[volt-nuts] Trimming the LTZ1000 tempco
willvolts at gmail.com
Sat Oct 1 14:19:59 UTC 2011
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.
> 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
More information about the volt-nuts