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Differential signal working

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Bjtpower

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Hello Friends

I was checking about the RS232 and RS 485 difference Points.
where everyone differentiate of noises.

By seeing attached pictures,at the comparator output is wrong
the output should be Vout=(V+)-(V-) and it would result zero signal but the image shows something else.

How differential signal actually works, i know the if the both trace distance is equal it would result same signal respectively.

Anyone knows better about the signaling,pls answer
diff signal.jpg

Regards
Marx
 

By seeing attached pictures,at the comparator output is wrong
Without a specification of the "RX" block, I can't decide if the waveform or your expectation is wrong. How do you derive that it's a "comparator"?

A RS232 or RS485 receiver would in fact show a digital output signal, input noise will be either suppressed or generate full logic level glitches.

Whatever is said about the schematic diagram, I believe it solely wants to visualize the differential signaling advantage of better suppressing common mode noise, which is a matter of fact.
 

Without a specification of the "RX" block, I can't decide if the waveform or your expectation is wrong. How do you derive that it's a "comparator"?

A RS232 or RS485 receiver would in fact show a digital output signal, input noise will be either suppressed or generate full logic level glitches.

Whatever is said about the schematic diagram, I believe it solely wants to visualize the differential signaling advantage of better suppressing common mode noise, which is a matter of fact.

Let say suppose it will differential amplifier,
And by seeing at the Input of Amplifier again the output signal would be zero i guess..

- - - Updated - - -

OK,
I understood.

i was considering +5V and -5V insted of 0
now by common mode noise will present equally on both the signal and amplitude will be same so it will either High or Low

Thanks for Explanantion
 

Hi,

the picture is correct.

By seeing attached pictures,at the comparator output is wrong
the output should be Vout=(V+)-(V-) and it would result zero signal but the image shows something else.
I can´t see anything wrong.

the output should be Vout=(V+)-(V-)
True

it would result zero signal
False

****
A RS485/RS422 receiver can be ssen as comparator.

****
Let´s assume all signals are 0V / 5V levels

Lets´call:
Single ended input: SI
Differential noninverted signal: DP
Differential inverted signal: DN
Single ended output: SO

SI = 0V --> DP = 0V, DN = 5V --> difference (DP-DN) = (0V - 5V) = -5V = negative --> SO = 0V
SI = 5V --> DP = 5V, DN = 0V --> difference (DP-DN) = (5V - 0V) = +5V = positive --> SO = 5V

now let´s assume we have +4V "noise" introduced in the differential lines:
SI = 0V --> DP = 4V, DN = 9V --> difference (DP-DN) = (4V - 9V) = -5V = negative --> SO = 0V
SI = 5V --> DP = 9V, DN = 4V --> difference (DP-DN) = (9V - 4V) = +5V = positive --> SO = 5V
Mind: the differential lines are considered to be routet in parallel. Therefore noise is introduced to both signals equally.

***
Note that a RS485 transmitter usually sends out signals with max. 0V 7 5V levels. But the receiver can handle at least -7V ... 12V levels.
So even if a receiver is supplied with 5V only, it is normal operational condition if input voltage is -7V up to +12V.
This also means that you should not limit input voltages to 0V/5V. This may decrease safe operation, because the current in the lines is not symmetric anymore.


Klaus
 
By seeing attached pictures,at the comparator output is wrong
the output should be Vout=(V+)-(V-) and it would result zero signal but the image shows something else.

How differential signal actually works, i know the if the both trace distance is equal it would result same signal respectively.

Anyone knows better about the signaling,pls answer
View attachment 140102

Regards
Marx

If you pay attention the signal and noise waveforms,you will see that the signals are differential so the phase is 180 degree but the noise has 0 degree.The signals are combined at the end by 180 degree, but the noises are combined 0 degree so the noise is easily slipped from the signal because the signal is differential, the noise is Common Mode.Briefly, the noise will be suppressed as much as CMRR of the amplifier.
 
What is this CMRR..??

i dint get the term.. Can anyone explain the same with Example.
always datasheet mention CMRR 50-100DB
What does that really tell to the user..?
 

What this stands for..??
What is this +-70V/+-30V in attached pictures

rs485.jpg
 
Last edited:

Hi,

Read datasheet section 8.3, especially 8.3.1
Then tell us what is unclear in this description.

Klaus
 

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