Isolation of analog signals is a major headache, and sometimes isolation is necessary.
Most are based on the following needs:
1. Isolate interference sources;
2. Separate high voltages.
There are many ways to isolate digital signals, but not as many ways to isolate analog signals.
Especially where precise measurements are required, the cost of isolating analog signals is prohibitively high.
I've been working on this kind of system development for years, and here's a little summary of what I know about isolation:
Digital isolation method:
1. Photocoupler;
2. ADI magnetic isolation chip,ADuMXXXX(XXXX is digital code, such as I2CADuM1250);
3, their own transformer isolation.
Digital isolation method, generally achieve one-way digital signal isolation, for the two-way digital signal, need two isolation units to achieve, the volume is very amazing;
It's hard to reduce the volume.
Compared with the cost of speed, if the speed is less than 100KHz, I recommend using Ps2501 and other common optical coupler to isolate digital signals, which is very cost-effective and highly isolated.
Generally, the degree of optical coupling isolation of Ps2501 is above 3000V/RMS.
However, if the frequency of the isolated digital signal is above 200KHz, the photocoupler like Ps2501 will not work. If the high-speed digital photocoupler is changed, the price and cost will also go up, which is not cost-effective.
Therefore, ADI magnetic isolation chip can be adopted.
The price of the cheapest magnetic isolation chip is $0.7 per channel, which is only 4~5 RMB. The high-speed optical couplers like 6N137 and 6N136 are no longer cost-effective, and a lot of PCB space is wasted for the isolation part.
The cost is about 4 yuan, or even higher, depending on your purchase amount of 6N137.
But ADuM's magnetic isolation chips are much smaller in size and cost competitive.
The only drawback is that the isolation voltage of the magnetic isolation chip can only reach about 1000V, which is a big headache.
If it is only to isolate the source of interference, natural no problem, if it is isolated high voltage, then you have to carefully consider the design.
The method that oneself use isolating transformer to isolate, average person is not used, because do not have economic benefits completely.
It has only one advantage, is the isolation voltage can be very high, generally only inverter, inverter and other IGBT drive, need to isolate a very large voltage, more than 5000V;
To use.
Because ordinary chips and optical couplers are not possible.
Isolation of analog signals:
1. Linear photocoupler;
2. Isolation amplifier;
3. Frequency and frequency conversion + digital isolation;
4. Flying capacitor;
5. DA/AD+ digital isolation method is adopted to realize the sampling and recovery of analog signals, so as to realize the isolation method;
6. Linear isolation realized by ordinary optical coupler.
Linear couplers, if you've ever done isolation.
The old TIL300, for example, no longer seems to be in production.
The Vishy IL300 is an alternative.
As a general analog signal linear isolation, is a good choice.
Generally speaking, linear optocoupler requires two operational amplifiers and some high-precision resistors to form an appropriate circuit to achieve signal isolation.
It's just one-way isolation.
The price of the IL300 is around ten yuan. If you want to get better temperature characteristics, you need two better operational amplifiers, so the cost of the whole isolation is 30RMB/Ch per channel, and the isolation frequency is about 200kHz.
Linearity is actually not ideal, the accuracy of 0.1% is difficult to ensure;
But the manual says you can get up to 0.01% servo accuracy.
In fact, because of the temperature, the linearity is at +/-0.5%, so the kids with higher isolation linearity cannot choose this method.
But for the most part it's ok;
Isolation amplifier, this is the ultimate baby ah, there are a lot of manufacturers have, small Chinese factories a bunch of production of this stuff.
The linearity can be up to 0.1%. The higher level is the isolation amplifier of TI and ADI. These things cost a lot.
TI has what it claims is the world's cheapest isolation amplifier, ISO124.
Isolated amplifiers have stable performance and good linearity. Most of them can reach or far exceed 0.1%, and all of them can reach 0.01%.
But the price is at least 40RMB/Ch, ADI's is even more outrageous, probably to 40dollar/Ch, for general industrial products manufacturers, it is prohibitive.
The third approach is similar to the fifth in that it converts the analog signal into a digital signal and then isolates the digital signal.
The digital signal is then converted back to the analog signal, so this also means that the circuit is very complicated.
But from the effect of isolation, should be a more reasonable way of isolation.
At least in terms of cost and accuracy, and isolated amplifier is the most competitive.
VVF conversion and VVF conversion are very expensive, so this device is not suitable for low-bandwidth signal conversion;
Voltage and frequency voltage conversion does not need the participation of MCU, circuit more concise, higher reliability.
If AD/DA conversion is used for recovery, the participation of control circuits such as single-chip microcomputer or FPGA is required. From the perspective of development, more efforts are needed.
Compared to the frequency voltage conversion method, the verbose, restored signal bandwidth and AD/DA bandwidth.
But choose the right chip and microcontroller, such as cortex-m0 32-bit microcontroller, about 40MHz, a few yuan.
With appropriate AD/DA, the cost can also be controlled at 30RMB/ch, but the effect is much better than that of IL300, mainly because the linearity of isolation can be guaranteed.
Flying capacitance, flying capacitance I have not tried, but flying capacitance is also a special idea of the isolation scheme.
Compared with other isolation schemes, first of all, there is no need to isolate the power supply.
Secondly, the circuit is simple.
The flying capacitor USES the analog signal as the source to charge an appropriate capacitor. After charging, the flying capacitor is switched to the side of the measuring circuit and completely disconnected from the circuit charging the flying capacitor.
Capacitance discharges the measuring circuit.
The measuring circuit measures the voltage of the capacitor.
The isolation of voltage signal is realized.
The core of this circuit is to switch capacitance and control the switching time.
The best use of relays, but the general life of the relay is limited, this kind of play must be scrapped.
Wet reed relay to be used;
Wet-reed relays are not supposed to be cheap, and I haven't tried this method, but it's theoretically possible.
But the signal bandwidth is pitifully small.
Maybe 10Hz.
Ordinary optocouplers only use special ideas, and another optocoupler is used as the feedback and compensation of the first one, as shown in the figure below.
The light coupling
In this way, I think it is ok to isolate the general signals, because without the guarantee of Datasheet, it is difficult to guarantee the consistency and stability in mass applications.
Only for very low degree of linearity.
The cost should be the lowest among all the analog signal isolation schemes, below 5 yuan /ch.