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Old 4th Apr 2018, 7:08 pm   #1
The Philpott
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Default LT1236 Series 10v reference chip, DIP-8

Having got round to using this chip kindly supplied by Nymrod121 (Guy) i am slightly befuddled by the PDF of the data sheet as it has gotten slightly corrupted and certain digits/letters are missing/shifted/substituted.

As far as i can tell,

Pin 4 GND is Ground for both input and Output (Load)
Pin 2 Vin is + input (Would 18v suffice?)
Pin 6 Vout is + output (10v)

Can i wire it in this manner for use as a 10v calibration reference (i don't need any more than 6mA on the output) or will it blow it's top!?

Thankyou
Dave
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Old 4th Apr 2018, 8:03 pm   #2
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Default Re: LT1236 Series 10v reference chip, DIP-8

Here's the datasheet: http://www.analog.com/media/en/techn...www.linear.com

48mW is going to heat up the chip somewhat, look at the thermal parameters in the datasheet to determine chip warm up, multiply this by the temperature coefficient and add ambient variations to get an idea of the worst case variation due to temperature.
Better to buffer it with an opamp with low temperature -input offset drift, to keep the heat out of the reference, and use an external pot to trim the output of the opamp, in that way you will calibrate away the initial offset of the opamp and reference.

When building a lab reference, better to NEVER turn it off.

To get the most out of these references, using a very stable supply LM723/UA723 is the go to chip when you want a power rail to feed a reference, the 723 itself is stable up to several mV.

Knowing what version you have will tell us the initial spread in output voltage of the reference.

Edit, your pinout is correct, this reference has the industry standard reference pinout, comparable with REF01 REF02 and a dozen others.

Last edited by Thatvalveguy; 4th Apr 2018 at 8:11 pm.
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Old 4th Apr 2018, 9:50 pm   #3
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Default Re: LT1236 Series 10v reference chip, DIP-8

Thanks! I will refer back if i get into trub with the circuit design. Any other comments welcome..

Although i quoted 6mA as a maximum, i would be happy with 50 to 250uA if that would simplify things.

Dave
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Old 4th Apr 2018, 10:26 pm   #4
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Default Re: LT1236 Series 10v reference chip, DIP-8

Hi Dave,

The reference will consume current itself, for maximum performance it is better to separate the reference from the circuits it is driving. For the opamp selection, look for something that has a small change in offset voltage over temperature, because this will determine the drift of the buffer amp. You can use a Resistor from the V+ rail to the output of the opamp to increase its current handling capability slightly, say it has to supply 6mA you can size the resistor to pass some of the current, the opamp will supply the rest. because the input of the opamp needs little current to bias up, you can use a RC filter in front of the op amp, to filter out some of the random noise from the reference(If the noise performance of the reference isn't up to scratch) you need an opamp with low offset voltage drift and a low input bias current drift for that (If you take a bipolar opamp)

From "Linear and interface application circuits 1986" "a bipolar opamp will have lower noise sub 10K source impedance, generally a Fet opamp is better above 10K source impedance" its a trade off between voltage noise or current noise.

please note that taking large resistance values can be worse then the noise from the opamp itself (due to resistor self noise), the C needs to be a film type with ample voltage rating to make sure that leakage doesn't influence the circuit, polystyrene capacitors are good in this regards if i remember correctly.

By the way, i meant using the external voltage trim of the 1236 to trim away any offset that the buffer might have, there is a circuit in the application data that uses some silicon diodes to compensate the temperature coeficient of the trim network.
Use a good quality 25 turns trimmer for that, not just any random carbon pot from the junk bin..
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Old 4th Apr 2018, 11:01 pm   #5
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Default Re: LT1236 Series 10v reference chip, DIP-8

Thanks again.

Just found some interesting data on www.analog.com 'Understanding and applying voltage references' on a pdf doc. by linear technologies, a useful summary of the pitfalls.

One of their examples uses a bipolar opamp & resistor in the way you mention, and also a cap. across the output. Much of the load is thus diverted away from the reference.

It looks like i can use 12v input.. I don't have any means of measuring (and trimming) the device for accuracy, so will be relying on the manufacturers spec.

The noise performance of this chip is said to be very good, and i think it has buried zener technology so i expect it to be very accurate if i do it right. Steep learning curve!

Dave
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Old 5th Apr 2018, 8:18 pm   #6
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Default Re: LT1236 Series 10v reference chip, DIP-8

The accuracy i am looking for can probably be judged by the fact that the highest resolution meter i am looking to calibrate using this circuit has a 4 1/2 digit display (and i do not think i will be upgrading)

(That PDF data sheet you have given a link to is still corrupted; do i need to use a newer version of adobe to view it or is there something wrong with the document itself? All other PDF's that i use look ok.)

I am now thinking of using (something like) the circuit shown below. R1 is 220 ohm, i think R2 is given as 5.6 ohm for use with the T1236-10 chip but could someone confirm as my data sheet is not clear.. Would a cap across the output still be required/recommended for my application?

Thanks
Dave
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Old 6th Apr 2018, 6:43 am   #7
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Default Re: LT1236 Series 10v reference chip, DIP-8

Found this Dave - http://www.analog.com/en/education/e...231599001.html and a datasheet onscreen here - https://www.digchip.com/datasheets/p...LT1236-pdf.php

See here for how to build a simple DIY reference here - http://frankshospitalworkshop.com/el...reference.html and here - https://hackaday.com/2015/03/23/buil...reference-box/ and lastly here - https://www.youtube.com/watch?v=9KtJr7Jv16I

Andy.
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Old 6th Apr 2018, 4:47 pm   #8
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Default Re: LT1236 Series 10v reference chip, DIP-8

I've tried the datasheet on a number of systems and it's been fine. Might be worth investigating another PDF reader, perhaps?

