4-Wire "Kelvin" Testing

4-Wire Kelvin Video Link

What is 2-wire and 4-wire?

If you've used an ohmmeter to make resistance measurements you've probably heard terms such as "2-wire measurement" and "4-wire kelvin measurement." This document explains how ohmmeters measure resistance, how 2-wire resistance measurements work, how 4-wire resistance measurements work, and the special considerations for each measurement type.

How do ohmmeters work?

When you use your ohmmeter to measure the resistance of a wire you touch one meter lead to each end of the wire and you get a resistance measurment (figure 1). How does the meter measure resistance? What resistance is it really measuring? To understand how ohmmeters work, start with Ohm's law; Resistance = Voltage / Current. This equation says "Put a current through the wire, measure the voltage drop along the wire, and you can calculate the resistance of the wire."

Your ohmmeter forces a current through the wire, measures the voltage that develops, calculates the resistance, and displays the result. To do all this your ohmmeter must have a current source and volt meter (see figure 2). What's important is where the current source and volt meter get connected together.

2-wire measurements

When you make a 2-wire resistance measurement your meter uses only two leads to connect to the device under test (DUT). Figure 1 shows a normal 2-wire test set up. This setup has the advantage of using just two wires to connect to the DUT but what is the actual resistance it's measuring? To measure just the resistance of the DUT you would want to measure just the voltage across the DUT. Figure 3 shows that the voltmeter is really measuring the voltage across the DUT and the test leads.

Two-wire measurements actually measure the DUT resistance plus the test lead resistance. What should you do when you really want to measure only the DUT resistance?

4-wire measurements

Some ohmmeters have four connections; two come from the current source (sometimes called the "force" leads), and two come from the voltmeter (usually called the "sense" leads). With an ohmmeter like this you can do a 4-wire measurement as shown in figure 4. With four connections you choose where to connect the voltmeter so you are in control over exactly what resistance you want to measure (see figure 5). If you connect the meter directly to your DUT you will measure just the resistance of the DUT.

The disadvantage of 4-wire testing is it takes four connections to do the test but it does give you an accurate resistance measurement of the DUT without the resistance of the test leads.

Resistance measurements in your cable tester

Your cable tester basically contains a high speed ohmmeter, with a current source and a voltmeter. Normally you do 2-wire measurements; you use two test points per measurement. More advanced testers allow you to also make 4-wire measurements; using four test points per measurement. To make a 4-wire measurement on your tester you generally need to create custom 4-wire test fixturing that combines the force and sense lines near your DUT, canceling the fixturing resistance.

You may not need 4-wire with a Cirris tester

Many continuity testers require 4-wire testing to accurately measure resistances under 1 ohm. The Cirris easy-wire CR, Signature CH+, 1100H+/R+, 1000H+/R+ and Touch1 testers use internal four-wire connections to reduce the fixturing (lead) resistance of the tester. All Cirris testers that measure resistance have this feature. Also, adapters that plug directly into Signature series testers eliminate much of the fixturing resistance that often occurs with adapting cables. If you need your resistance measurement to be accurate to only 0.1 ohms you won't need to use 4-wire on your Cirris tester.

Why not just subtract fixturing resistance?

Fixturing resistance is sometimes referred to as a "tare value" that could be removed to meet a specification for maximum resistance in the DUT. While the tare value can be used to adjust your measurements, it's not as simple as it first appears. First the accuracy of the tester is reduced by the ratio of fixturing to DUT resistance. This means that a .1 ohm DUT measurement with 2 ohm of fixturing and a 2% tester accuracy has (2 + .1) ohm x 2% = 0.042 ohms of variation or 42% measurement error (adjusted measurement error = Tester measurement error X (fixture resistance + DUT actual resistance) / DUT actual resistance). In this example the threshold for a good cable would need to be adjusted to .1 x (100%- 42%) = 0.062 ohms.

There is a more serious danger if you "tare out" the fixture resistance. Try measuring the resistance of a piece of wire with your VOM. You will find that the resistance varies depending on how hard you hold your lest leads to the wire ends. This variation in resistance comes from the point of contact between the DUT and the fixturing. This resistance variation from measurement to measurement can add significantly to a learned resistance and will get worse as the mating connectors wear. The effect of this variation could be that resistance thresholds are set too high and defective cables are allowed to pass.

