Hi,
I'm trying to do a 4 wire measurement. I have the problem that due to a large contact resistance, I cannot measure the test resistance very precicely with a Keithley 2400 Sourcemeter. I can reproduce the problem with this simple circuit, which I have taken from the Keithley manual with the additional pre-resistor R:
I'm using a resistance Rs = 9 Ω, which I can easily measure as long as there is no pre-resistor R in the current lead. Also with a pre-resistor R similar to Rs the measurement works.
But with R = 1 kΩ as shown in the figure above and Rs = 9 Ω it doesn't work. Only if I manually chose a suitable current I from 1 mA to 7 mA, I can measure Rs= 9 Ω with the SourceMeter. For other currents, I get these false readings:
7,1 mA, Rs = 9.5 Ω
7,4 mA, Rs = 36 Ω
20 µA, Rs = 9.8 Ω
10 µA, Rs = 10 Ω
1 µA, Rs = 20 Ω
0.1 µA, Rs = 100 Ω
Using another 2400 SourceMeter the errors get even larger, e.g. at I = 1 µA, Rs = -100 Ω (at small currents the resistence suddenly gets negative!). The same problem occurs with a pre-resistor R = 100 Ω and Rs = 9 Ω.
Does anybody have an idea what am I doing wrong? I've always thought that a 4 wire measurement is just the right methode for measuring a small resistance with high contact and lead resistances.
I'm trying to do a 4 wire measurement. I have the problem that due to a large contact resistance, I cannot measure the test resistance very precicely with a Keithley 2400 Sourcemeter. I can reproduce the problem with this simple circuit, which I have taken from the Keithley manual with the additional pre-resistor R:
I'm using a resistance Rs = 9 Ω, which I can easily measure as long as there is no pre-resistor R in the current lead. Also with a pre-resistor R similar to Rs the measurement works.
But with R = 1 kΩ as shown in the figure above and Rs = 9 Ω it doesn't work. Only if I manually chose a suitable current I from 1 mA to 7 mA, I can measure Rs= 9 Ω with the SourceMeter. For other currents, I get these false readings:
7,1 mA, Rs = 9.5 Ω
7,4 mA, Rs = 36 Ω
20 µA, Rs = 9.8 Ω
10 µA, Rs = 10 Ω
1 µA, Rs = 20 Ω
0.1 µA, Rs = 100 Ω
Using another 2400 SourceMeter the errors get even larger, e.g. at I = 1 µA, Rs = -100 Ω (at small currents the resistence suddenly gets negative!). The same problem occurs with a pre-resistor R = 100 Ω and Rs = 9 Ω.
Does anybody have an idea what am I doing wrong? I've always thought that a 4 wire measurement is just the right methode for measuring a small resistance with high contact and lead resistances.