Voltage Measurement

Course overview

The course provides a comprehensive introduction to acquiring precise voltage data across a wide range of contexts using DewesoftX and compatible DAQ hardware.

You’ll start with the fundamentals—understanding voltage types and how to quantify waveforms using peak, RMS, and crest factor metrics. The training uses analogies (e.g., water pressure for voltage) to make concepts intuitive.

Next, you’ll explore amplifier hardware:

• Low-voltage amplifiers (±50 V) ideal for sensor outputs

• High-voltage amplifiers (up to 1200–1600 V) for power applications

• Voltage divider/probe options for very high-voltage scenarios

A key module addresses measurement safety and accuracy: isolation, common-mode voltage, and bandwidth considerations—especially important for high-frequency or PWM signals.

Hands-on setup lessons guide you through DewesoftX channel configuration: selecting voltage type, range, amplifier, and optional scaling for transducers or dividers. You’ll learn to visualize waveforms and synchronize voltage data with other sensor types—enabling multi-domain analysis.

By course end, you’ll be able to configure optimal voltage measurement setups for small signals, AC waveforms, high-voltage lines, or sensor outputs—ensuring accurate acquisition, secure operation, and seamless integration into broader measurement systems.

Voltage, sometimes referred to as electromotive force (EMF), is what causes electric charges to move. It represents the electrical potential difference between two poles: the positive and the negative.

Voltage can be either AC or DC, depending on the type of current carrying it. In DC systems, the current never changes direction—it is unidirectional and does not change polarity. In AC systems, however, the current alternates direction, crossing 0 V in the positive direction, then reversing and crossing 0 V again in the negative direction.

Both DC and AC voltage (and current) are represented in the graphs below:

Electricity is difficult to visualize because we cannot physically see whether voltage is present or whether current is flowing. To make it easier to understand, we can use a water analogy to explain how electrical circuits function. Water systems are simpler to grasp because we can directly observe the water and its behavior.

Consider how a water system behaves. It is a well-known physical fact that, for water to flow out of a pipe, the water must be pressurized—this is usually achieved with a pump. In the electrical analogy, current corresponds to the flow of water, voltage corresponds to the water pressure, and the pump represents the battery.

This means that voltage drives the flow of current, just as water pressure causes the flow of water through a pipe.

The device used for measuring voltage is called a voltmeter. Since voltage represents the potential difference between two points, the voltmeter is always connected in parallel to the circuit (see image 2). To minimize its influence on the circuit, the input impedance of the voltmeter must be very high. The typical input impedance of a voltmeter is 10 MΩ.

Measuring voltage is the most basic operation with Data Acquisition (DAQ) devices, as most Analog-to-Digital Converters (ADC) use voltage as their input value. Measuring voltage with DAQ seems simple if the voltages are within the range directly supported by the ADC. However, when measuring very small voltages in the microvolt (µV) range or very high voltages up to several kilovolts (kV), an amplifier is required to condition the signal for AD conversion. Dewesoft provides solutions for both challenges.

• Low Voltage Amplifiers (LV and HS-LV): In combination with 24-bit ADC technology, they enable precise measurement of very low voltages, even at high measurement ranges (e.g., µV resolution at a range of ±10 V).

• High Voltage Amplifiers (HV and HS-HV): Allow direct measurement of voltages up to 1600 V DC (1200 V DC on HS modules). For voltages higher than 1600 V DC, voltage probes/dividers or voltage transducers must be connected to the instrument.

Isolation voltage

When measuring voltage, it is critical to select the correct amplifier range. Using an incorrect range can destroy the amplifier if the measured voltage exceeds the isolation voltage of the integrated amplifiers.

For example, measuring the public grid voltage (230 V RMS / 325 V peak) with an STG module can destroy the module, since the isolation voltage is rated at 200 V peak for ranges below 10 V, and 300 V peak for ranges above 10 V. Therefore, to measure voltages higher than ±100 V, the use of HV amplifiers is mandatory.

Measurement range

Selecting the appropriate measurement range is essential for both accuracy and reliability. Each amplifier provides multiple ranges that can be configured in the Dewesoft X  channel setup.

• If the selected range is too low, the signal will exceed the input range, causing errors and channel overloads.