What does 4-wire testing buy you?

1. It eliminates the resistance of your interface cabling. If fixturing resistance is a significant part of the total resistance then using 4-wire will greatly improve accuracy.
2. It allows you to measure lower resistance values than 2-wire testing. On Cirris hipot testers we use a higher current (up to 1Amp) when performing 4-wire Kelvin tests. This allows us to more accurately measure lower resistances, all the way down to 1 mΩ (0.001 ohm). Our low voltage testers that are 4-wire capable (CR, 1100R+) can measure down to 5 mΩ , but can still resolve to 1 mΩ . (You lose mΩ resolution once the DUT resistance is > 10 ohms)
3. If you make 4-wire connections on the DUT, not just the connector that mates to the DUT, you can eliminate all sources of fixturing resistance. This extra effort however may not be feasible.

What 4-wire testing will cost you

1. Complexity in fixturing, more wiring, more work building and maintaining the test fixture
2. Twice the number of test points are required in the tester.
3. Increased complexity in setting up a test that performs these specific measurements
4. Slower testing speed.

What you need to know before you can build a 4-wire test fixture

• You will pair 2 wires from the tester together for each test point of the device that is to be tested with a 4-wire measurement.
• You must combine the proper "force" and "sense" lines from the tester. Not just any 2-points can be paired to create 4 wire fixturing. Don't build it wrong! Check the 4-wire instructions for your tester. There are simple rules that tell you what points need to be paired.
• If you want higher levels of current to be applied for the highest level of resistance resolution then keep your fixturing resistance low. The fixturing resistance plus the DUT resistance should be below 2 ohms.

Which Cirris tester should I use?

Important "Fixture Setup" Information

Signature 1000H+: All test points must be wired as 4-wire points. You must use the Signature AV4W-64 adapter. This adapter has 2 VME connectors on it. You must pair A1 with C1, A2 with C2 and so on, for each 4 wire connection on the adapter. As you build your fixture keep in mind that you can not skip over any points in the adapter. The test points used must be contiguous.

Signature 1100H+, 1100R+, and Touch 1 Testers: These testers are more flexible in the 4 wire mode than the 1000H+. These three testers let you use Signature Adapters in any combination you choose and you can mix 2 wire and 4 wire points in the same assembly. This means you can use 4-wire test points only where you wish to.

Keep in mind that you still must 'pair points' using one type "A" and one type "B" test point for each 4-wire pair. You can be sure of using one of each type point for each 4-wire pair by:

• Using an A1KM-64 adapter and pairing A1 with C1, A2 with C2 and so on (note; of you use this approach, the fixturing will be compatible with CH+ testers also).
  
  
• Using the 'AHED Family' of adapters where pin 1 is paired with pin 2, Pin 3 with pin 4 and so on.
  
  
• Using IDC ribbon cable for all terminations by using two identical double high AHED adapters (i.e. (2) AHED-64's). Pair wires between the 2 adapters offset by 2 pin positions: J1-1 to J2 pin 3, J1-2 to J2 pin 4... This can be done by offsetting ribbon cable in the connector before crimping. An IDC connector crimped between the two ends of this cable forms a 4-wire connection. If you don't want to waste the first 2 pins on J2 then take the last 2 pins from J1 and connect them in sequence to the first 2 pins on J2.
  
  
• Using the help screen of the Touch 1 and follow the 4-wire constraints.

easy-wire CR Tester: You can mix 2 wire and 4 wire points in the same assembly on this tester also. Presently, 4-wire measurements cannot be made using "easy-wire adapters" that plug into the easy-wire transition boards.

(If you use a ribbon cable with 64 points, you can pair pin 1 with pin 33, pin 2 with pin 34 and so on.)

Signature CH+: You can also mix 2 wire and 4 wire points in the same assembly with this model tester. Pair point A1 with Point B1, point A2 with B2 and so on.
(Use your test probe to identify the test points.)