How to accurately measure the capacitance content of CAN nodes

　　The automotive CAN bus design specification has strict regulations on the input capacitance of CAN nodes. Too many capacitive devices are not allowed to be added to each node. Otherwise, when the nodes are combined together, the bus waveform will be distorted and communication errors will increase. The details are shown in Table 1. This is the input capacitance standard in the automotive test standard GMW3122.

　　Table 1 GMW3122 input capacitance standard

　　Therefore, each manufacturer must test the input capacitance of the CAN node DUT (device under test) from CANH to ground, CANL to ground, and CANH to CANL before installing it on the vehicle. The method is generally to use the CAN method in the GMW3122 automotive test standard. As shown in the figure.

　　However, this test method has great limitations. It can only measure and calculate the discharge time of a waveform, which leads to large manual errors. It is very time-consuming to perform multiple statistics and then average them. In addition, since capacitors are nonlinear devices, using square wave measurement cannot effectively eliminate the DC component.

　　Therefore, Guangzhou Zhiyuan Electronics Co., Ltd. has improved the test method. After using the CANScope-Pro bus analyzer and the CANScope-StressZ expansion board, the test waveform sent is a sine wave, and the test accuracy is higher than that of a square wave. It will not be affected by the internal resistance of the transceiver and the cable, and there is no need to use a load resistor. It can also be measured automatically, reducing manual errors and workload.

　　1. Test Сin

　　Make the test connection as shown in Figure 4. Note that the terminal resistance RHL should be set to 0Ω, that is, CANH and CANL should be short-circuited.

　　If Cin is between 40 and 150 pF, the Cin test passes. [page]

　　2. Test Cdiff

　　As shown in Figure 6, make the test connection. Note that the RHL terminal resistor must be disconnected and RL must be enabled and set to 0Ω before connecting to Vdis-.

　　Open the CANStress configuration and set the interference source to internal. Ensure that CANL is short-circuited to ground. See Figure 7.

　　As shown in Figure 8, click Start in Impedance Measurement, the tested Cp is Cbusin2, and then Cdiff = Cbusin2 - Cin.

　　If Cdiff is between 0 and 90 pF, the Cin test passes.

　　CANScope Analyzer is a comprehensive professional tool for CAN bus development and testing developed by Guangzhou Zhiyuan Electronics Co., Ltd. It integrates a mass storage oscilloscope, a network analyzer, a bit error rate analyzer, a protocol analyzer and a reliability test tool, and organically integrates and relates various instruments; it redefines the development and testing methods of the CAN bus, and can conduct a multi-angle and comprehensive evaluation of the correctness, reliability and rationality of CAN network communication; it helps users quickly locate faulty nodes and solve various problems in CAN bus applications. It is the ultimate tool for CAN bus development and testing.