• If the selected range is too high, the resolution will be reduced, leading to inaccurate results.

The most precise measurement is achieved when the measurement range matches the DAQ amplifier input range. This ensures the highest resolution since the full number of ADC bits is utilized.

Example:
If only one-tenth of the ADC input range is used, the resolution will also be reduced to one-tenth of the converter’s potential. A 16-bit converter can resolve 65,536 discrete values, but in this case, only about 6,500 values would be used—resulting in low resolution.

Measuring a signal from 0–7 V with a 1200 V range (resolution ≈ 18 mV) would be ineffective. Instead, using a module with a 10 V range (resolution ≈ 0.15 mV) would provide much higher accuracy. For this reason, different modules with diverse input and measurement ranges are available.

There are various types of voltage, so it is important to first identify which type is being measured. Voltage types include peak, peak-to-peak, average, RMS, AC, and DC. The differences between them can be better explained using the image below.

The average voltage, as the name suggests, is the average value over a certain time period. For pure sinusoidal (AC) signals, the average is zero.

The RMS voltage (root-mean-square voltage) is the square root of the arithmetic mean of the squared function values that define the continuous waveform. It is the most commonly used value for defining AC voltage at a given point, as it produces the same energy in an ohmic load as an equivalent DC voltage.

The peak voltage represents the highest voltage reached during a given period. In datasheet specifications, the term peak voltage often refers to the same value as the DC voltage of an input. To calculate the RMS value for sine waves, the peak value must be divided by the square root of 2.

The peak-to-peak voltage describes the amplitude between the maximum positive and maximum negative peaks within one period.

The crest factor is defined as the ratio of the peak amplitude to the RMS value of the waveform.

The rectified mean is the average of a rectified signal. For an AC signal, it represents the average of the absolute values of voltage or current.

The rectified mean is commonly used in applications such as transformer testing, since it is proportional to the magnetic flux.

There are three main types of DAQ amplifiers: single-ended, differential, and isolated amplifiers. Each is briefly explained below.

Single-ended

Single-ended amplifiers have only one input pin, as the second pin is connected directly to ground. Because of this configuration, they are suitable only for measuring floating voltage sources where one output point can be connected to ground.

This type of amplifier is simple to use but has two major disadvantages:

• Susceptibility to unwanted ground loops

• The amplifier is not isolated

A ground loop is an unwanted current flowing from the sensor ground to the instrument ground due to a small difference in ground potentials. Although these potential differences are typically in the microvolt (µV) range, they can introduce a significant amount of noise into the measured signal.

Example:
Consider a sensor with an output range of 10 V connected to a single-ended amplifier, where a dynamic range of 140 dB is required. After a simple calculation, it is determined that the maximum allowable potential difference between the sensor and the instrument ground is 1 µV. To address this, the solution is to isolate either the sensor or the instrument.

Differential amplifier

A differential amplifier has two inputs that are electrically separated from ground. This is the most common type of amplifier, as it amplifies the voltage difference between the two inputs. With this configuration, ground loops can be avoided. However, care must be taken with the common-mode input voltage, as it can still affect measurements if not properly managed.

What is common-mode input voltage? One way to describe the input common-mode voltage (VICM) is as the average voltage of the inverting and non-inverting input pins.

Another way to imagine VICM is as the voltage level common to both inputs, Vin(+) and Vin(-). In other words, for the differential inputs of a DAQ system, which measure the difference between the two signals, the differentially measured value is small, but the common-mode input voltage itself can still be very large—sometimes in the range of hundreds of volts (e.g., current measurement with shunts).

Another important term describing differential amplifier inputs is the input common-mode voltage range (VICMR). This parameter, frequently specified in datasheets, deserves particular attention. VICMR defines the range of common-mode input voltages within which the amplifier operates correctly and indicates how close the inputs can approach either supply rail. This means the potential of the input pins must remain between the supply voltages (V+ and V-).

Isolated amplifier

Isolated amplifiers eliminate the disadvantages of both single-ended and differential amplifiers. They are unaffected by ground loops, common-mode voltage, short circuits, and similar issues. These modules are electrically isolated from the housing and the main board of the measurement device. As a result, the amplifier measures only the difference in absolute voltage. The high isolation voltage (relative to the measurement range) ensures safe and reliable operation even during voltage peaks or faults, making isolated amplifiers suitable for a wide range of applications.

