Fluke 8842a instruction manual DC Voltage Measurement, Circuit Loading Error

8842A

Instruction Manual

4-1. INTRODUCTION

This section discusses considerations and techniques to help you use the 8842A effectively. Among other things, this section discusses sources of error which are an inherent part of the measurement process and which occur for all multimeters. By understanding why and when these errors occur, and by knowing how and when to correct for them, you can make accurate measurements with confidence.

This section also discusses the relative benefits of 2-wire and 4-wire ohms, describes special considerations for making ac measurements, and presents some unusual applications--for example, using the test current in the 2-wire ohms function as a troubleshooting tool in itself.

4-2. DC VOLTAGE MEASUREMENT

When measuring dc voltages in high-impedance circuits, there are two possible sources of error to consider: circuit loading and input bias current.

4-3. Circuit Loading Error

Whenever a voltmeter is connected to a circuit, the voltmeter’s internal resistance changes the voltage of the circuit under test. The resulting error is called circuit loading error. The error is negligible as long as the resistance of the circuit under test (the source impedance) is small compared to the input impedance of the meter. As the source impedance approaches the input impedance of the voltmeter, the error can be considerable. The percentage of error can be calculated using the formula in Figure 4-1.

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Figure 4-1. Circuit Loading Error Calculation

4-2