The isolation of analog signals is very headache, and sometimes it has to be isolated. Most of them are based on the following needs:
1. Isolate interference sources;
2. Separate high voltage.
There are many ways of isolating digital signals. The method of isolating analog signals is not as many as they have imagined. The key is the cost of isolation, which is much higher than imagined. Especially when precise measurement is required, the isolation of analog signals is even more costly than expected. I have been engaged in this type of system development for many years and have made a small summary of what I know about isolation methods:
Digital isolation method:
1, optocoupler;
2, ADI's magnetic isolation chip, ADuMXXXX (XXXX is a digital code, such as I2CADuM1250);
3, use their own transformer isolation.
The digital isolation method generally implements the isolation of one-way digital signals. For two-way digital signals, two isolation units are required to realize the volume. The volume is very amazing; it is difficult to reduce the volume. Relative to the speed is very cost, if the speed is less than 100KHz, personal recommend using the common optocoupler such as Ps2501 isolated digital signal, very good price, isolation is also very high. General optocoupler isolation such as Ps2501 is above 3000V/RMS.
However, if the frequency of isolated digital signals is above 200KHz, optocouplers such as Ps2501 will not work. To change the speed of digital optocouplers, the price and cost will go up, which is not worth it. So you can use ADI's magnetic isolation chip. The price of the cheapest magnetic isolation chip per channel is $0.7, and the renminbi is only 4-5 yuan. Selecting high-speed optocouplers such as 6N137 and 6N136 has no cost-effectiveness, and wastes a lot of PCB space for isolating parts. . The cost is about 4 blocks or even higher, mainly depends on your 6N137 purchase volume. However, the size of the ADuM series of magnetic isolation chips is much smaller and the price is also very advantageous. The only fly in the ointment is that the isolated voltage of the magnetic isolation chip can only reach about 1000V, which is a very problematic headache. If you just isolate the source of interference, naturally no problem. If you are isolated from high voltage, you must carefully consider the design.
The method of isolation by an isolation transformer itself is not used by most people because there is no economic benefit at all. It has only one advantage, that is, the isolation voltage can be made very high. Generally only the drive of inverters, inverters and other IGBTs needs to isolate a very large voltage, exceeding 5000V; it is used only. Because the general chip and optocoupler can not be achieved.
Isolation of analog signals:
1, linear optical coupling;
2, isolation amplifier;
3, frequency pressure conversion and frequency conversion + digital isolation;
4, flying capacitor;
5, the use of DA / AD + digital isolation method to achieve the analog signal sampling recovery, and then achieve the isolation approach;
6. Linear isolation of common optocouplers.
Linear optocouplers, who have done isolation, all know. Like the previous TIL300, it seems that it has not been produced yet. Vishy's IL300 can be chosen as an alternative. As a linear isolation of the general analog signal, it is a good choice. In general, linear optocouplers require two op amps and some high-precision resistors to form the appropriate circuit to complete the signal isolation. Also only one-way isolation. The price of IL300 is about ten yuan or so. If you want to get better temperature characteristics, you need two better operational amplifiers. Therefore, the cost of isolation is 30RMB/Ch per channel, and the isolation frequency is about 200kHz. The linearity is not ideal, 0.1% accuracy is more difficult to guarantee; but the manual says that can reach 0.01% servo accuracy. In fact, due to the influence of temperature, the linearity is +/- 0.5%, so children's shoes with higher isolation linearity cannot choose this method. But for most occasions it is possible;
Isolation amplifiers, this is the ultimate baby ah, there are many manufacturers have, China's small factories produce a bunch of this stuff. Linearity can reach 0.1%, compared with the upper level of TI and ADI's isolation amplifier, these things are costly. TI has a so-called isolation amplifier with the lowest price in the world, ISO124. Isolated amplifier, performance, stability, good linearity, most can reach or far exceed 0.1%, can reach 0.01%; but the price is at least 40RMB/Ch, ADI is even more outrageous, probably to 40dollar /Ch, for manufacturers of general industrial products, it is really prohibitive.
The third method and the fifth method are similar in that they convert the analog signal into a digital signal and then isolate the digital signal. The digital signal is then restored to an analog signal, so this also means that this circuit is very complex. However, in terms of isolation effect, it should be a relatively reasonable isolation method. At least in terms of cost and accuracy, isolation amplifiers have the best match. Voltage-frequency conversion and frequency-voltage conversion are very expensive, so this device is not suitable for low-bandwidth signal conversion; voltage and frequency conversion does not require the participation of the microcontroller, the circuit is more concise and higher reliability. If you use AD/DA conversion recovery, you need the participation of control circuits such as a microcontroller or FPGA. From a development point of view, more effort is required. Compared with the method of frequency-to-frequency conversion, the signal bandwidth of recovery is related to the bandwidth of AD/DA. But choose the appropriate chip and MCU, such as 32-bit microcomputer Cortex-M0, 40MHz or so, a few renminbi yuan. With the appropriate AD/DA, the cost can be controlled at 30RMB/ch, but the effect is much better than that of the IL300. The main reason is that the isolation linearity can have a very good guarantee.
The flying capacitor, I did not try flying capacitors, but the flying capacitor is also a special isolation scheme. Compared with other isolation schemes, there is no need to isolate the power supply first; secondly, the circuit is simple. The flying capacitor uses the analog signal as a source to charge an appropriate capacitor. After charging is completed, the flying capacitor is switched to the side of the measurement circuit and completely disconnected from the circuit that charges the flying capacitor. The capacitor discharges the measuring circuit. The measuring circuit measures the voltage of the capacitor. That is, the isolation of the voltage signal is achieved. The core of this circuit is to switch the capacitor and control the switching time. The best use of relays, but ordinary relay life is limited, this game will certainly be scrapped. To use wet-spring relays; wet-spring relays should not be cheap stuff, and I haven't tried it this way, but it's theoretically perfectly feasible. Only the signal bandwidth is very small. It may only be 10Hz.
The ordinary optocoupler only uses the special thought, uses another optocoupler to do the first optocoupler's feedback and the compensation, like the following picture.
This method, personally feel that the isolation of the general signal can also be, because there is no guarantee such as Datasheet, in fact, high-volume applications are difficult to ensure consistency and stability. It is only used in applications where linearity is not very high. The cost should be the lowest in all analog signal isolation schemes, and can be less than 5 bucks/ch.
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