Hi Mickle,

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

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

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

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

Regards,

M K

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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