Take stock of wonderful designs

I was looking at the CAN bus a few days ago. There were so many devices connected to a single information bus, and they all wanted to talk, but there was no leader. Isn’t it a mess? After understanding it, I found that they assigned an ID number to each access device— — ID cards with different sizes, relying on the binary 01 level to launch unlimited competition, realizing single-bus competition and occupation of multiple devices without a leader at once. After reading it, I felt wonderful. These foreign devils seem to be smart, at least not stupider than me.

Let’s look at the amplifier again. To detect the current consumption of a load, one method is to connect a detection resistor in series with the loop. As long as the voltage difference across the resistor is obtained, the flowing current can be calculated. Everyone knows this. But where is the series resistor string? Is it the high side, which is the top of the load, or the low side, which is the bottom of the load? So, I know that there are two detection methods, namely High side and Low side. Both methods have their own advantages and disadvantages: the biggest advantage of low-side detection is that there is almost no common-mode voltage across the series resistor. For example, one end is 0V, the other end is 0.1V, and the voltage difference is 0.1V. This can be detected directly using an instrumentation amplifier. , very convenient. But it also has a disadvantage, that is, the bottom of the load is no longer 0V, but 0.1V. If the current fluctuates, this 0.1 will be unstable, just like standing on the first floor, but the floor is shaking, and the result is that the load is very uncomfortable. . You are a testing instrument, and you want to detect the current in the load, but it makes the load very uncomfortable, just like a doctor makes his patient very uncomfortable, which is a bit bad.

Therefore, a large number of designs adopt high-side detection. But there are also troubles in high-side detection. For example, the working voltage of the load is 100V. During normal operation, the bottom of the load is 0V and the top of the load is 100V. Now you connect a small resistor in series to the top of the load, and there is a voltage difference of 0.1V, which makes The high end of the resistor is 100V, and the lower end of the resistor is 99.9V (that is, the potential at the top of the load). Judging from the effect, the load is actually very comfortable. It is very stable under its feet, with 0V. Yes, the top of its head floats a bit, almost around 99.9V. We know that general loads are not very sensitive to voltage fluctuations above their heads, so it is very Comfortable.

But if the load is comfortable, the measuring instrument will be uncomfortable. The measurement amplifier circuit must detect the voltage difference between the two lines. They are 100V and 99.9V respectively. The common mode is 99.95V. If such a large common mode voltage is loaded onto any instrumentation amplifier, the amplifier will be burned immediately.

How to do it?

Foreigners have designed a differential amplifier, such as ADI's AD628. The circuit is as shown below. It uses two sets of voltage dividing resistors to divide 100V to within 10V. The actual voltage loaded to the internal op amp pin is only about 10V, which is safe. However, we found that the differential voltage to be detected of 0.1V was also attenuated by 10 times, it became 0.01V, so they added a level of 10 times amplification to the output end of the subtractor, which not only protected the internal operational amplifier from being burned, but also ensured that the voltage difference of 0.1V was not attenuated, and the output This is the 0.1V we need to detect.

Wonderful. In fact, it's not good at all. The good thing is later.

We all know that shrinking something first and then enlarging it always makes people feel uneasy. Is there a circuit that can achieve: first, resistance to high common mode input, and second, no attenuation of differential modulus.

At this time, I began to admire foreigners. They designed an AD629, which is the younger brother of the AD628, and it meets this requirement. The circuit structure is as shown in the figure. It is claimed to be able to withstand common-mode voltages up to about 270V and achieve one-to-one differential voltage detection. How did they come up with this? It seems that their beef is not in vain.