I would always advocate pragmatism here. We're talking about a 0.1% (10mV) chip after all. Let's start by saying that a 4.5 digit multimeter is going to read 10V with 1mV resolution

While it's not a bad idea to regulate the input to this IC, it's not necessary to go mad. Line regulation for this IC is typically 0.5ppm/V providing you're above 14.5V. That's 0.000,05%! So if the input voltage changes by 1V, the output might change by 5uV. Take a "jellybean" 7815: the output of that will be easily within 100mV over time and temperature, so that amounts to perhaps 0.5uV drift at the output of the reference. In practice, the 7815 will be much more stable than that.

The maximum input voltage is 40V, but the higher you go, the more heat will be generated inside the IC. Not that this IC takes much current (about a milliamp), but it makes sense to avoid this. I'd run it in the 15 to 18V range. A pair of PP3s would be ideal, perhaps with a low-voltage indicator (see here for a suggestion).

The tempco of this IC is pretty reasonable at typically 5ppm/C, or 50uV per degree C. This is likely to be better than the reference in the multimeter, frankly. The self-heating from the 1mA supply current is likely to be low (15mW isn't much), but as I don't immediately spot a thermal resistance figure, I'd be cautious about taking too much current from it for this reason - not without careful testing and characterisation first. But if we need half a millivolt to get our DMM to change the LSD,

If you buffer the output with an op-amp, then you'd have to carefully consider the DC specifications of the chosen op-amp. While it'll side-step the drift caused by heating in the reference IC, will it actually have more drift of its own? It's difficult because the datasheet might not cover all the parameters you'd need to know. Do not underestimate the need for careful testing and detailed engineering - while it's by no means impossible, it might well be better to just forget about it

Boosting the output of the reference using a PNP transistor is a tempting trick, but do be aware that if the transistor overheats and fails s/c between collector and emitter, then the input voltage will be applied to the output pin of the reference IC. Now, I'm really not sure how this IC will respond to this, but with the 78xx regulators, that's the fastest way to destroy them! So at the very least, consider using the version that includes current limiting (using an LED to limit the voltage seen at the base), and check that the transistor doesn't overheat when passing the chosen s/c current. BTW, R2 on your schematic should be 5k6 - 5R6 would be rather cruel on the transistor, don't you think

Finally, I'd really recommend that you find a way to measure the voltage with a better multimeter. Can you borrow one, or send it away to someone who has one? If no-one else offers, I have a Keithley 2015THD. The reason for suggesting this is simple: the IC has a similar spec to your multimeter, so it'll be difficult to know which to trust.

Hope this helps, and apologies in advance if I'm out by the odd power of 10 in the calculations above (they were done in a hurry), but hopefully you'll follow the thought-processes

Mark
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Old 6th Apr 2018, 5:15 pm   #9
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Default Re: LT1236 Series 10v reference chip, DIP-8

Better to update your version of reader, old versions might have security issues, i have no problem opening the PDF, nor would anyone else i think as Analog is a major corporation and corrupted PDF's on the webpage is unacceptable from a brand confidence standpoint.

DC accuracy of the supply rail is not the only banner spec, you should also look at noise, and the ability for the reference to reject this (power supply rejection ratio)

For all intents and purposes, i fully agree with you that a 7815 is more than adequate, however i suggested the 723 because its awfully good and inexpensive. The Ti 723 has marginally better ripple rejection, 84dB versus 70db ( I used the National datasheet, modern ones will likely be better)

The 723 itself is a buried zener supply, hell this IC itself could make a decent ish reference its a buried zener regulator, for a 50c part that its impressive. Side note: elektuur actually did in ''82
note the 84dB figure is true for a bypassed reference pin on the 723 5µF

The 317 does better then the 723 or 7815 in therms of ripple rejection, the 783 is even slightly better, however the 723 is still king of the hill in therms of DC stability, and i doubt any of the other bandgap reference based IC can beat it in therms of reference noise.

Put a 1n4004 over anything that might go boom if reverse polarity is applied is my mantra, this includes the input of the regulator as well as the input of the reference. These parts are the cost of a metal film resistor.

When it comes to op amp selection, the DC stability is indeed the most important spec, AD709 is one part some people use if memory serves me well, The EEVBLOG forum has a metrology page that has hundreds of topics dedicated to voltage references and precision meters.
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Old 6th Apr 2018, 5:24 pm   #10
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Default Re: LT1236 Series 10v reference chip, DIP-8

The 723 is great, but complicated. It's a "kit of parts" that needs a lot of external connections and components to make work. Whereas the LM7815 requires...

Yes, I'd add a diode too. But still use the current-limited version of the boost circuit, and test it thoroughly
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Old 6th Apr 2018, 9:07 pm   #11
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Default Re: LT1236 Series 10v reference chip, DIP-8

Plenty to think about there, i like the closing comment in the video clip- along the lines of

'Don't make one of these, make two- someone is bound to walk off with one of them'

I don't need to rush it as already have a DCV reference (it's just that the values it provides aren't round numbers.) I appreciate your help with this. Until now I hadn't considered a divider circuit to give more than one reference value, but fortunately i think i know where i can swapsie something for precision resistors.
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