this has come up a couple of times by e-mail and would be good to post the answer...
"What determines the limit of the number of physical nodes on the CAN bus?"
The maximum number of nodes on a CAN bus is limited by the driving capability of a CAN transceiver.
A typical example for the ADM3054
Table 1 in the ADM3054 datasheet specifies a minimum Receiver Differential Input Impedance of 20 KΩ.
If the load of a transmitter on the CAN bus is composed of 110 nodes of the ADM3054 in parallel (with 20 KΩ input impedance), this would give a combined input impedence of 180 Ω.
When this 180 Ω is in parallel with two 120 Ω terminations on the CAN bus, the final combined impedance is 45 Ω.
The drive current capability of the ADM3054 for this case is the Driver Differential Output Voltage 3.0V/45Ω= 67mA.
This is less than the maximum current of 75mA as given in the ADM3054 datasheet, Table 1.
It should be noted that the 75mA typical maximum specification allows for process variation, and variation across temperature and power supply for the CAN Transceiver (3-5.5V for Vdd, and also 4.75 V ≤ VDD2 ≤ 5.25 V for the ADM3054). The 75mA was measured using a 60 Ω load on the bus.
The customer should also look at the Receiver Differential Input impedance values for other manufacturer’s CAN transceivers, if they are also on the bus. This will allow a calculation for the number of nodes possible on the bus.
Another typical example for the ADM3052
Table 1 in the ADM3052 datasheet specifies a minimum Receiver Differential Input Impedance of 20 KΩ.
The receiver input impedence can vary between 20 KΩ minimum and 100 KΩ maximum.
If 160 nodes of ADM3052 (with 100 K Ω input impedance) are on the CAN bus, and also two 120 Ω termination resistors, then the combined impedence would be 55 Ω.
For a Driver Differential Output Voltage 3.0V, the drive current would be approximately 55mA. This 55mA current is the same as the typical value stated on the ADM3052 datasheet for a TxD/RxD Data Rate of 1 Mbps.
So, in a field application, where the ADM3052/3054 receiver input impedence is variable (20-100K Ω), the number of ADM3052/3054 nodes used can be greater than or equal to 110 nodes.
Analog Devices CAN305X data sheets specify in the Features Section - ‘Connect 110 or more nodes on the bus’.
It must be noted that the customer must also look at other factors in optimizing his CAN transceiver application - like transmission losses (cable length) and stub lengths.
A maximum bus cable length of 40 meters is specified for CAN at a data rate of 1 Mb, with longer bus cables possible at lower data rates. The individual nodes on a CAN bus must be connected by stub lengths of 0.3m or less for a 1Mb data rate.
The customer can refer to the Analog Devices Application note AN-1123 for a Controller Area Network Implementation Guide.
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