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# Voltage measurement

Electricity is very hard to imagine because we can not see if a voltage is present or if a current is flowing. If we want water to flow out of a pipe we need some water pressure which is achieved with a water pump. In electricity, our flow is the current, water pressure is the voltage and pump is the battery. This means that the voltage is the cause of the current.

## What is voltage?

Electricity is very hard to imagine because we can not see if a voltage is present or if a current is flowing, so let us try to explain electricity with water analogy. Water systems or circuits are simple to understand because we can see water and it's the one thing we are used to. Now let's see how the water system works. We all know if we want water to flow out of a pipe we need some water pressure which is achieved with a water pump. In electricity, our flow is the current, water pressure is the voltage and pump is the battery. This means that the voltage is the cause of the current like the water pressure is the cause of volume flowrate of water.

## Voltage measurement

A measurement device for measuring the voltage is called a voltmeter. To measure the voltage, otherwise known as the potential difference between two points, the voltmeter is always connected in parallel to the circuit (see picture). To influence the circuit as little as possible, the input impedance of the voltmeter has to be very high. The typical input impedance of Voltmeter is 10 MΩ.

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Measuring voltage is the most basic measurement with DAQ devices because most of the AD converters use voltage as the input value. That's why measuring voltage with DAQ seems simple, right? The answer is yes if we are measuring voltages in the range that is directly supported by the AD converter. But when measuring very small voltages of some micro-Volts (µV) or very high voltages up to several kilo-Volts (kV), an amplifier is needed to prepare the signal for the AD conversion. For both challenges Dewesoft has the right solution.

On one hand the Low Voltage amplifier (LV and HS-LV) together with the 24-bit ADC technology allows measurements of very low voltages also at high measurement ranges (e.g. µV resolution at a range of ± 10V).

On the other hand, the High Voltage amplifier (HV and HS-HV) allows to directly measure voltages up to 1600V DC (1200V DC at HS-module). For measuring voltages higher than 1600 VDC, voltage probes/dividers or voltage transducers can be connected to the device.

## Isolation Voltage

When measuring a certain voltage it’s important to choose the right amplifier. Using an incorrect amplifier can destroy the amplifier if the measured voltage exceeds the isolation voltage. For example measuring the voltage of the public grid (230 Vrms / 325V peak) with a STG module can destroy the module because the isolation voltage is just 200 Vpeak for the measurement range below 10V and 300V peak for the measurement range above 10V. For measuring voltages higher than ±100 V the use of the HV-amplifiers is mandatory.

## Measurement Range

The appropriate selection of the measurement range is essential for high accuracy and reliable measurement results. There are a number of measurement ranges available with every amplifier which can be configured in the Dewesoft X channel setup. If the measurement range is too low, the signal will exceed the input range and errors and channel overloads will appear instead of correct values. On the other side, if the measurement range is too big, the inaccuracy will be too high to make correct readings.

The most precise measurement can be achieved when the measurement value range coincides with the DAQ amplifier input range. In this case we get the highest resolution of our measurement with the same number of bits used by the AD converter.

Let’s take a look at an easy example measuring voltage. If we only use one tenth of AD converter input range, our resolution of the outcome will only be one tenth of the actual performance of the AD converter. A 16-bit converter can read 65536 different discrete values, but our measurement will only consist of 6500 different values which are very low resolution. Measuring a signal from 0-7V will be pointless with a 1200V range (resolution 18 mV) if we can use a module with 10V range instead (resolution 0,15 mV). That's why we have more modules with different input and measurement ranges on disposal.

## Differences between Peak, Average and RMS voltage

When we are talking about voltages, it must be declared about which voltage we are talking. There are several different types of voltages like peak, peak-to-peak, average, RMS, AC or DC voltage. See the difference between them in the picture below.

The average voltage is, as the name already states, the average value for a certain time period. For pure sinusoidal signals, the average will be zero.

The RMS voltage is the root-mean-square voltage and it 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 to define the AC voltage at a certain point and produces the same energy as the DC voltage at an ohmic load.

The peak voltage describes the highest voltage in a period. In the datasheet specifications the peak voltage or the DC voltage of an input is given which means the same. To calculate the RMS value for sine waves, the peak value has to be divided by the square root of 2.

The peak-to-peak ratio shows the amplitude of positive and negative peak in a period.

The Crest factor is the peak amplitude divided by the RMS value of the waveform

## Why use isolated amplifiers

Basically, we have three different "types" of DAQ amplifiers: Single ended, differential and isolated amplifiers.

We will shortly explain these types of amplifiers.

Single ended amplifiers have only one input pin because the second input pin is connected directly to the ground. Because of this kind of connection this amplifier is only suitable for measuring floating voltage sources where one output point can be connected to ground. This type of amplifier is easy to use but has two major disadvantages:

• unwanted ground loops,
• the amplifier is not isolated.

