Chapter 7 Measurement Tutorial

Common Mode Rejection (CMR)

Common Mode Rejection (CMR)

Ideally, a multimeter is completely isolated from earth-referenced circuits. However, there is finite resistance between the multimeter’s input LO terminal and earth ground as shown below. This can cause errors when measuring low voltages which are floating relative to earth ground.

Vtest

Vf

HI

Rs

LO

Ideal

Meter

Ci Ri >10 GΩ

Vf = float voltage

Rs = DUT source resistance imbalance

Ri = multimeter isolation resistance (LO-Earth)

Ci = multimeter input capacitance:

≈200 pF (LO-Earth)

Error ( v ) = Vf x Rs Rs + Ri

Noise Caused by Magnetic Loops

If you are making measurements near magnetic fields, you should take the necessary precautions to avoid inducing voltages in the measurement connections. You should be especially careful when working near conductors carrying large currents. Use twisted-pair connections to the multimeter to reduce the noise pickup loop area, or dress the test leads as close together as possible. Loose or vibrating test leads will also induce error voltages. Make sure your test leads are tied down securely when operating near magnetic fields. Whenever possible, use magnetic shielding materials or physical separation to reduce problem magnetic field sources.

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Agilent Technologies 34401A manual Common Mode Rejection CMR, Noise Caused by Magnetic Loops
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34401A specifications

Agilent Technologies 34401A is a highly reputable digital multimeter (DMM), renowned for its accuracy, versatility, and performance in a wide range of measurement applications. First introduced in the early 1990s, the 34401A has become a staple in laboratories, manufacturing environments, and educational institutions due to its robust build and comprehensive features.

One of the standout characteristics of the 34401A is its impressive accuracy, boasting a basic DC voltage accuracy of 0.0035%, making it ideal for precise measurements in electronic testing. The multimeter offers a wide range of measurement capabilities, including voltage (DC and AC), current (DC and AC), resistance, frequency, and continuity test, making it a highly versatile tool for engineers and technicians.

The 34401A utilizes a 6½-digit resolution, enabling the user to measure small changes in electrical signals with remarkable precision. With a sampling rate of up to 1000 readings per second, it can handle dynamic test requirements, providing timely results without sacrificing measurement integrity.

In terms of connectivity, the Agilent 34401A includes a GPIB (General Purpose Interface Bus) for remote operation, allowing users to automate their testing processes and integrate the multimeter easily into larger measurement systems. This feature is particularly beneficial in automated test environments, where speed and accuracy are paramount.

Additionally, the DMM incorporates advanced measurement functions, such as statistical analysis, including mean, standard deviation, and minimum/maximum readings, which provide users with valuable insights into their data. The instrument's user-friendly interface, complete with a clear digital display and intuitive controls, enhances usability and ensures that users can easily navigate its various functions.

Robust software support further extends the capabilities of the Agilent 34401A. It is compatible with a range of software tools for data logging and analysis, enabling users to efficiently document and analyze their measurements over time.

In summary, Agilent Technologies 34401A stands out in the realm of digital multimeters due to its high accuracy, extensive measurement functionalities, and user-friendly features. Whether for research and development, quality control, or educational purposes, the 34401A remains a preferred choice for professionals demanding reliable and precise measurement solutions. Its enduring legacy in the industry speaks to its performance and reliability, making it an essential tool for anyone involved in electronic testing and measurement.
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