[volt-nuts] LTZ1000 reference appnote question

[Fabio Eboli]

Thank you Andreas, very interesting points you made.

In the meantime the original OP on eevblog thread
reported that Linear answer (from Bob Dobkin)
was about startup behaviour:

"It is for start up. If the amp has a negative supply,
it can start in the wrong direction.

Dobby"

Remains the doubt if he referring to negative offset,
or psu negative transient.

Fabio.

Andreas Jahn <Andreas_-_Jahn at t-online.de> ha scritto:

> Hello together,
>
> what about another theory:
> J.W. first developped the negative reference cirquit in the datasheet.
> In the negative cirquit a diode is essential that on no circumstance  
> the diode between reference and heater within the LTZ is not forward  
> biased.
> Later on J.W. gave the order to his apprentice to develop a positive cirquit.
> The apprentice did not dare to remove a "holy part" from the cirquit.
>
> 3rd theory:
> The diode helps to limit inrush current on startup through the zener  
> which is not specified above 5mA.
> I have a modified cirquit with a J-FET after the LT1013 and had to  
> add a limiting resistor to limit to below 14 mA.
>
> 4th theory:
> the startup pull up resistor after the diode has been forgotten in  
> the final cirquit.
> (like the ground connections and the "star" point).
>
> On the other side: the cirquit in my opinion lacks on stability. (at  
> least for large signal behaviour).
> The 22nF is charged by a high ohmic resistor and discharged by a low  
> ohmic transistor.
>
> The DATRON reference cirquit is using some kind of integrator for  
> the current regulation.
>
> With best regards
>
> Andreas
>
>
> ----- Original Message ----- From: "Fabio Eboli" <fabioeb at quipo.it>
> To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Sent: Thursday, November 08, 2012 2:41 AM
> Subject: Re: [volt-nuts] LTZ1000 reference appnote question
>
>
>> Mitch,are you referring to startup behaviour,
>> like Bob Smither suggests?
>>
>> Just now I was trying to imagine the startup
>> (it's too late for firing up ltspice, I have to sleep :) )
>> and I was thinking that on startup both inputs
>> are zero and output can take both ways: go down (remain approx zero)
>> or go up, depending on input offset.
>> Is this correct?
>>
>> If so what determines the correct startup
>> of the circuit?
>>
>> Until the output diode is not forward biased
>> the opamp has no feedback and the output should amplify
>> any input noise some million times (open loop gain),
>> so I imagine that is only waiting that the noise on
>> inputs will steer the output the rignt way to forward-bias
>> the diode and start the reference.
>> Does all this makes sense?
>>
>> Fabio.
>>
>> Mitch Van Ochten <mitch at vincentelectronics.com> ha scritto:
>>
>>> Positive input voltage range for an LT1006 is typically 1.2V below  
>>>  the positive rail.
>>>
>>> mitch
>>>
>>>
>>> ----- Original Message ----- From: "Fabio Eboli" <fabioeb at quipo.it>
>>> To: <volt-nuts at febo.com>
>>> Sent: Wednesday, November 07, 2012 7:02 PM
>>> Subject: [volt-nuts] LTZ1000 reference appnote question
>>>
>>>
>>>> Hello, in this thread on eevblog:
>>>>
>>>> http://www.eevblog.com/forum/projects-designs-and-technical-stuff/does-anyone-know-what-this-diode-is-for/
>>>>
>>>> a guy named DiligentMinds.com asked the question
>>>> about why there is a 1N4148 diode on the output of
>>>> the opamp in the LTZ1000 reference circuit, the one
>>>> designed by Jim William in the appnotes, that can
>>>> also be seen on the first page of the datasheet:
>>>>
>>>> http://cds.linear.com/images/product/1203_app_1.JPG
>>>>
>>>> Now I'm curious,
>>>> anybody knows the answer?
>>>>
>>>> Thanks,
>>>> Fabio.
>>>>
>>>> ----------------------------------------------------------------
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>>>>
>>>>
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>>
>>
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