Hi, Mr. Mickle.T and Mr. Kleinstein ,
Thanks a lot!
I had added a line on the back of the PCB to reduce the res. of the track , but seems no obviously improvement by checking the data.
Could you help to identify some of the part from DC amplifier ,like Q102,Q104,U103,U104,U105 ,U102 and U101. Which are for the pricision channel? I am still trying to drawing the schematic for reference. It's not like the one in 6871 or 3458.
About the transfer error and positive-negative check error , I am not expect more than the specification. Most of the 6581 might keep the spec. limit when first in using. But as your mentioned , the design are more sensitive to the component aging or changes by some surge events. Some of them are of still accuracy enough like Mickle's and some are bad like mine. What I try hard for is to find out the most sensitive one and replace them with the new one. But I know maybe I can't find out the solution even with your help ,because it is too complex or can't find the replacement parts like the MUX or the JFET in DC amp. The 6581 is still a reliable and easy-to-use tool when debug in the 8.5 digit world., despite the issue we found.
I agree that the cap might be the sensitive one to aging. So I had tried the original SOSHIN cap together with the COG ceramic cap , which are the best two in my testing, and got some data as attached.The MAX error is around 10uv at 1v input of 10v range and nearly no big change with the original and the ceramic one. It is very interesting that the 1v range perform much better than the 10v range, why? I will try the new cap like the one using in 7480, which was producted recent years by SOSHIN, and report.
For the rundown resistor , it is the ceramic film type and packaged in one chip. The temp. coefficient will be better except the truth-value. I had checked both the external and internal calibration, it seems no timing for the directly resistor calibration. Maybe they do it by the indirect way like the voltage calibration? It 's unlikely not to calibrate the res. as the aging of the resistor maybe 10-20ppm/year.
I remembered that Mickle have given me one picture before ,which are the measured zero voltage by the ADC in the diagnostic mode. You can see that the X10zero and x100zero of mine are much bigger than the others. Are these the hint for debug?
Many thanks to your help these day!
Best Regards,
szszjdb