[volt-nuts] Trimming the LTZ1000 tempco
m1k3k1 at hotmail.com
Fri Oct 7 21:15:19 UTC 2011
Well that expert on 38hot also got a +40ppm tempco.
This page shows that he was right to say a much lower collector current would be required. A smaller zener current and a lower collector current will reduce a + tempco, but at the expense of higher noise, so a tradeoff is that we must increase the noise to reduce the required accuracy of the temp control loop.
It seems to me that a lower set temp would reduce the sensitivity to orientation.
> Date: Sat, 8 Oct 2011 07:40:12 +1300
> From: bruce.griffiths at xtra.co.nz
> To: volt-nuts at febo.com
> Subject: Re: [volt-nuts] Trimming the LTZ1000 tempco
> 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
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