to see beyound this rage.

http://www.national.com/an/AN/AN-184.pdf

Regards,
Silvio
 

niz_se

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No worries Silvio, I´m not offended!
I´m well aware of my low knowlage in electronics, but my little project is based on the PIC16F870, so most problems are taken care of digitaly by programming. And that, I know more aboute.

If I just fix the hardware-problems with the built in A/D converter, I think that I can do this. Besides, if I don´t try, I never learn.

So, thanx for that document! I will read it. Now!
By the way. What I don´t understand in that algoritm for TS431, is how do I know the values of Iref and Vref?

Regards!
 

silvio

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Because you're honest I'll give you a help hand.

If you look into the documentation, you'll see that for a 10 mA current flowing through anode - cathode, the TS431 needs typical 70 nA as reference input current. That current is similar with ordinary BIAS current required by usual amplifier. It's like your tongue. It must be wet to eat your meal.

Now if you tied the reference input to cathode, the output voltage follows the reference voltage (typical 1.24 v) sucking from the main current a fraction of 70 nA. But you need more than 1.24 v
Thus, if you are able to keep the reference input current as closer as possible to 70 nA you can count on 1.24 on reference input. All you need now it's a divider build from R1 and R2, as per your drawing. The voltage developed across the R2 is 1.24 V and current equal with 1.24 v / R2. That current flows through R1 as well, plus 70 nA BIAS current. If you choose the resistors value in order that divider current to be higher than BIAS current you can forget about Iref. All that you must count is the value of R1 in order to get the required 2,048 v on cathode. That means a voltage across R1 of 2,048 - 1.24 = 0.808 v. If you know the current through R2 you can figure out the value of R1.

Maybe a little bit confusing explanation, but if you don't like mathematical in one equation described in the documentation you point to........

If you want to get deep you can pay attention to Ratio of Change in Reference Input Voltage to Change in Cathode to Anode Voltage and Reference Input Current Deviation Over Temperature Range.

If you didn't understand I'll try to tell you other way.

Regards,
Silvio
 

PPG78

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Try to use LM336-2.5. It's the best in relation to price&drift.
 

Tornado

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Voltage Reference

Hi Niz_se, you can use the series LM4040 from National Semiconductors
they come in various voltages and precision and very good stability.
I use them in my projects at work many times with very good results.

Regards

Tornado
 

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