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R&S®Essentials | Power supplies fundamentals

Understanding Remote Sense

Author: Paul Denisowski, Test & measurement expert

The following will explain how remote sense is used in benchtop DC power supplies.

What is remote sense in power supplies?

Benchtop power supply users often enter the voltage they want to be present at the load. However, the power supply lead cables will reduce the voltage seen by the load because they leads have a non-zero resistance. This resistance increases as the leads become longer and/or thinner. In many cases, lead resistance and the resulting voltage drop can be safely ignored, but this decrease in voltage can become significant when working with higher current or smaller load resistances.

Power supplies offering remote sensing, also known as four-wire sensing, are able to compensate for these voltage drops. To achieve this, the voltage actually present at the DUT is measured using a pair of sense lines in addition to the force lines. The measured value is used to regulate the power supply’s output voltage so that the desired voltage is present at the DUT input. Depending on the model, the Rohde & Schwarz power supplies provide remote sensing for each output channel.

Backpanel of an R&S®HMP4040 with force and sense connections.

The most common solution to this problem is to enable the supply to measure the output voltage at the load instead of at the output terminals. This methodology is called “remote sense”.

How does remote sense work?

In remote sense, an additional set of sense leads are used to measure the voltage as close as possible to the load. These leads are connected to a very high impedance in the supply, meaning that (almost) no current flows and therefore (almost) no voltage drop in these sense leads. Using the remotely sensed voltage, the power supply can then adjust its output to obtain the desired voltage at the load.

In the example below, the 0.3V drop was corrected by the power supply increasing output voltage to 5.3V.

Built-in measurements

Rohde & Schwarz DC power supplies come with a range of built-in measurements that can in many applications replace additional instruments such as an or a multimeter to measure e.g. instantaneous power. Since no extra load is connected to the power supply, no additional burden voltage needs to be taken into account – a major advantage offered by built-in measurements. In addition, they are very convenient and simplify the setup.

R&S®SMU200A provide further analysis functions such as statistics, including min./max. and average values for power, voltage and current as well as an energy count.

High-precision power supplies such as the R&S®NGL200, R&S®NGM200 and R&S®NGUoffer measurements with 6½ digit resolution. If in addition a power supply features a high-speed ADC, like with the R&S®NGM200 and R&S®NGU, even fast transients can be detected and measured.

Measured values can be recorded versus time using the logging functions. Some power supplies with a large display also offer graphical analysis functions.

Digital voltmeter functionality

Some power supplies can be equipped with an optional integrated digital voltmeter (DVM), which even further expands the instruments’ built-in measurement capabilities. The DVM allows measuring the voltage at any desired point in the DUT circuitry. This renders an additional multimeter superfluous.

The DVM option is available for all R&S®NGM200 models and the R&S®NGU.

This is an automatic / dynamic process. Once the sense leads have been connected and remote sense has been enabled, the power supply will automatically and continuously adjust the output voltage as needed.

Logging

While measurement results provide a fast overview of the actual voltage, current and power measured on the DUT, it is in many applications desirable to monitor and view variations of these values, i.e. the DUT behavior, over long periods of time, or with high time resolution, or both.

Standard logging and fast logging

Most Rohde & Schwarz power supplies offer standard logging functionality, and some even feature fast logging functionality. Standard logging collects measurement data from all active channels of a power supply simultaneously and stores it to one common file. The settings for standard logging are therefore found in the DC power supply’s device menu.

The R&S®NGM200 and R&S®NGU power supplies additionally offer fast logging. Fast logging (FastLog) with a sample rate up to 500 ksample/s allows capturing voltage and current values versus time in an oscilloscope-like manner. It can help analyze energy consumption of different functional blocks of a DUT and detect faults in the DUT’s circuitry.

Fast logging collects measurement data from individual channels of a power supply and stores it to separate files. The settings for fast logging are therefore found in the channel settings menus. By logging the values of the current flowing through analog sensors like photo cells and thermistors, environmental data like illuminance and temperature can be determined.

Summary

  • Lead resistance can reduce the voltage seen at the load.
  • In many cases this can be ignored, but it can be significant for small load resistances or high current.
  • Remote sense uses an additional pair of leads to measure the voltage directly at the load.
  • Based on remote sense reading, the supply can increase voltage to overcome the voltage drop in the supply leads.
  • This process is typically both automatic and dynamic and requires no additional user intervention once configured.

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