Many engineers encounter common-mode voltage issues during their daily debugging work, especially when a two-wire current transmitter is connected to an analog input channel, which can easily lead to misunderstandings and ambiguities. Therefore, it is necessary to explain the issue of common-mode voltage.
As shown in the image above, the red and black lines form a power rail. The term "rail" is a translation of the English word. In Western culture, two power lines, one positive and one negative, are like two railway rails. These two rails must always be parallel and cannot intersect, otherwise the train will derail. Similarly, the positive and negative wires must never be short-circuited, otherwise the power supply will burn out. That's why we call the power supply a rail.
The blue line represents signal 1, with an amplitude of 20V; the purple line represents signal 2, with an amplitude of 16V. The amplitudes of both signal 1 and signal 2 are within the power rail range.
For a system with a common ground, under stable conditions, the amplitude of any signal cannot exceed its power supply range; otherwise, the system would be unstable and an e-factor would occur.
The differential voltage between signal 1 and signal 2 is defined as the difference between the two, which is equal to 20V - 16V = 4V.
The common-mode voltage of signal 1 and signal 2 is defined as the sum of the two, divided by 2, which here equals (20V + 16V) / 2 = 18V.
The diagram above can be represented by the following simple circuit:
Node 1 is signal 1, which equals 20V.
Node 2 is signal 2, which equals 16V.
Now let's connect node 1 and node 2 of this circuit to our UN 231-0HC22 module for measurement and see what the results are, as shown in the figure below:
The UN 231-0HC22 is a 4-channel analog measurement module. Its internal measurement circuit is a differential circuit, which means that it amplifies and measures the signal difference between the two input terminals A+ and A-.
In this example, the DIP switch is 101, which corresponds to a range of 10V. This means that the difference between A+ and A- can be measured within a range of 10V. If it exceeds 10V, then it is out of range. For example, if it is 12V, then the reading will be fixed at 32000, and there will be no linear relationship.
Now, A+ equals the voltage at node 1, which is 20V.
A - equals the voltage at node 2, which is 16V.
The difference between A+ and A- is 4V, which is within the 10V range. According to the above statement, UN 231-0HC22 can accurately measure this 4V difference signal, and the corresponding AIW0 reading should be 12800.
However, don't get too excited yet. Please think carefully, is there something wrong? That's right, we've been focusing too much on the difference between A+ and A-, but have forgotten about the common-mode voltage values of A+ and A-.
It is precisely this common-mode voltage value that prevents the UN 231-0HC22 from processing these two signals. With such a measurement, AIW0 would definitely not be 12800; those interested can conduct their own experiments. Now, let's return to the UN 231-0HC22's technical specifications, as shown in the following figure:
As you can see in the red box, there are two indicators related to common-mode voltage.
1. Common-mode rejection: This parameter is for AC signals. Since there are DC common-mode voltages and AC common-mode voltages, this parameter mainly targets AC common-mode voltages. 40dB refers to the ability to suppress AC common-mode voltages. The larger the value, the stronger the ability to suppress common-mode voltages, and the better the performance of the module.
DC to 60Hz refers to the frequency range of AC common-mode voltage. In field applications, power frequency interference is frequently encountered. The rated frequency for industrial power in my country is 50Hz, but there are certain fluctuations in reality. The higher the frequency of this parameter, the wider the common-mode voltage suppression range of the module, and the better the performance of the module.
Of course, the price will be higher. 60Hz is common, but some high-end measuring instruments can reach 100Hz or even 120Hz. This is because after taking full-wave rectification into account, the frequency doubles to 100Hz.
2. Common-mode voltage: This parameter applies to both DC and AC signals, especially the DC common-mode voltage range. Here it is: Signal voltage + Common-mode voltage ≤ 12V
This parameter indicates that the common-mode voltage range allowed by the UN 231-0HC is less than 12V. In the example above, the common-mode voltage is 18V, which exceeds 12V. Therefore, the UN 231-0HC22 is not capable of handling this voltage.
Some curious users might ask, "What should I do if I want to increase the common-mode voltage range of the UN 231-0HC22?"
First, increasing the common-mode voltage range will lead to a rapid increase in the price of the UN231-0HC22 module.
Secondly, the higher the common-mode voltage, the less safe it is for engineers handling the wiring.
Secondly, you need to find a chip whose power supply is equivalent to, or even greater than, your common-mode voltage. For example, if your common-mode voltage is 100V, then you need to find a chip with a 120V power supply.
It's not very practical in principle, but I still want to use the UN 231-0HC22 to handle very high common-mode voltages. What should I do?
Two words: isolation. Isolation is the best solution. (See image below.)
Okay, next we will apply the above explanation to the two-wire current transmitter we actually encounter, as shown in the following two diagrams:
The difference between Figure 1 and Figure 2 lies in the different connection methods of the two-wire current transmitter to the UN 231-0HC22. The comparison between Figure 1 and Figure 2 is equivalent to:
Having read this far, you should all know which connection method is incorrect and which is correct, right? That's right, the connection in Figure 1 is incorrect; this connection method essentially raises the common-mode voltage. Now let's calculate the common-mode voltage of the connection in Figure 1.
Since two-wire current transmitters are generally 4-20mA, 4-20mA flows into A+ and then out of A-. Because RA and A+ are shorted, the resistance between A+ and A- is equivalent to 250Ω. Therefore, when 4mA of current flows into A+ and out of A-, the voltage difference between A+ and A- is equal to 1V. A+ is directly connected to 24V+, and its potential to ground is equal to 24V; the potential of A- to ground is equal to 24V - 1V = 23V.
Their common-mode voltage is equal to (24V + 23V) / 2 = 23.5V, which is far beyond the range that UN 231-0HC22 can withstand;
When a current of 20mA flows into A+ and out of A-, the voltage difference between A+ and A- is 5V, the potential of A+ to ground is 24V, and the potential of A- to ground is 24V - 5V = 19V.
Their common-mode voltage is (24V + 19V) / 2 = 21.5V, which is far beyond the range that UN 231-0HC22 can withstand;
The connection in Figure 2 is correct. With the connection in Figure 2, the common-mode voltage between A+ and A- will not exceed 2.5V, which is far less than the rated range of UN 231-0HC22. If you are interested, you can calculate it yourself.
By now, I believe everyone understands the power of a two-wire current transmitter.
