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Firmware rev 01.06.4C,
last week it arrived, I was playing around with it in the continuity function, pressed MENU and the screen got blue.
Today an other continuity problem, now not a blue screen but it hang up the meter. I changed the limit to 1.5V . After that I could change to the function screen but no response, until I pressed another function the round in progress thingy showed up and stayed then I had to re-power it.
OT: Great meter, I really like it. For me the speed, the sampling part (even better as I hoped), the dry circuit ohms, the nOhm-Gohm , pA resolution and the easy user interface are the winners but I do not know if I like the continuity and diode functions.
2 functions that are very important for me. (I really like the continuity from my 2000)
Continuity is fast enough but it latches much to long and the beep sounds like a mix of bird with a cold and a choked mouse.
Diode test has two levels and a beep (the same choked mouse), what is great on its own but not for my use. (component level repair work) My Agilent beeps once for a diode and stays beeping on a short so I do not have to look to the screen. The 14V is also great but not always for use in situ.
I still like the meter, it is so comfortable in use. I normally have a handful of meters powered up so I do not need to switch functions, but it is a bit sick a meter in this class after 3 years still has bugs. My Siglent scopemeter has some bugs too but not in a way it stalls and need a repower cycle (and is probably the cheapest isolated scopemeter on the market instead of a high end meter like the 7510)
I have a fully shielded probe station from Signatone with triaxial cables in and out of the station to the DUT. I saw some weird behavior with a net negative or positive offset current that was entirely dependent upon the load that the Keithley was measuring. I did a sanity check and measured different thick film resistors at zero Voltage bias and found that the Keithley said there was a net bias current that could be up to nA's (see plots below). This is frustrating as I am trying to measure a non-zero current (nA scale) for zero-bias applied voltages, but the Keithley shows a non-zero current for even a passive resistor which should show '0' current at 0 Voltage.
I noticed 3 weird things that should not be happening and am curious if each of these are a systematic issue with the Keithley or a measurement error on my side.
1. Every ~3 seconds with the 10 nA and 100 nA ranges, whether Output was OFF or ON or whether PLC = 1, 3, and 5, the current and voltage would spike semi-randomly to positive or negative values higher than the noise floor. I saw this on both 2450's and included an image below. Has anyone else experienced this and have a recommended fix? 2. There is a large difference in the current & voltage measurements when the Output is OFF or ON at the lowest range of 10 nA. The change in current measurement is about ~1 nA for a 1kOhm load and ~20 pA for a 10 MOhm load. The source readback returns ~0.2 milliVolts when a 10 MOhm load is connected for Output is OFF versus only 1 microVolt when Output is ON. This effect is significantly reduced (not eliminated) at the 100 nA range, but there are slight differences in the measurement between when Output is ON vs OFF. Is this common with the 2450's that the Output has to be ON for accurate measurements?
3. The net DC current at zero applied voltage depends upon the load only and does not depend upon the measurement window/time (# of PLCs). This is very counter-intuitive to me why a basic feedback Ammeter on the front end of the Keithley 2450 should show a consistent positive or negative current at zero voltage bias for a resistor! The data showed above has resistors from 1 kOhm to 10 MOhms and each shows a consistent current. This is not good for our measurements specifically because we are trying to measure the nA current for devices of different resistance at zero bias voltage without any systematic error that is introduced from the Keithley. I would greatly appreciate input on this issue specifically as to whether this is a common problem with 2450's or what would be a better instrument for this measurement!
Measurement Setup: Rear terminals, Triax cables to probe shielding (2-wire mode - Force Hi & Low connected), triax terminated with alligator clips from probe box to resistor (only center conductor on triax connected to resistor leads, others left floating), Sourcing Voltage & Measuring Current, Voltage setpoint = 0V, Source Readback On
Variables: Load resistance, Output OFF/ON, Manual Range 10 nA/100 nA, PLC 1/3/5
- Keithley Applications
- Posts: 1548
- Joined: October 15th, 2010, 10:35 am
- Country: United States
The assumption about 0V applied to the resistor should be reconsidered. The voltage sourcing for the 20mV range allows as much as 200uV of offset. Even with voltage read back, the measure spec has offset of 150uV. If the read back says only 1uV of actual voltage applied, the error bars are quite high on that measurement. If actual voltage is not zero, it would explain a load dependent current.
I suggest if you want to evaluate the current floor, turn output on and make an open circuit condition.
The 10nA measure range has offset error of +/- 50pA.
https://download.tek.com/document/SPEC ... 2017_0.pdf
Thanks for the quick response! The load dependent current definitely could result from the source accuracy being +/- 0.2 mV. The voltage needed to source the currents in those resistors varies between 2-20 microVolts so the sourcing accuracy specification seems very conservative.
I read through the operation manual multiple times and there is no mention about the discrepancy between the measurement accuracy when Output is On/Off, however I should have read the specifications sheet with more detail. I would like to note though, in the specs document it does not mention anything about specifications/operation with the Output OFF. I was confused about this when purchasing and thought the 2450's could be used as a Source and/or Measurement Unit meaning I could use the source and measurement functions independently, but I am learning a lot about the instrument which I appreciate!
I have a few follow-up questions, do the provided specifications apply for all DUT input resistances or what is the DUT resistance range where these specifications apply? I guess I am asking what procedure is used for generating the accuracy specification (i.e. what reference is the 2450 measured against to provide a 'true' '0' V or A). Secondly, is there a reasoning why is the measurement resolution on the 2450's is 500-10000x greater than the measurement accuracy?
Thanks in advance for your help.
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