DMM Resistance Measurement Analysis Notes 1

(1) The constant current voltage drop method is used. The advantage of this method is that it ensures linear measurement and reduces measurement errors. When using the 4-wire Kelvin connection method, it can also eliminate the influence of the additional resistance of the lead wire.

(2) Ratio method: Connect a standard resistor in series with the resistor to be measured, use the voltage across the standard resistor as the reference voltage, and then measure the voltage across the resistor to be measured. The advantages are: the circuit is simple, the measurement accuracy is closely related to the standard resistor, and it is not dependent on the power supply reference. It is generally used by low-end digital multimeters. The disadvantage is that since the stability of the reference voltage depends on the standard resistor, and a good standard resistor is relatively expensive, it is not suitable for high-end digital multimeter measurements.

(3) Operational amplifier method: The standard resistor and the resistor to be measured are placed in the feedback loop of the operational amplifier to form an inverting amplifier to measure the standard voltage. The disadvantage is that the protection circuit is difficult to make, and if you are not careful, the operational amplifier will burn out. Therefore, it is rarely seen in digital multimeters.

(4) Constant voltage method: Conductance is obtained by measuring current, and then resistance is calculated. It is generally used for high-resistance measurement of high-end multimeters.

The most famous high-end six-and-a-half-digit digital multimeter in the industry is Agilent's 34401. Let's see how it measures resistance: