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Differential voltage of LTC2855

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dany.1986

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Hi,

Could someone please help me with understanding what the differential voltage is of the LTC2855 driver. From the datasheet I know that minimum VOD is 2 V and maximum VOD is VCC (3.3 V in this case). Maximum VOC is 3 V, so using the equations VOC+VOD/2 and VOC-VOD/2 and maximum values I get 4.65 V and 1.35 V with reference to ground. Am I right to say that because of that the voltage on pin Y is 4.65 V and on pin Z it is 1.35 V. The supply voltage for the device is 3.3 V. If I am wrong then could someone explain to me what the voltage would be on those pins, please? Another thing, if the voltage on one of those pins is 4.65 V then how is it generated, as the device supply is only 3.3 V?
 

KlausST

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Hi,

With rs422/rs485 drivers only the differcial voltage is of interest. The voltag referenced to gnd is not very informative.

When a min diff voltage of 2V is given at a 3.3V system, then maybe the one is at 0.5V and the other at 2.5V.
Max possible output voltage is, when both outputs are at their supply rails. So one is at 3.3V , while ghe other is at 0V.
This is max diff voltage if Vcc, = 3.3V.

Vocm that is given with 3.0V makes no sense with 3.3V supply. When one output is at ghe upper rail of 3.3V, then the other must be at 2.7V, but this doesn't fit to the 1.5V min diff voltage. I don't know...

Klaus
 

dany.1986

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Thank you for your answer. What I am trying to figure out is what voltage would be seen by other device connected to pins Y and Z (for example differential inputs of another LTC2855). Do you mean that the maximum voltage on those pins could be up to 3.3 V? And according to the datasheet the minimum voltage would be 2 V? If that is the case, then does it mean that as per minimum VOD (2 V) the voltage on pin Y could be 1.5 V and on pin Z 0.5 V (or 1.2 V and 0.8 V or 1.7 V and 0.3 V, and so on as long as both voltages add up to minimum VOD which is 2 V)?
 

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Hi,

on pin Y could be 1.5 V and on pin Z 0.5 V (or 1.2 V and 0.8 V or 1.7 V and 0.3 V, and so on as long as both voltages add up to minimum VOD which is 2 V)?
No, they don't ADD up to 2V, the DIFFERENCE is 2V. So if one is 1.2V, then tge other has to be 3.2V. 0.8 --> 2.8,
1.7 is not possible. 0.3V --> 2.3V

Klaus
 

dany.1986

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Thank you for your help.I understand it finally.
 

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