Analog Switch for Shunting in Resistor on RS485 Lines

[HarryRai1984]

Hi,

I am trying to implement a circuit in which a 120R line termination resistor needs to be connected between the RS 485 A/B lines of a transceiver, driven by a Microcontroller Pin. So the logic is that if Micro Pin = +5V, then analog switch closes, and 120R termination resistor is connected between the A/B lines, and if Micro Pin = 0V, then switch opens, and 120R resistor is no longer in circuit...

The trouble I am having is that most Analog Switches are CMOS logic based, and have an acceptable range of -0.5V to VDD+0.5V (typically). The VDD for this circuit is +5V so my concern is that using an Analog Switch (like the MAX4626) would not be sufficient as the differential voltage across the A/B lines can be > +7V. The MAX4626 has a typical 0.4Ohm ON Resistance and a low RON is needed as well.

As such, is the best option to use a relay (with low contact resistance)?? I have looked at the PhotoMOS relay series from Panasonic and Avago, and am leaning towards these, but if anyone can suggest an alternative approach for consideration, that would be appreciated.

Regards,
Harry

 

[Skogsgurra]

The termination resistor and switch do not need to handle DC. It is there to avoid impedance mismatch. So, it would be possible to add an HF capacitor in series with the resistor. That eliminates the DC restrictions but you still have to handle the full signal swing.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[benta]

Use a photorelay, then all your problems are gone.

http://www.toshiba-components.com/ParamSearch/photo_photorelay.html

Benta.