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# ADM2491E MAX Output Current

Category: Datasheet/Specs

In the maximum absolute rating column of data sheet P6
Average Output Current, per Pin = ± 35mA.

From this specification, I understand that the maximum number of bus connections is up to 7 units when using two 120Ω terminations.
Is it correct?

Calculation process that I thought was a 7-unit load:

・120Ω Termination resistance // 2 + Unit load x X <= 35mA
⇒X <= 35mA-1.5V ÷ 60Ω
* Unit load = 1mA

If my understanding above is correct
Datasheet 1P says that up to 32 nodes can be connected, but is this the case without a terminating resistor?

Parents
• ADM2491E will support 32 nodes (or 32 unit loads) on a network which is double terminated with 2x 120Ω resistors.

In the RS-485 standard, a double terminated network of 2x 120Ω resistors with maximum allowed of 32 unit loads is represented as a 54 Ω load.

The min. single ended input resistance of an RS-485 transceiver is 12kΩ, this represents 24kΩ min. differential input resistance. For 32 devices: 24kΩ / 32 = 750Ω. 120Ω || 120Ω || 750Ω = 55.6Ω, which is rounded to 54Ω by the RS-485 standard.

You will see this 54Ω load number in most VOD specifications for RS-485 transceivers , including the ADM2491E.

At 1.5V VOD the average current into a fully loaded network is 27.8mA, within the max specified in table 7.

Regards,

Neil

• ADM2491E will support 32 nodes (or 32 unit loads) on a network which is double terminated with 2x 120Ω resistors.

In the RS-485 standard, a double terminated network of 2x 120Ω resistors with maximum allowed of 32 unit loads is represented as a 54 Ω load.

The min. single ended input resistance of an RS-485 transceiver is 12kΩ, this represents 24kΩ min. differential input resistance. For 32 devices: 24kΩ / 32 = 750Ω. 120Ω || 120Ω || 750Ω = 55.6Ω, which is rounded to 54Ω by the RS-485 standard.

You will see this 54Ω load number in most VOD specifications for RS-485 transceivers , including the ADM2491E.

At 1.5V VOD the average current into a fully loaded network is 27.8mA, within the max specified in table 7.

Regards,

Neil

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