The main advantage of differential amplifiers is their lower cost. They are well suited for measurements with isolated sensors such as strain gauges or current clamps. Differential amplifiers can also provide high-quality measurements for non-isolated sensors, but engineers must be mindful of sensor behavior—particularly common-mode range and isolation requirements—to ensure accuracy.

Isolated amplifiers are more expensive but provide a worry-free solution up to their rated isolation voltage, ensuring safe and accurate measurements even in demanding conditions.

Let's take a look at how a low-voltage measurement of up to 50 V can be performed. Voltages in this range can be connected directly to several different Dewesoft amplifiers. These amplifiers differ in terms of measurement range, isolation, bandwidth, noise performance, and extended functionalities.

Each measurement channel supports multiple input voltage ranges. The most precise measurement is achieved when the selected input voltage range of the channel closely matches the voltage level of the measured signal.

The table below lists the Dewesoft amplifiers that can be used for measuring voltages up to 50 V. It also provides information about the maximum sampling rate, bandwidth, and the available measurement ranges for each amplifier.

Common low-voltage amplifiers support a sampling rate of up to 200 kS/s per channel with a maximum bandwidth of 75 kHz. The High-Speed (HS) series is designed for applications requiring both high sampling rates and wide bandwidths, such as voltage or current measurements in inverters. The HS series offers a sampling rate of 1 MS/s.

Once the appropriate amplifier has been selected, the signal simply needs to be connected to the amplifier.

Measuring voltages higher than 100 V requires the use of the SIRIUS HV or HS-HV module. The SIRIUS HV module supports measurements up to 1200 VDC, while the HS-HV module allows measurements up to 1600 VDC.

The table below compares these two high-voltage amplifiers, providing information about their measurement range, sampling rate, bandwidth, and isolation.

Just as with low-voltage amplifiers, the HS series is designed for measuring very fast signals, such as pulse-width modulation (PWM) regulated voltages in inverters. Inverters operate at switching frequencies of up to 200 kHz, which requires both a high bandwidth across the entire measurement chain and a high sampling rate.

To support analysis of all types of applications, the HS-HV module offers a maximum sampling rate of 1 MS/s.

HV vs. HS-HV

To simplify the selection of the most suitable module for a given application, both modules were used to measure the voltage of a PWM-regulated 3-phase servo motor. The results were then compared. Both modules were set to their maximum sample rates: 200 kS/s for the HV module and 1 MS/s for the HS-HV module. While the coarsely measured data appears similar, the real differences become clear when analyzing the PWM-modulated sine wave voltages in more detail.

The first noticeable difference appears in the “chopped” sine wave. With the HV module (200 kS/s), an overshoot occurs when the signal “jumps.” In the image below, the measured motor voltage with the HV module at 200 kS/s is depicted. The overshoot is clearly visible as a small spike at the beginning of the voltage level shift.

When the signal is zoomed in for a closer look at the resolution of individual samples, it becomes clear that the overshoot is caused by an insufficient sample rate. Because of the short rise time of the voltage, only one—or sometimes no—samples are captured on the slope. This results in measurement errors at the edges of the signal.

The HS-HV module has a higher bandwidth and therefore provides five times more samples within the same measurement period. This eliminates overshoot in the measured signal. The cleaner transitions are the result of capturing multiple samples along the slope, which provides better resolution at the edges of signal jumps.

The issue of overshoot with the HV module begins at switching frequencies of around 2 kHz. At higher switching frequencies, the differences between the dual-core HV and the HS-HV modules become even more pronounced. To illustrate this, a second measurement was performed using the voltage output of the frequency converter, but this time the switching frequency was increased to 16 kHz.

The first clear difference is in the bandwidth of the measured signal. With the dual-core channel (blue), the frequency response begins to attenuate at around 66 kHz, and no frequencies above 100 kHz appear in the measured signal. In contrast, with the HS module (green), frequency attenuation starts much later. As shown in the image below, the HS module is capable of measuring a much wider bandwidth than its dual-core counterpart.