A ground loop is an unwanted current from sensor ground to instrument ground because of a small difference in ground potentials. When we say small we can talk about microvolts but these µV causes a large amount of noise in measured signal. Let's see a simple example of problems with noise when facing ground loops. Let us take a sensor with an output range of 10V and connect it to a single-ended amplifier and assume that we need a dynamic range of 140dB. After a simple calculation, we get the results that the allowed potential difference between sensor and instrument ground is only 1µV! The perfect solution for the potential difference - isolation of the sensor or the instrument.

The differential amplifier has two inputs separated from ground. This type of amplifier is the most common and amplifies the voltage difference between both inputs. This technology gives us the possibility to avoid ground loops, but the first thing we should be careful about is common-mode input voltage.

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

Another way to imagine a VICM: It is voltage level common to both inputs Vin(+) and Vin(-). That means if we use differential inputs of DAQ which measures difference between inputs, differentially measured value is the small, but common mode input voltage can still be in hundreds of volts (current measurement with shunts).

Another term which describes differential amplifier inputs is input common-mode voltage range (VICMR). This is the parameter most often used in datasheets and is also the one we should pay the most attention to. VICMR defines a range of common-mode input voltages in which amplifier will work properly and describes how close the inputs can get to either supply rail. This means the potential of the input pins must be between supply voltages (V+ and V-).

## Isolated amplifier

Using isolated amplifiers eliminates the disadvantage of single-ended amplifiers and differential amplifiers. They are independent of ground loops, common mode voltage, short circuits etc. These modules are isolated from the housing and the main board of the measurement device. Therefore, the amplifier will only "see" the difference of the absolute voltage. The high isolation voltage (compared to measurement range) allows safe and reliable operation also at voltage peaks, faults etc. and so enables the usage for a lot of different applications.

After short descriptions of amplifiers, we can say that the main advantage of differential amplifiers is the lower price. They are perfect for measurements with isolated sensors like strain gauges or current clamps. Differential amplifiers also provide the high-quality measurement for non-isolated sensors, but engineers also need to know sensor behavior like common mode range or isolation to provide correct measurements. On the other hand, isolated signal conditions are more expensive, but a worry free solution up to isolation voltage.

## Voltage measurement up to 50V

Let's take a look at how a low voltage measurement of up to 50V looks like.

Voltages of up to 50V can be connected directly to a couple of different Dewesoft amplifiers. The amplifiers differ from each other in measurement range, isolation, bandwidth, noise, and extended functionalities.

Every measurement channel supports a number of input voltage ranges. The most precise measurement will be achieved when the input voltage range of the measurement channel is set in a way that it coincides with the voltage of the measured signal.

The table below shows which Dewesoft amplifiers can be used for the measurement of voltages of up to 50V. The table also contains information about the maximal sampling rate and the bandwidth as well as the available measurement ranges for each amplifier.

The common low-voltage amplifiers allow a sampling rate of up to 200kS/s per channel with a maximal bandwidth of 75 kHz. The high-speed series (HS) is used for applications that require high sampling rate and high bandwidths like voltage or current measurement in inverters. The sampling rate of the HS series is 1 MS/s and the bandwidth 2 MHz.

After choosing the right amplifier, the signal only has to be connected to the amplifier.

## Voltage measurement up to 1kV

Measuring voltages higher than 100 V requires the use of the Sirius HV or HS-HV modules. The Sirius HV module allows measuring voltages of up to 1200V DC while the HS-HV module allows measuring voltages of up to 1600V DC.

The table below shows the two different amplifiers for measuring high voltages with information about the measurement range, sampling rate, bandwidth and isolation.

Just like with low-voltage amplifiers the HS-series is designed for measuring very fast signals like voltages (PWM) of an inverter. Inverters operate at a switching frequency of up to 200 kHz which requires high bandwidth of the whole measurement chain and a high sampling rate. To allow analysis of every kind of application the maximal sampling rate of the HS-HV module is 1 MS/s at a bandwidth of 2 MHz.

Please take care that the measured voltage doesn’t exceed the isolation voltage of the amplifier otherwise it can quickly become dangerous. Depending on the level of the voltage, the measurement system can be destroyed and the people around the device are in danger.

## HV vs. HS-HV

For easier understanding which of both modules fits better to your application we will compare the modules by measuring the voltage of a PWM regulated 3-phase servo motor.

For both types of HV modules the sample rate is set up to the maximum, which is 200kS/s on the HV module and 1MS/s on the HS-HV module. Coarsely measured data will be the same while further analysis of the PWM modulated sine-wave voltages reveals the differences.

