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 <[email protected]> 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" <[email protected]>
To: "Discussion of precise voltage measurement" <[email protected]>
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 <[email protected]> ha scritto:
Positive input voltage range for an LT1006 is typically 1.2V below the positive
rail.
mitch
----- Original Message ----- From: "Fabio Eboli" <[email protected]>
To: <[email protected]>
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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