When comparing both measured signals on the scope, we can see that the HS module responds a few microseconds faster than the dual-core module.

Another important difference when measuring high-frequency voltages is the handling of transients. The HS module is significantly better at capturing transients, providing more accurate oscillation coverage after each peak.

While voltage measurements up to 1 kV are relatively straightforward, measuring voltages above 1600 VDC becomes more complex. In such cases, voltage probes/dividers or voltage transducers are required to scale and reduce the voltage to a level suitable for the amplifier.

Voltages probes (voltage dividers)

There are two main types of voltage probes: the pure resistor voltage probe (for both AC and DC measurements) and the resistor-capacitive voltage probe (for AC measurements only). The input impedance of a voltage probe should be as high as possible—meaning the input resistance should be high and the input capacitance kept low.

Voltage probes are available in three categories: active, passive, and differential.

• Passive voltage probes: These are simple, inexpensive, and robust. However, they typically have high input capacitance and can present challenges when measuring low voltages.

Active voltage probes have a high input resistance and low input capacitance, but they require an external power supply. They are significantly more sensitive—and also more expensive—than passive probes.

Differential voltage probes: Passive and active voltage probes function as single-ended amplifiers referenced to earth. However, when a differential signal needs to be measured, a differential voltage probe must be used.

The operation of a voltage probe can be explained using a simple resistor probe, consisting of two high-resistance resistors connected in series.

When using a voltage divider with equal resistance values (R1 = R2), the voltage drop across each resistor will be half of the applied voltage. For example, when measuring high voltages up to 2000 V, this simple transducer reduces the voltage to 1000 V. This is low enough to be measured with a SIRIUS high-voltage module, which supports input ranges up to 1200 V.

If the resistors have too low an impedance compared to the circuit where the voltage is measured, a substantial current spike may occur. This current can affect the measured circuit and reduce measurement accuracy. To avoid this, the input impedance of the measurement instrument must be 100 to 1000 times higher than the value of R1. Otherwise, the divider ratio will change, and the input impedance of the instrument must also be considered.

Another factor to take into account is the short-circuit resistance, which equals the sum of R1 + R2. If this value is too low, it may cause a short circuit.

When using a voltage probe, the ratio between the input and output voltage must be calculated and entered in the channel setup of the software. This ratio is critical because the measured voltage from the device must be multiplied by it to obtain the actual voltage (Vin, as shown in the image).

Voltages transducers

Voltage transducers are primarily used to monitor voltages on the public grid. A voltage transducer can be thought of as a transformer operating under no-load conditions. On the input (primary) side, a high-voltage signal is applied, while on the output (secondary) side, a low-voltage signal proportional to the input voltage is produced.

In public grid applications, the secondary voltage is standardized at either 100 V or 100 V/√3. The level of 100 V/√3 is used in unipolar isolated voltage transducers in star connections, while the level of 100 V is used in bipolar isolated transducers (line-to-line voltage).

Voltage transducers are categorized into different measurement classes, which define their accuracy and phase shift. These classes range from 0.1 to 3:

• Class 0.1: Accuracy of 0.1% with a phase shift of ±5 minutes.

• Class 3: Accuracy of 3% with a phase shift of ±120 minutes.

Following the theory, let’s look at a practical example of how Dewesoft measurement instruments operate. In this case, the voltage of the public grid will be measured.

To select the appropriate amplifier input, the value of the public grid voltage must be considered. In Europe, the grid is specified at 230 V RMS. However, to ensure the safe operation of measurement instruments, the peak voltage of the grid must also be taken into account when determining the input range.

The peak value of the European grid is calculated as the RMS value multiplied by the square root of two, as shown in the equation below.

With a peak value of 325 V, we can directly use a SIRIUS HV-HS module, which supports voltages up to 1.6 kV. This allows for a simple measurement without the need for additional voltage dividers or amplifiers, using a straightforward connection as illustrated below.

In this example, Channel 4 will be used, as it has a SIRIUS HV-HS amplifier integrated into it. The other channels can remain inactive (unused in the software), as they are not relevant to this measurement.

The next step is to configure the measurement channel setup in the software, as shown in the illustration below.

