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. Watch this video (above) and/or read below to learn more. 

• Greater Precision Resistance Testing
  Allows resistance measurements down into the milli ohm range.
• "Masks out" Fixturing Resistance
  Very helpful when fixturing is complex or long and has high resistance.
• High Current Resistance Testing
  Uses 1 amp of current to make more precise measurements.

How do ohmmeters work?

To you use your ohmmeter to measure the resistance of a wire you touch one meter lead to each end of the wire and you will 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."

Figure 1. 2-wire resistance measurement

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 voltmeter get connected together.

Figure 2. Meters contain a current source (I) and a voltmeter (Vm).

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.

Figure 3. A 2-wire measurement really measures the DUT resistance plus the meter lead resistance.

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.

Figure 4. A 4-wire measurement. Notice the meter has four connections.

Figure 5. A 4-wire measurement gives you control of where the voltmeter connects.

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 0.1 ohm DUT measurement with 2 ohm of fixturing and a 2% tester accuracy has (2 + 0.1) 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 0.1 x (100%- 42%) = 0.058 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 test 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?

Tester	Max Current	R Accuracy	Special Notes
CH2	2 AMP	+/- 0.001 +/- 2%	Point 1 gets paired with point 2, 3 with 4, etc... for16 points per VME connector.
Touch 1 and 1100H+	1 Amp	+/- 0.001 +/- 2%	Pairing of points is programmable within constraints.
1100R+	6 ma	+/- 0.005 +/- 2%	Pairing of points is programmable within constraints.
1000H+	1 Amp	+/- 0.003 +/- 4%	All points must be 4-wire in DUT. Must use the Signature AV4W-64 Adapter.
CH+	1 Amp	+/- 0.001 +/- 3%	Point 1 paired with point 2, 3 with 4... for 16 points per VME connector.
CR	6 ma	+/- 0.005 +/- 2%	Point 1 paired with point 33 Point 2 with 34... for each pin in 64-pin header connector.