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> mk> _______________________________________________ mk> volt-nuts mailing list -- [email protected] mk> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts mk> and follow the instructions there. _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