The first difference seen here is for the "chopped" sine. With an HV module (200kS/s) we get an overshoot when the signal "jumps". In the picture below we can see the measured motor voltage with the HV module with 200kS/s rate. So this overshoot is clearly seen as little tick at start and stop of the voltage level shift.

When we zoom the signal into the resolution of single samples, we see that the reason for the overshoot is a too low sample rate. Because of the short rise time of the voltage there is only one or even no sample on the slope, which causes an error of measurement on the edges of our signal.

The HS-HV module has more bandwidth and gives us 5 times more samples for the same time period we don't get an overshoot on the same measured signal. This cleaner transition is the result of more than one sample on the slope, which gives us a better resolution at the edges of a signal jump.

The problem with the overshoot begins at switching frequencies at around 2 kHz. At higher switching frequencies, we should expect to see even more differences between the dual core and the HS amplifier. For this purpose, we will measure another voltage output of the frequency converter with the switching frequency set to 16 kHz.

The first obvious difference is seen in the bandwidth of the measured signal. With the dual core channel (blue) we can see the start of damping at the frequency of 66 kHz and there are absolutely no frequencies above 100 kHz in the measured signal. With the HS module (green) we have measured higher frequencies in the same signal as seen in the picture below.

When observing both measured signalsh on the scope we see that the HS module is few microseconds faster than the Dual core module.

The last difference we must bear in mind when measuring high-frequency voltages are the transients in the signal. The HS module is much better at transient measuring and gives us a very well oscillation coverage after every peak.

## Voltage measurement higher than 1 kV

While voltage measurements up to 1kV are really simple, things are getting more complicated with measuring voltages over 1600 VDC because voltage probes/dividers or voltage transducers are necessary which adjust and reduce the voltage to a level which is suitable for the amplifie.

Please be especially careful using voltage probes or voltage transducers. There are several things which have to be taken into consideration when using voltage probes or voltage transducers.

### Voltages Probes (Voltage Dividers)

There are two different types of voltage probes: The pure resistor voltage probe (for AC and DC measurement) and the resistor-capacitive voltage probe (only for AC measurement). The input impedance of the voltage probe should be as high as possible, therefore the input resistance should be as high as possible and the input capacitance as low as possible.

There are active, passive and differential voltage probes available.

Passive voltage probes are simple, cheap and robust but have a high input capacitance and problems measuring low voltages.

Active voltage probes have a high input resistance and low input capacitance but need an external power supply. They are a lot more sensitive and more expensive than passive ones.

Differential Voltage Probes: Passive and active voltage probes are single-ended amplifiers with reference to earth. If you want to measure differential signals you have to choose a differential voltage probe.

Voltage probes have no galvanic isolation. If the GND connection is interrupted the full voltage potential is on the measurement device and can destroy the device and can be hazardous for people around.

The function of the voltage probe can be easily explained using a simple resistor voltage probe with the serial connection of two resistors with high resistance.

If we use the voltage divider with the same resistance value, the voltage drop on one resistor will be half of the connected voltage. So if we want to measure a high voltage in the range up to 2000V, this simple transducer reduces the voltage to 1000V. This is low enough to measure it using a Sirius high voltage module with an input range up to 1200V.

If we have resistors with too low impedance, compared to a circuit in which we are measuring voltage, we will cause a substantial current. This current affects the measured circuit and reduces the accuracy of the measurement. We also must ensure that the input impedance of the measurement device is 100 to 1000 times higher than the value of R1, otherwise the ratio will change and the impedance of the measurement device also has to be considered. We must also bear in mind that R1+R2 is the short-circuit resistance – if this sum is too low it will cause a short circuit.

If you use a voltage probe, the ratio between the input and the output voltage has to be calculated and adapted to in the channel setup of the software. This ratio is important because the measured voltage of the measurement device has to be multiplied by this ratio to become the real voltage (Vin in the picture).

### Voltages Transducers

Voltage transducers are mainly used to monitor the voltage on the public grid. A voltage transducer is easily explained as a transformer in a no-load operation. On the input (primary) side, a high voltage signal is connected. On the output side of the voltage transducer, we get a low voltage signal which is directly proportional to the input voltage. In public grid operation the secondary voltage is standardized with a level of 100V respective 100V/sqrt(3). The level of 100 V/sqrt(3) is used in unipolar isolated voltage transducers in star connection. The level of 100V is used in bipolar isolated transducers (line-line voltage).

There are different measurement classes of voltage transducers which describe the accuracy and phase shift of the transducers. The classes span from 0.1 to 3. Class 0,1 means that the accuracy of the measured amplitude is 0,1% and the phase shift is ± 5 minutes. At class 3 the accuracy is 3% and the phase shift ± 120 minutes.

