[volt-nuts] LTZ1000 reference appnote question

Thu Nov 8 17:36:24 UTC 2012

Hello Chuck, I understand that for dual supply, but is
this valid also for single supply like that one?
I was thinking that the lowest voltage in the circuit,
at least after powerup transient, will be ground
potential that is also negative rail's potential.

I agree that, without the output diode, if the out
is for some reason externally tied few mV below ground
there is really no hope to start the reference.

Fabio.

Chuck Harris <cfharris at erols.com> ha scritto:

> Assume that the diode isn't there, and the output of the
> opamp starts out at negative 0.1 volt. None of the
> semiconductor junctions will be active, so the 30K
> resistor will feed negative current into the "+" input
> of the opamp (positive feedback), and force the output
> of the opamp to the negative rail.  The opamp will happily
> stay that way because there is nothing to prevent it.
>
> The diode keeps any negative voltages from the output
> pin from reaching the noninverting input, and allows the
> circuit to only start up in the positive direction.
>
> -Chuck Harris
>
> Fabio Eboli wrote:
>> 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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