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THVD2450: THVD2450D termination resistors

user5847884
Prodigy 40 points
Part Number: THVD2450

It is about suitability of very universal THVD2450D use in configuration with one termination resistor 470R at the receiver (This is to limit the current drawn by the transceiver line).

We want to use this RS485 transceiver only in one direction point to point application.

Usually we will have 10cm to 0.5m connection in noisy environment with flat ribbon tape. 

What values range of this resistor are allowed?  Any simulation/calculation I could perform?

  • 0
    Guru 243220 points

    As shown in the electrical characteristics, the resistor must be large enough to have a voltage drop of at least 200 mV. The device will work with any larger resistor (even ∞ = no resistor). However, if the termination does not match the characteristic impedance of the cable, you will get reflections, which limit the achievable speed. And if you make the resistor too large, a small noise current will be able to flip the signal, so in a noisy environment, you want to make the resistor as small as possible.

    Simulation is possible if you know the characteristics of your boards and cables. Measurements are likely to be easier.

  • 0
    TI__Mastermind 47895 points

    Hi User,

    I'll echo what Clemens said here. RS-485 compliant drivers such as THVD2450 are capable of driving a valid differential signal ( > 200mV) across a loaded and terminated system. Typical termination for such systems is two 120-ohm resistors at each end of the network (60-ohms equivalent load). Each transceiver also presents slightly more load to the bus. For this reason, THVD2450 specifies several of its driver parameters under the conditions of a bus load (RL) of 100 to 54-ohms. 

    In a single ended system, it is still important to include termination at the receiver to reduce signal reflections. However, because most of the signal energy will only be traveling in one direction, it is less important (bus still acceptable) to include termination at the transmitting node as well. In such cases, a designer may choose to only terminate the receiving end of the system using a resistor value that matches the characteristic impedance (typically 120-ohms) of the cable. 

    As Clemens says, such loads should pose no problem for THVD2450 which is capable of driving valid RS-485 signals through much larger loads. If your system could use a long cable or you are expecting to use high data-rates, you may want to consider a termination value closer to the cable's characteristic impedance to reduce the impact of signal reflections. 

    Let me know if you have any other questions.

    Regards,
    Eric Schott