How to accurately measure the input voltage threshold of a CAN node

The CAN bus design specification has strict regulations on the input voltage threshold of CAN nodes. If the input voltage threshold of the node does not meet the specification, it is easy to have abnormal working conditions after the on-site networking, and communication failures between nodes. The specific requirements are shown in Table 1, which is the test standard "ISO 11898-2 output voltage standard".

Table 1 ISO 11898-2 input voltage threshold standard

Therefore, every manufacturer must perform an input voltage threshold test on the CAN node DUT (device under test) before the product is put into use. Generally, the CAN test method based on the ISO 11989-2 input voltage threshold standard is used, as described below:

Under the load and common mode conditions shown in Table 1, select the adaptability conditions of the DUT under test, as shown in Figure 1, where Rtest is the network load resistance, which is 60Ω.

Adjust U to make V -2V and 6.5V respectively, then adjust I to make Vdiff = 0.5V, and observe whether the DUT can communicate normally. If it can communicate normally, it passes the implicit Vdiff input voltage test.

Then adjust U so that V is -2V and 6.1V respectively, and then adjust I so that Vdiff=0.9V, and observe whether the DUT can stop communicating. If it can stop communicating, it passes the explicit Vdiff input voltage test.

Figure 1 Input voltage threshold test principle

It can be seen that although this method can measure the input voltage threshold of the CAN node, in order to simplify the operation and improve the accuracy of the measurement results, Guangzhou Zhiyuan Electronics Co., Ltd. improved the test method and used a combination of CANScope-Pro bus analyzer, CANScope-StressZ expansion board and programmable DC power supply DCP8325L for measurement.

The test plan is as follows:

1. Make the test connection as shown in Figure 2. Adjust the RHL value on the CANScope-StressZ analog expansion board to 60Ω, and the RH and RL to 0Ω. Make the DUT actively send CAN messages, uncheck the bus response on CANScope, and the black test lead (ground) should be grounded with the CAN transceiver of the DUT.

Figure 2 Input voltage threshold test connection diagram

2. As shown in Figure 3, configure the interference source to be external, then start the CANScope and CANScope-StressZ analog expansion board to start the test

Figure 3 Adjusting the CANScope-StessZ interference source to external

3. Adjust U of DCP8325L to make V -2V and 6.5V respectively, then adjust I of DCP8325L. When Vdiff = 0.5V is observed on the oscilloscope of CANScope, observe whether DUT can send messages normally. If it communicates normally, the implicit Vdiff input voltage test passes.

4. Then adjust U to make V -2V and 6.1V respectively, then adjust I and observe Vdiff = 0.9V on the oscilloscope of CANScope. Observe whether the DUT can stop communication. If it can, the dominant Vdiff input voltage test is passed.

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