4-wire(Kelvin measurement) with very thin lead wires - Page 1
Forum section: electronics / general

author: poinciana
author_rank: Newbie
timestamp: November 16, 2016, 06:16:07 pm
content: I am doing a 4-wire(Kelvin) resistance measurement on a sample of a few milliohms. I have to use very thin lead wires(42AWG). I am seeing the length of the lead wire is affecting the measurement. Longer lead wire shows higher measured resistance.4-wire measurement is supposed to remove the effect of lead wire resistance. Can anyone think of why this is happening.

author: SeanB
author_rank: Super Contributor
timestamp: November 16, 2016, 06:19:32 pm
content: You probably either have noise pickup, or your current source is running out of headroom driving the long leads and is having a reduced current drive. Another thing is the differential amplifier might have common mode issues with the voltage developed across the long lead.

author: Vgkid
author_rank: Super Contributor
timestamp: November 16, 2016, 06:39:07 pm
content: Even with a 4 wire measurement, most dmm's will specify the max lead resistance. As seanB said, those could be other reasons to consider as well.

author: The Soulman
author_rank: Super Contributor
timestamp: November 16, 2016, 06:42:06 pm
content: Quote from: SeanB on November 16, 2016, 06:19:32 pmYes I was also thinking that, or the meter could have a relative low input impedance.But both these scenarios would cause a lower reading not higher as the OP stated.So noise remains a candidate.What meter is used? And how long is a "longer" lead?

author: 2N3055
author_rank: Super Contributor
timestamp: November 16, 2016, 06:52:58 pm
content: There is only so much voltage drop that 4 wire setup can make up for.. Se eventually it runs out of headroom...Same thing with sense terminals on PSU.. It is sometimes specified in datasheet..

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: November 16, 2016, 07:44:36 pm
content: DVMs do have limits to compensating 4-wire connections, there is usually a specification called out in the manual for this and whatever effects it may have on the accuracy, DVMs measure and mathematically subtract out lead errors but it is not entirely accurate. It still depends on the DVM's accuracy of measurement which is limited at such low values. The Kelvin bridge was developed to MINIMIZE errors due to interconnecting lead resistance, it does not eliminate the effects. In more modern bridges such as the ESI 242D, there are two adjustments which compensate for lead and yoke resistance up to 0.1 ohms by modifying resistance in the bridge arms to compensate for the extra resistance, these adjustments come very close to eliminating errors within the limits. There is also a 6-wire modification of the Kelvin bridge which further compensates for other errors at very low resistances, the NIST website has information on the various methods of resistance measurement.No measurement technique is entirely free of error, each one has its own sources of error and knowing just what those errors are can help in making more accurate measurements.

author: VintageNut
author_rank: Frequent Contributor
timestamp: November 17, 2016, 02:18:35 am
content: In a SMU, the spec is maximum voltage drop between FORCE and SENSE.For the OP, please disclose what instrument or instruments you are using to accomplish the measurement.The datasheet of the instrument(s) should state under what conditions a 4-wire measurement will be in spec or out of spec.

author: grifftech
author_rank: Frequent Contributor
timestamp: November 17, 2016, 04:07:13 pm
content: Quote from: poinciana on November 16, 2016, 06:16:07 pmwhy so thin?

author: poinciana
author_rank: Newbie
timestamp: November 17, 2016, 05:23:15 pm
content: Quote from: grifftech on November 17, 2016, 04:07:13 pmThe measurement is on a thermoelectric material. I inject an AC current pulse to the sample, and measure the voltage with an amplifier and oscilloscope, there are 2 components to the voltage, one is due to the ohmic resistance of the sample(instantaneous), the other is a peltier component(has a time constant). I have to use very thin wires to minimize thermal leakage from the lead wires.

author: VintageNut
author_rank: Frequent Contributor
timestamp: November 18, 2016, 07:45:00 am
content: Quote from: poinciana on November 17, 2016, 05:23:15 pmThe input impedance of the amplifier should be very high compared to the impedance of your device. If not, you must calculate the error that the amplifier causes. If the impedance of device is not a constant, then the error will be dependent on the behavior of the device.

author: Assafl
author_rank: Frequent Contributor
timestamp: November 18, 2016, 08:45:52 am
content: Quote from: poinciana on November 17, 2016, 05:23:15 pmIf your device has a "time" constant (i.e. a component that has a thermal lag, or a reactive component such as a capacitive device that is spending time setting up an electric field or - conversely, an inductive device that is spending time setting up a magnetic field) - using a thinner and longer force lead (with a higher resistance) will indeed cause the measured voltage to change more slowly... Simple RC or RL or dR/dT...

author: VintageNut
author_rank: Frequent Contributor
timestamp: November 18, 2016, 09:19:51 am
content: Good point! My comments assumed that the device and measurement cables and measurement instrument are at steady state.If it requires many seconds or minutes or hours for everything to reach steady state, then you cannot rely on the measurement at all until steady state is reached.Touching the connections to set up the measurement induces body heat into the metal of some of the circuit connections. This temperature differential must settle before an accurate measurement can be taken.The technique to see this behavior is to short the device and then press REL (or its equivalent) to make the voltage measurement zero. Then look at the measurement. If the measurement changes from zero and continues to change, then the system is not at steady state and you must wait for the change to stop.Quote from: Assafl on November 18, 2016, 08:45:52 am

author: Assafl
author_rank: Frequent Contributor
timestamp: November 18, 2016, 10:58:27 am
content: Quote from: VintageNut on November 18, 2016, 09:19:51 amOr, conversely, decrease the force current as much as possible so as to measure steady state without affecting the reading (since he is pulsing the voltage to the device my guess is he is trying to do peek through...). Pick a higher range, and add as much resistance on the force line (and not DUT) as the DMM is willing to accept. Like you would do a thermistor or RTD reading (without causing self heating).

author: nfmax
author_rank: Super Contributor
timestamp: November 18, 2016, 11:03:09 am
content: What are the thin wires made of? You be running into thermo-electric potential problems...

author: poinciana
author_rank: Newbie
timestamp: November 18, 2016, 06:17:37 pm
content: Quote from: nfmax on November 18, 2016, 11:03:09 amI am using copper wires soldered onto the sample

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 28, 2016, 08:37:36 pm
content: You might be able to improve the situation by using short pieces of manganin where you don't want the thermal transfer. It has lower conduction than copper, but obviously more resistance. Still, a moderate gage will be low, and it has low thermal emf to copper. Then you can increase the gage of the wire to the meter. It's probably a calculus problem to optimize the lengths and gages, but I'll bet an improvement can be had.

author: nfmax
author_rank: Super Contributor
timestamp: November 30, 2016, 03:07:44 pm
content: Does the instrument you are using have an 'offset compensated ohms' mode? That would remove the effects of stray voltages arising from thermoelectric or electrochemical causes. If it doesn't eleiminate the issue, it may at least help narrow down the cause