# [volt-nuts] Trimming the LTZ1000 tempco

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Oct 7 18:40:12 UTC 2011

```Andreas Jahn wrote:
>> However the tempco matching isnt perfect with a "typical" (note
>> sample size = 1!!!!) residual of +50ppm/K (+350uV/K)
>
> I would not talk of a sample size = 1.
Even a sample size of 2 is too small to be useful in predicting the
tempco of the entire population of LTZ1000s
There will be variations due to manufacturing tolerances.
Perhaps your 2 LTZ1000s came from the same batch/wafer?
Measuring the tempcos of a statistically significant sample would be
more useful.
> My LTZ #1 has  48ppm/K with 70k and 52ppm/K with 50K
> My LTZ #2 has  54ppm/K with 70k
> Both have a setpoint divider of 12K5/1K.
>
> and:
>
> from the datasheet you can calculate for the 13K/1K nominal
> temperature setpoint
> 100 ppm in voltage divider change R4/R5 will give 1 ppm output change.
> 100 ppm change of 0,5V VBe will give around 50uV temperature setpoint
> change.
> 2mV VBE/K or 50uV/2000uV/K = 0,025K temperature setpoint change.
>
> So from datasheet with nominal divider you can calculate
> 40 ppm / K typical temperature gradient.
The major problem with that analysis is that the sign of the tempco
cannot be deduced and this only gives an approximate upper bound for the
magnitude of the tempco.
The tempco could lie anywhere within the [-40, +40] ppm/K range.

>
>> From Mickles "LTZ-guru" we can calculate 39ppm/K for the
> unheated reference. (0.95ppm output for 100ppm divider change)
>
> So there seems to be less tempco (but faster ageing)
> with higher temperature setpoints.
>
> With best regards
>
> Andreas
>
Bruce

```