Hi everyone, As part of my checking over of the behaviour of the 7081 I ran "mode,testk" and "nines=8" overnight, the result is a swing larger than I would expect! max 166.00455 min 166.00040 this means a 25 ppm shift The circuit is meant to generate a constant current, so if that constant current can vary by 25ppm over a 20-23 degree house temperature then it is only accurate to 25ppm for resistance measurement except when used as a transfer standard. anyone else find a similar shift? M K

How long had the meter been powered on for when you started the test? Think days rather than hours to stabilise. D. -----Original Message----- From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On Behalf Of m k Sent: 20 January 2013 09:06 To: volt-nuts@febo.com Subject: [volt-nuts] 7081 resistance accuracy issue Hi everyone, As part of my checking over of the behaviour of the 7081 I ran "mode,testk" and "nines=8" overnight, the result is a swing larger than I would expect! max 166.00455 min 166.00040 this means a 25 ppm shift The circuit is meant to generate a constant current, so if that constant current can vary by 25ppm over a 20-23 degree house temperature then it is only accurate to 25ppm for resistance measurement except when used as a transfer standard. anyone else find a similar shift? M K _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.

Hi David, Yes, the meter has been on for days already. It had just done a long run into a shorting plug that shows much better stability, I suspect a tempco in the opto-isolators that transfer the reference volts, but will carefully consider all possible sources first. The other option is the input bias current that Mickle found. Regards, M K > From: david.partridge@perdrix.co.uk > To: volt-nuts@febo.com > Date: Sun, 20 Jan 2013 11:13:00 +0000 > Subject: Re: [volt-nuts] 7081 resistance accuracy issue > > How long had the meter been powered on for when you started the test? Think days rather than hours to stabilise. > > D. > -----Original Message----- > From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On Behalf Of m k > Sent: 20 January 2013 09:06 > To: volt-nuts@febo.com > Subject: [volt-nuts] 7081 resistance accuracy issue > > > Hi everyone, > > As part of my checking over of the behaviour of the 7081 I ran "mode,testk" and "nines=8" overnight, the result is a swing larger than I would expect! > > max 166.00455 > min 166.00040 > this means a 25 ppm shift > > The circuit is meant to generate a constant current, so if that constant current can vary by 25ppm over a 20-23 degree house temperature then it is only accurate to 25ppm for resistance measurement except when used as a transfer standard. > > anyone else find a similar shift? > > M K > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there.

Hi, M K! It is almost impossible to check any of the Ohms range performance via TESTK mode. While in this mode a constant current is applied to the ADC with a low input resistance equal to 160 kOhm (R201). R201 paired with R202 is a Vishay VHA51x custom metalfoil resistors set. It have a very low TCR tracking error (needed for ADC slope stability), but a large absolute TCR value, up to 5 ppm/C. Regards, Mickle T.

Hi Mickle, That is good to know, I know it is a positive tempco I have, I will just have to try a stable resistor and see what it gives me, but there is more checking to do first. Regards, M K > Date: Sun, 20 Jan 2013 17:02:04 +0400 > From: timka2k@yandex.ru > To: volt-nuts@febo.com > Subject: Re: [volt-nuts] 7081 resistance accuracy issue > > Hi, M K! > It is almost impossible to check any of the Ohms range performance via TESTK > mode. While in this mode a constant current is applied to the ADC with a low > input resistance equal to 160 kOhm (R201). R201 paired with R202 is a Vishay > VHA51x custom metalfoil resistors set. It have a very low TCR tracking error > (needed for ADC slope stability), but a large absolute TCR value, up to 5 ppm/C. > > Regards, > Mickle T. > > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there.