Chapter 8 Tutorial

Measurement Fundamentals

True RMS AC Measurements True RMS responding multimeters measure the “heating” potential of an applied voltage. Unlike an “average responding” measurement, a true RMS measurement is used to determine the power dissipated in a resistor. The power is proportional to the square of the measured true RMS voltage, independent of waveshape. An average responding ac multimeter is calibrated to read the same as a true RMS meter for sinewave inputs only. For other waveform shapes, an average responding meter will exhibit substantial errors as shown below.

The internal DMM’s ac voltage and ac current functions measure the ac-coupled true RMS value. This is in contrast to the ac+dc true RMS value shown above. Only the “heating value” of the ac component of the input waveform is measured (dc is rejected). For sinewaves, triangle waves, and square waves, the ac and ac+dc values are equal since these waveforms do not contain a dc offset. Non-symmetrical waveforms, such as pulse trains, contain dc voltages which are rejected by ac-coupled true RMS measurements.

An ac-coupled true RMS measurement is desirable in situations where you are measuring small ac signals in the presence of large dc offsets. For example, this situation is common when measuring ac ripple present on dc power supplies. There are situations, however, where you might want to know the ac+dc true RMS value. You can determine this value by combining results from dc and ac measurements as shown below. You should perform the dc measurement using at least 10 power line cycles of integration (61⁄2 digit mode) for best ac rejection.

ac + dc = √ac 2 + dc 2

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34970A specifications

Agilent Technologies 34970A is a versatile data acquisition and measurement system that has gained popularity in various industries, including research and development, manufacturing, and education. The primary purpose of the 34970A is to offer an efficient solution for data collection, monitoring, and analysis, which can significantly enhance productivity and accuracy in testing applications.

One of the main features of the 34970A is its modular design, allowing users to customize the system according to their specific measurement needs. The mainframe can accommodate up to three plug-in measurement modules, which can include various types of measurements such as analog, digital, temperature, and frequency. This modularity provides flexibility for users to tailor the system to their requirements, making it highly adaptable to different applications.

Another notable characteristic of the Agilent 34970A is its impressive channel configuration. The system can support up to 120 measurement channels when fully configured, enabling extensive data acquisition without the need for multiple devices. This capability is essential for applications requiring simultaneous monitoring of multiple parameters or locations.

The 34970A utilizes advanced signal processing technologies to ensure high-accuracy measurements. The internal 6.5-digit resolution multimeter provides precise readings, while the device also supports various input types, such as thermocouples and RTDs for temperature measurements. The ability to perform mathematical functions, such as summation and averaging, on the acquired data further enhances its usability.

For data storage and management, the Agilent 34970A features built-in memory for storing up to 20,000 readings. Users can also easily transfer data to a PC through the RS-232 interface or GPIB (IEEE 488) for further analysis and reporting. Additionally, it offers the capability to program automatic data logging and scheduling of measurements, which streamlines the testing process and reduces manual intervention.

In terms of user interface, the 34970A is equipped with a large graphical display that provides clear visualization of measurement data and easy navigation through settings and options. The menu-driven interface makes it accessible for users of all skill levels, simplifying the setup and operation of the device.

Overall, the Agilent Technologies 34970A stands out for its modularity, high channel capacity, advanced measurement technologies, and user-friendly interface. As a reliable and efficient data acquisition system, it is well-suited for a wide range of applications, making it an essential tool for engineers and technicians looking to enhance measurement precision and efficiency.
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