[volt-nuts] Mickles 7081 drift issues, some ideas.

[Михаил]

Hi, M K!
7081 drift correction cycle is a ultra-slow chopper, integrated to the ADC.
Timed drift correction is performed by DMM (with drift=on) every 15 minutes if
nines=6,7,8. If nines=6 drift correction is useless, because of low resolution
of the ADC. Also drift correction cycle is performed every time, after changing
nines to any of the 6-8.
7081 reference have a two precision wirewound dividers. First of it (R306b:R306a) needs
for 6.2 to 20 V conversion with IC305 opamp. Second (R305a:R305b) - provides a
virtual ground for +/- 10 V. All modification I made with Cu wire have nothing to
do with zener current or reference voltage and apply only to second divider. Any
changing in the R305a:R305b ratio immediately lead to a change in ADC zero
point. Without such correction I made, ADC zero drift while DMM startup heating
was about 70-80 uV (ADC range is 10 V, drift=off). Of course, with drift=on any
temperature drifts in ADC input zero and reference virtual ground is corrected.
But short-term DMM stability is not sufficient.

Regards,
Mickle T.

Friday, January 18, 2013, 2:20:37 AM, you wrote:
mk> Hi Mickle,

mk> I have been looking at the documentation and circuits of the 7071/7081 and
mk> I think you have overlooked something.

mk> According to the manual, if nines=6 or greater then drift=off does not do anything.

mk> Also looking at the large tempco that you had I suspect that the reference
mk> zener had drifted enough that the minimum drift current setting was now
mk> incorrect for your unit, you should have been able to correct the drift by
mk> tinkering with the current setting number, and not by adding copper at
mk> +3700ppm c-1. OR the current delivering circuit had changed enough that the
mk> zener current was wrong. They are designed to have a minimum tempco at
mk> approximately 7.5mA, but for any individual item it may be different from that current.

mk> I used to be a service agent for Dranetz and their 305c's used a reference
mk> that was good for 6 to7 digit stability using the in829's and the required
mk> current could vary quite a bit. There was no particular use of any high
mk> stability resistors in the reference they designed, but it worked quite well.

mk> Regards,

mk> M K
mk>                                           
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