Attention: Never operate voltage transducers with a short-circuit on the secondary side. This creates high currents which will destroy the transducer.

Tip: Voltage dividers and transducers always have a frequency dependent behaviour concerning amplitude and phase. Using the Dewesoft sensor editor allows to correct this behaviour and increase the accuracy of measurement. See details in the "Current Pro Training" in the chapter "Sensor Editor".

## Voltage measurement with Dewesoft X

Now let's see how all this theory works with the Dewesoft equipment by measuring the grid voltage.

First of all we should think about the value input voltage to see what kind of amplifier input range we need for our measurement. The European grid voltage is declared with a value of 230Vrms, but for the input range we need to know the peak voltage value.

With this peak value of 325V we can directly use Sirius HV module which supports voltage up to 1.2kV. This means that we can make a simple measurement without any additional voltage dividers or amplifiers and a simple connection which is on a picture below.

We will use the channel 4 which has a Sirius HV amplifier. This means we will leave the other channels disconnected. The next step is to configure the measurement channel setup.

Here we have two sides of a possible setup, one is the Amplifier side and the other is the Sensor side.

At the Amplifier side, we can toggle between 50V and 1200V range, In our case 1200V range will be used. We can also use a Lowpass filter to cut off the higher frequencies, but we must be careful with that. If we choose a frequency lower than half of the sample rate it will cut the signal already in the range of the measurement, sometimes this is necessary but most of the time this configuration is set by mistake.

Setup at the Sensor side is about which sensor is used for measurement. In this case we are measuring voltage directly without a sensor, so we just need to set the physical quantity as Voltage and unit as Volts(V). In this part of the setup, we can also set the scaling factor if we are using some sensors or dividers. It has a value of 1 since we are measuring voltage directly like in this case.

Settings for these measurements are done so it's time to start measuring. This can be done in the tab "Measure". The best way to observe a waveform is in the scope. When we first open the scope we can only see a running wave which is impossible to analyse. That's because it's running in Free mode, we need to "hold" the measurement somehow. This can be done by setting the Trigger on Norm trigger and defining the trigger source and trigger level. It's ok for now to leave it as it is, trigger source is the Grid voltage channel and the level is 0.

That's how simple it is to measure voltage in Dewesoft X.

## DUAL-CORE mode

We talked a lot about proper amplifier measurement range selection before. Now it's time that we take a look at the impressive option offered by dual core mode in the Sirius amplifiers. If we are using the Sirius dual-core mode we will get a better resolution (less noise) in low amplitudes. That is solved with two 24-bit AD converters with different range on each channel. One AD converter has a full input channel range and the range of the other AD converter is only 5% of a full channel range. This technology measures the signal with low and a high gain at the same time which means that we can measure the signal with a relatively high amplitude but at the same time a perfect resolution at low amplitudes of the same signal.

Let's look at the difference between dual core mode and normal mode when measuring low signals with a high range:

So we will measure a 0.3V DC signal from the calibrator on two ACC amplifiers. On both amplifiers, a 10V range will be chosen (which is a complete nonsense) but it's the easiest way to see the difference between dual core mode on or off. This can be toggled in the channel setup where also range can be set.

On the first channel, we will turn Dual core mode off, on the second this mode will be turned on. Now we just take a look at the noise level at each of those channels. The difference can be perfectly seen in the picture below where the two channels are shown on the graphs with a same scale range.

By the noise level, it's not hard to guess where dual core mode is doing its job(right), and where it's turned off(left). With dual core mode turned on we get the same noise level in 10V measurement range as it would be if we were using 0.5 V range. This gives us a better look at lower signals.

## Fun voltage measurement

Now it's time that we do some practical voltage measurements and have some fun with that. Meanwhile, we will explain what a great job Dewesoft X is doing automatically instead of us.

We will start with a simple voltage measurement of a discharging capacitor. We connected two capacitors with rectifying diodes like in the picture below and then connected them to the grid voltage between the points L and N. This connection allow us to charge the capacitors with the peak voltage in both polarities. This gives us around 700V on both capacitors together. So we will use the HV module on the Sirius to perform this measurement. For discharging the capacitors we will use the input impedance (10MΩ) of the module as a discharging resistor which will be connected to points IN+ and IN-.

The measurement gives us the result shown in the graph below.

In this measurement, we can perfectly explain the good side of dual core mode amplifier use. Sure we must use the 1200V range to perform this measurement correctly, but the voltage on the capacitors will fall fast and that means we will get a noticeable noise level. This is where the dual core amplifier kicks in and when the voltage level drops under 50V, it starts to measure it with the lower range which highly reduces the noise level on the measurement. This switch between ranges can be clearly seen when we zoom-in on the part of measurement when the voltage drops below 50V and that is exactly what has happened in the picture below.