The setup window is divided into two sections: the left side, which displays the amplifier settings, and the right side, which displays the sensor settings.

On the Amplifier side, we can toggle between the 50 V and 1600 V ranges. In this example, the 1600 V range will be used. A low-pass filter can also be applied to cut off higher frequencies, but caution is advised. If the cutoff frequency is set lower than half of the sample rate, the signal within the measurement range will be cut. While this may be useful in certain applications, in most cases it results from an incorrect configuration.

The setup on the Sensor side involves selecting which sensor is used for measurement. In this case, the voltage is measured directly without a sensor, so only the physical quantity needs to be set to Voltage and the unit to Volt (V). In this part of the setup, a scaling factor can also be defined if sensors or dividers are used for measurement. Here, the scaling factor is set to 1, since the voltage is measured directly and no scaling is required.

For this example, the setup is now complete and the measurement can begin by clicking the Measure button. The best way to observe a sinusoidal waveform is with the scope. When the scope is first opened, a continuously running waveform will appear, making analysis difficult. This happens because the software is operating in free-running mode and the measurement needs to be stabilized. To address this, it is recommended to add a trigger—typically the norm trigger—and define the source and trigger level. For this example, however, the default settings can be kept, with the trigger source set to the U1 channel and the trigger level at 0.

DualCoreADC mode

In the previous section, much was said about the importance of selecting the proper amplifier measurement range. Now, let’s take a look at the impressive capabilities offered by the dual-core mode in the SIRIUS amplifiers. When using dual-core mode, it is possible to achieve higher resolution (less noise) at low amplitudes. This is accomplished with two 24-bit AD converters, each operating with different ranges on the same channel.

The first AD converter operates over the full input channel range, while the second AD converter uses only 5% of that range. This technology measures the signal simultaneously with both low and high gain, which means the signal can be captured at relatively high amplitudes while still maintaining excellent resolution at low amplitudes.

Let’s now compare the difference between dual-core mode and normal mode when measuring low-level signals across a high range:

In this example, a 0.3 V DC signal from the calibrator will be measured using two ACC amplifiers. On both amplifiers, a 10 V range is selected (which is clearly inappropriate, but it provides the easiest way to demonstrate the difference between dual-core mode being on or off). This option can be toggled in the channel setup, where the range can also be defined.

On the first channel, dual-core mode will be turned off. On the second channel, dual-core mode will be turned on. The resulting image, shown below, illustrates the difference in noise levels. Both graphs are displayed with the same scaling for comparison.

From the noise levels, it is clear where dual-core mode is active (right) and where it is disabled (left). With dual-core mode enabled, we achieve the same low noise level in the 10 V measurement range as we would if a much smaller 0.5 V range were used. This allows for far better resolution and visibility of low-level signals.

Now it’s time to perform some practical voltage measurements—and enjoy the process along the way.

It is also worth emphasizing how much Dewesoft automates to streamline and accelerate the measurement workflow.

As an example, we will carry out a simple voltage measurement on a discharging capacitor. The circuit consists of two capacitors with rectifying diodes, as illustrated in the image below. These are connected to the grid voltage between the L (live) and N (neutral) conductor cables. This configuration allows the capacitors to charge to the peak voltage in both polarity directions, resulting in a combined voltage of approximately 700 V across both capacitors.

The instrument used in this example is the SIRIUS LV-HV, with the HV channel selected for handling the higher voltages. To discharge the capacitors, an input capacitance of 10 farads on the module will be used as the discharging resistor, connected to the IN+ and IN- terminals.

The measurement yields the result shown in the image: the capacitor charges instantaneously and then discharges exponentially.

In this example, the advantage of the dual-core amplifier becomes clear. For the voltage applied, the 1200 V measurement range must be selected to ensure accurate measurement. However, as the capacitors discharge, the voltage decreases rapidly, which introduces a noticeable level of noise.

When the voltage falls below 50 V—entering the low-voltage spectrum—the dual-core amplifier demonstrates its strength. At this point, it automatically switches to the lower measurement range, which greatly reduces noise. This transition can be observed in the image below, which has been zoomed in to highlight the exact moment the voltage drops below 50 V and the second core takes over to measure in the lower range.