[volt-nuts] Trimming the LTZ1000 tempco

[m k]

Looking at the data from various manufacturers of Zeners it seems that increasing the current makes the tempco become more positive.

> Date: Sat, 1 Oct 2011 17:19:59 +0300
> From: willvolts at gmail.com
> To: volt-nuts at febo.com
> Subject: Re: [volt-nuts] Trimming the LTZ1000 tempco
> 
> The 3458A reference board has 111 ohms and 74.25 kohms which are not a
> standard values either. It is also running 30 degrees higher than the
> datasheet circuit, so maybe it minimises the tempco around that higher
> temperature (pure guessing). Or HP was just stocking those values for
> other purposes at the time the board was designed.
> 
> It would be interesting to do some voltage vs temperature plots with
> different zener currents (from 3 to 8 mA for example). A potentiometer
> between the power rails and a high value resistor from the wiper to
> the temperature control voltage divider will do the job. 2mV/C change
> is probably an assumption accurate enough to calibrate the
> potentiometer scale in degrees. We don't need high accurary or good
> long term stability, just an idea of the overall behavior which tells
> us the direction to go when looking for the lowest tempco.
> 
> It is hard to believe that such a small variations in current would
> affect stability. Except through selfheating in unheated references
> but that is not an issue in this case.
> 
> Will
> 
> 
> > Hi Will,
> >
> > I was wondering about this too, it is not clear to me how important the
> > actual circuit values are. In the canonical "7V positive reference
> > circuit" we have R2=70k, R1=120 ohms.
> >
> > OK, R1=120 ohms gets you 5mA zener current at the operating point (when
> > it starts to turn on the reference transistor Q1 at ~0.6V Vbe).
> >
> > And R2=70k gets you a 100uA Q1 collector current, when Q1 is turned on
> > such that its collector voltage equals its base.
> >
> > But these are awkward values when shopping for precision
> > resistors. R1=100 ohms and R2=50k would be easier. Yielding 6mA diode
> > current and 130uA. Is there anything wrong with that? Or are the
> > 5mA/100uA magic values in some way? Perhaps they have been designed for
> > zero TC at 5mA - like a reference diode is selected for 7.5mA.
> >
> > The (unstabilised?) "low noise reference" on page 1 has a 30k collector
> > resistor. And the "averaging" dual reference on P6 has 150ohms zener
> > resistor and a 5mA collector current!
> >
> >
> > John Devereux
> >
> 
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