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The previous AFE started and wrote another article with the heat.

The most important reason we use the sampling chip is its high accuracy, but we should also use it carefully, because every link on the whole sampling circuit may cause sampling accuracy deviation, such as the resistance on the sampling link.

First look at the sampling circuit diagram below. What are the resistances on the sampling link?

In the figure above, r represents the sum of the internal resistance of the core and the connection impedance of the copper bar of the core; Rc represents the sum of the contact impedance of the connector and the impedance of the external sampling harness; Rm represents the series resistance of the sampling channel recommended by the manufacturer; Ri represents the equivalent resistance inside AFE. R Generally μ Ω level, Rc is generally m Ω level, Rm is 100 Ω to 10K Ω, Ri is M Ω level.

Rc is the sum of the impedance of the sampling line and the contact resistance of the connector. The worst case is open circuit, but generally AFE has a broken wire detection mechanism, which can identify this fault; Another case is poor connection. At this time, the impedance may be very large. We should be careful. When the Rc impedance can be compared with Ri (as shown in the figure below, a part of the resistance is omitted), the voltage of the cell will be divided, resulting in the deviation of the voltage sampling results.

The above situation also often occurs in daily tests. For example, when using battery simulation tooling, if the tooling is connected to the same channel in parallel with many sampling harnesses, it is easy to cause sampling deviation, because of poor contact.

In addition, even if the Rc is in good contact, we should also pay attention to the problem of sharing the AFE power supply line with the sampling line. As shown in the figure below, because AFE needs to be powered by the cell, the current is about 10mA. If it shares a line with the sampling line, there will be a voltage drop on Rc, which may cause sampling deviation. You can calculate it.

So many products actually go through a separate line for power supply, as shown in the following figure; Of course, don't forget that there is also a power supply line. This operation may actually introduce another problem, that is, the correct identification of wire break detection, which needs special confirmation.

Rm is the series resistance above the sampling channel. Generally, the manufacturer will have the recommended range; The recommended value of the manufacturer is mainly considered in three aspects: first, set a fixed filter cut-off frequency, match the filter sampling circuit and time inside AFE, and achieve high accuracy; For example, the Rm recommended by ADI is 100 Ω, the TI recommended is 100 Ω~1K Ω, and the NXP recommended is larger. The second is to consider the leakage current of AFE channel. If the leakage current is large, Rm must not be too large, otherwise there will be a voltage drop at Rm that will affect the sampling accuracy. The third is hot plug protection, which will be discussed in detail later.

Ri refers to the equivalent resistance inside the AFE sampling channel, which may be given by the manufacturer in the form of channel leakage current. Its size directly affects the sampling accuracy, because its product with Rm is the voltage drop above the channel; The following figure shows the nominal value of leakage current in TIADINXP specification; So if you want to add a stabilizer tube in the circuit for hot plugging protection, you must select the leakage current of the stabilizer tube.

R is the sum of the internal resistance of the core and the impedance of the copper bar. With the charging or discharging of the core, the current flowing through r is bidirectional, so there will be a voltage drop on r ± Δ 5. This was also mentioned in the previous article; The voltage value collected by the actual AFE is ± Δ The superimposed value of V and cell OCV; So when r becomes larger, it may affect the sampling accuracy, or even damage the channel. The principle is very simple.

There is also a more extreme case of r open circuit, as shown in the figure below. At this time, a very dangerous scenario will be involved: when r is disconnected, the entire high voltage will fall on the disconnected channel, and then the channel will be burned, which is very dangerous; So we try not to sample across modules in the design, but some modules can't be avoided, so we need to work hard at the connection.

Summary:

This article basically introduces the various resistances involved in the AFE sampling circuit. Knowing that their existence is only the first step, and more importantly, how to deal with these resistances; During the weekend, I wrote some words and drew some pictures. Maybe not many people can see this, but those who want to see it will help your design.

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