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Keithley 2450  Zener diode test

robrenz

I just had to try out the sweep functionality of the 2450 on a 12V zener.  Very easy to set up and the graph mode is very functional.The pan and zoom is a little slow and seems to have a glitch if you zoom in too far. Still very awesome.

 

As graphed with auto on X and Y axis Y is in linear mode

 

As graphed with auto on X and Y axis Y is in Logarithmic mode. Notice Amperage is shown as all positive ??????

 

Zoomed on the knee of the Linear Y graph

 

Zoomed on the knee of the Logarithmic Y graph

 

Zoomed on the forward conduction of the Logarithmic Y graph

Time to get back to getting used to this unit and doing the review.

  • Keithley tech support says the log scale is as intended and that there was a lot of discussion on how to show it. They said that that format is somewhat common in the industry and why they picked it. I suggested the flipped "negative log" scale or shabaz's suggestion of a different color for the plotted values that are actually negative.  These will be considered for enhancements. I also reported some graphing bugs that will be addressed.

     

    RR

  • in reply to shabaz

    IMO it is very simple, it just needs a negative log scale below zero.  The fact that technically you cant plot negative values on a log plot is irrelevant. just take the points that are <0 that they have plotted as positive and show them inverted below the x axis on a "negative" log scale. I have contacted Keithley tech support about it.

     

     

    Here is a better setup for measuring the diode that almost eliminates the cable leakage issue

     

    Much better now only about 18pA offset at the voltage reversal point.  Its current measuring accuracy is only +/- 50pA at this level so it is doing a pretty good job.

  • in reply to robrenz

    Since the instrument is allowing you to take measurements with forward and reversed current on a single graph, the implementation could have gone several ways:

    1/ flag an error and not allow you to plot the complete graph in log mode

    2/ Display a graph with a broken y-axis

    3/ Display two graphs

    4/ Display a single graph as it currently does (as shown in the screenshot above)

    5/ Display a single graph as in 4, but perhaps with two colors, one for each leg

     

    I think 1, 4 or 5 is the best implementation - option 1 and the others have these disadvantages:

    1 - Restrictive

    2 - (a) May be open to misinterpretation, and (b) means less detail can be shown on a given sized LCD

    3/ Less detail can be shown on a given sized LCD

     

    But, everyone may have a different choice. Which one would you have chosen, or some option 6? Just curious, in case there is a better way of representing the data that I've not thought of.

    I'm curious because at one time I was planning to design a home instrument to do a similar thing (cut-down of course...) and I would have drawn a graph in two colors, i.e. option 5 (and also curious because I'm always interested in graphs :-).

  • in reply to michaelkellett

    Thanks for concurring on the log behavior. Most of that apparent current offset is probably from not using the traxial connections with the driven guard. The worst case measuring accuracy spec at the lowest resolution of 10fA on 10fA is +/- 50pA which is about half of the offset in the graph.  Also the fact that the trace exactly touches the Y axis before it goes the wrong direction leads me to believe that it is the leakage current of the standard test cables I used for the test added to the diode leakage. 
    RR

  • in reply to robrenz

    I don't think you are wrong here, the negative current should be shown below the X axis - the graph as plotted shows two different voltages for each current level which is obviously nonsense. With a zener diode the user can work out what it really means but with some other devices this might be really difficult.

    I'm also slightly concerned that there is no indication (on the graph) that the current offset error dominates the current readings below about 1nA - it might be interesting to devise some tests to explore this in more depth.

    MK