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3.5 volt reference + or - .020 volts

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rfranzk

Member
Hello All,

I am working on a project at work where an instrument we operate has a sensor that utilizes an auto id function. The id volts for this particular sensor is 3.5 volts dc + or - .020 volts. These particular sensors are prone to the id function going out of spec and that stops the instrument from functioning as it thinks there is no sensor attached. A new sensor is $1350.00 and when these fail the other sensor functions are just fine. I have disassembled and inspected one of these and it appears to be a simple voltage regulator with a voltage divider providing the id voltage. I just haven't had time to troubleshoot the real source of the problem and attempt repair as the components are all surface mount and difficult to work with.

I am looking for a work around until I can come up with a permanent solution. The instrument has a +15 volt supply that I can tap into for a source that measures 15.8 volts dc. I measured current draw on the id circuit of 0.00ua so it must be high impedance.

I was thinking just a 3 wire add on that utilizes the available + 15 vdc and provides the needed 3.5 volts by either a 5 volt regulator or reference with a voltage divider or a zener diode. The temperature environment has swings from 5 degrees C to 40 degrees C so it needs to be stable in that range.

I can assemble just about any kind of circuit but don't have the knowledge to design what is most effective.

Any assistance would be appreciated.

Thanks,

rfranzk
 

alec_t

Well-Known Member
Most Helpful Member
I'd use a regulator based on a TL431.
 

unclejed613

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i agree, a TL431 would probably be your best bet here. since the tolerance is about 0.6%, you want the "B" grade part (TL431BC) which has a tolerance of 0.5% as shown on page 12 of the [datasheet].
 

rfranzk

Member
Thanks for the input. I will review the datasheet more carefully for more questions concerning resistor values for the voltage regulator drawing and design parameters on page 25
Thanks

rfranzk
 

Pommie

Well-Known Member
Most Helpful Member
Replace the existing voltage divider with a multiturn pot. What is the voltage at the top of the existing divider?

Using the normal power rail will probably vary too much.

Mike.
 

kubeek

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+/-20mV at 3.5V is just a bit less than 0.5% accuracy, which the ordinary tl431 cannot provide. I suggest finding a variant that can do that, or some other precision reference.
 
Last edited:

AnalogKid

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The LM4040 comes in a 0.1% version, but it doesn't multiply up like the 431. You would have to start with the 4.096 V part and divide it down. This should work as long as the calculated value of the two divider resistors in parallel is 0.1% or less of the input impedance of whatever this thing drives.

ak
 

Pommie

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Could it be that the accuracy of the sensor is dependant on the voltage reference? Can you measure a good and a bad sensor both at the top of the divider and at the junction?

Mike.
 

ChrisP58

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The LM4041 comes in an adjustable 0.2% version. It's basically a TL431 with a 1.23V internal reference. http://www.ti.com/lit/ds/symlink/lm4041c.pdf

Keep in mind though, if you're using a pot to set the voltage, the real accuracy will depend on that of the voltmeter you're using when making the final adjustment.
 

rfranzk

Member
Could it be that the accuracy of the sensor is dependant on the voltage reference? Can you measure a good and a bad sensor both at the top of the divider and at the junction?

Mike.
The sensor in question is a temperature/baro press sensor and I have not had a new one here long enough to tear down and measure for comparison. The temp and bp sensors do however function linearly and as expected even when the id function doesn't.

+/-20mV at 3.5V is just a bit less than 0.5% accuracy, which the ordinary tl431 cannot provide. I suggest finding a variant that can do that, or some other precision reference.
The +/- 20mV is the stated specification by the manufacturer. In real life application I have just used a 10 turn precision pot to adjust a voltage with success at around +/- 40 mV. I think the TL431bc variant at .5% will suit my needs. I would at least like to start with that as it is pretty simple.

Here is an online 431 calculator
Thanks for the calculator. That will help. Just a quick question on the one from Texas Instruments. The bottom yellow input cell asks for IDiv/IRef. I don't see exactly what that input should be from the datasheet. I haven't had a chance to review it closely and probably won't till later tonight.

Thanks for everyones help and input!!
 

rfranzk

Member
Hello All,

I used the TL431b and things work just as I needed. A little more testing to confirm things stay stable with temperature swings and we are set. Thanks again for everyones input.

rfranzk.
 

crutschow

Well-Known Member
Most Helpful Member
The data sheets shows <10mV change in the 2.5V reference voltage over your temperature range, which should be well within your 20mV requirement for the 3.5V output.
 
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