Keithley 2400 in manual ohms mode:  differing readings for R - Page 1
Forum section: electronics / testgear

author: smoothVTer
author_rank: Regular Contributor
timestamp: July 12, 2016, 07:00:47 pm
content: This question is specific to the Keithley 2400 SMU in manual ohms mode.I'm using a 4-wire Kelvin sense arrangement to measure a 0.1% 1-ohm 1/4 watt resistor. Remote sense is on, filtering is on, source readback is on. In Auto ohms mode, I see the 2400 sources 100mA and measures 0.9996 ohms. This figure agrees 100% with a Keithley 2000 DMM. Both are fully newly calibrated.In manual ohms mode, I'm varying the force current from 100mA, to 50mA, to 10mA, to 1mA.In each step, I see a different value for resistance computed depending on the current supplied. For example, for each value listed the computed ohms values is 0.9996, 0.9845, 0.9655 , 0.9538 ... etc. This is an anomaly, something I was not expecting, way outside the tolerance of the instrument. The computed value in ohms should be the same because the instrument computes V/I no matter what current is forced.Then I turned offset compensation on. This is the setting on the SMU which alternately applies the force current, then zero current, and subtracts the later voltage from the former to eliminate thermal EMF's. When this offset compensation was turned on, the calculated values for R were 0.9996, 0.9991, 0.9982, 0.9974 Closer together, and still much better, but not perfect. These values are closer to the instruments rated accuracy.Since thermal EMF depends on the materials and the temperature difference between them, and not on current, I do not understand why turning offset compensation on reduced the error in voltage read between different force currents.Anyone have an explanation to the described behavior above?

author: HighVoltage
author_rank: Super Contributor
timestamp: July 12, 2016, 09:23:43 pm
content: What kind of Kelvin clips do you have and where on the resistor are you hooking them up?

author: David Hess
author_rank: Super Contributor
timestamp: July 13, 2016, 05:34:46 am
content: Quote from: smoothVTer on July 12, 2016, 07:00:47 pmThat is exactly the result I would expect.At lower force currents, the voltage drop across the resistor is proportionally lower but the thermocouple voltages remain the same so they become a larger part of the result. Offset compensation works to remove the effects of the thermocouple voltages but it is not magic and as they become a larger part of the result, accuracy suffers.

author: smoothVTer
author_rank: Regular Contributor
timestamp: July 14, 2016, 09:13:32 pm
content: Quote from: HighVoltage on July 12, 2016, 09:23:43 pm2 sets of Pomona grabber clips used. Force leads on outside. Sense leads butting up to resistor body. Its a thru-hold 1/4 watt resistor.

author: smoothVTer
author_rank: Regular Contributor
timestamp: July 14, 2016, 09:15:27 pm
content: Quote from: David Hess on July 13, 2016, 05:34:46 amAh yes! Okay, thank you for your prompt edification.

author: Vgkid
author_rank: Super Contributor
timestamp: July 14, 2016, 09:43:12 pm
content: When you are running at a lower current w/ oc ohms enabled. You could take an averaged reading. That might help you get a better reading. Though with this, you still run into the same resolution errors that you face when running at a lower current.@ smoothVTer. Did you go to Virginia Tech, or are you from Vermont?