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I am using this MAX9278 deserializer.

The inputs to the IN+, IN- pins of the deserializer are twisted pair inputs.

On the IN+ and IN- serial link, I have 2 AC coupling capacitors of 220nF 50V on each link and parallel termination resistors of 49.9K from the IN+,IN- line to GND.

I am just trying to figure out what would be the voltage on the serial link (IN+ and IN-) lines so as to calculate the power dissipation of the 49.9k parallel termination resistors and also find whether the capacitors are properly rated in voltage.

What would be the voltage on the serial link?

enter image description here

JRE
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  • It depeds on what DC bias the GMSL transmitter device applies on the bus. Do you know how the transmitter you have works? – Justme May 01 '21 at 08:59
  • You should not connect 49.9k resistors from either input pin to ground. – Andy aka May 01 '21 at 09:16
  • I have edited the question and added an image. That's the resistor and capacitor I am talking about. The image can be found on page 74 of the datasheet attached in the question –  May 01 '21 at 10:36
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    @Newbie The schematics you posted verifies the DC bias voltage is set by the transmitter and is unknown. The resistors see an average voltage set by the transmitting side. You can easily find it out from the transmitter datasheet which you can open yourself, but it does not answer the general case with another GMSL transmitter. – Justme May 01 '21 at 13:22
  • @Justme, thank you for the clarification –  May 02 '21 at 06:28

1 Answers1

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Consider the smallest resistors in size (taken from here): -

enter image description here

A typical 0603 resistor is rated at 100 mW and a safe power dissipation will be 50 mW. So, to dissipate 50 mW in a 49.9 kΩ resistor requires a voltage across it of: -

$$V = \sqrt{\text{power x ohms}} = 50 \text{ volts}$$

So, the question really boils down to how much voltage can the serializer produce? For certain, it will be a few volts maximum or it would potentially destroy the deserializer: -

enter image description here

Given that the deserializer cannot survive more than 1.9 volts on the input, the power dissipation can be seen to be trivial.

Andy aka
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  • thank you for the answer. It had never occurred to me to approach the answer in this reverse way. Thank you for the help. –  May 02 '21 at 06:28