LVDS vs Various RS types

Thread Starter

Metalfan1185

Joined Sep 12, 2008
161
Hello all
I have recently been learning about LVDS and using it in some sample circuits i have made. I have a question:
I see a variance in LVDS and similar type of ICs with different categories. For instance: Comparing a DS90 series IC like the DS90LV011 (LVDS receiver) and a DS89C21 (RS422 Transceiver):
Can I drive the LVDS receiver with signal from the RS422 IC?
Can I drive a LVDS Tx into a RS422 receiver?
The signals look very similar.
I have also looked at the SN65HVD52, which is a RS485 transceiver. What is considered when trying to select these ICs for a particular application? The speed is far less than the max for each of these (my circuit is likely around 400kHz-800kHz) I just want to move data using a low-noise differential signaling on some twisted pair wiring.
 

SteveSh

Joined Nov 5, 2019
103
There are a number of app notes out there that address interfacing between rs-422/485 and LVDS. But at a top level, you can run a ‘422 diff pair into an LVDS rcvr providing you use resistor dividers to reduce the input voltage to the LVDS rcvr. But you cannot reliably drive a ‘422 rcvr with LVDS because the diff voltage swing is too low.
 

RPLaJeunesse

Joined Jul 29, 2018
64
I think of LVDS as best for 30MB/s and up, where the signal lines are treated as RF paths at low impedance and can be AC coupled with relatively small capacitors. That combination gives the differential pair good common mode rejection to things from DC to many 10s of kHz. RS-422 (and -485) are best at 10MB/s and down, and are almost always DC coupled so they will work down to DC. As such these need to deal with significant common mode (think TX ground-to-RX ground) noise & voltage, and will have much higher input voltage range to provide that wide common mode range. If the environment is especially noisy or high current (think big industrial motors, theater lighting, etc.) then add optical isolation to the RS-422/485 path. Now the common mode rejection jumps to 100s of volts or more.
 
Last edited:

andrewmm

Joined Feb 25, 2011
144
Good on you for looking into this,

Two type of signal categories, single ended and differential,
LVDS is the differential type, meaning you send a signal as two signals , a and b , and its the difference between the two signals that indicate a 1 or a 0.

The signals could be say +1 and -1 volt , which would give you a 2 volt differential, with a common voltage of zero volts,
for a 1 , you would put +1 on a, and -1 on b, for a 0 you would put -1 on a , and +1 on b,

The great thing about differential, is both a and b pick up interference the same, so for instance if you pick up 1/2 a volt noise, for a 1 a would now be 1.5v, b would be 0.5 v, which gives you the same 2 volts differential , and you ignore the interference.

Hold with this, Im getting there.

Most differential systems, are not ground referenced,
so for instance, you could have signal of 1v and 2 volts, gives a system with a common voltage Vcom of 1.5 volts, and if a is 1 and b is 2, thats -1v differential, a 0.

Now you have all the information you need

A driver has a Vout max and min, and a Vcom
your receiver has a Vcom and a Vmax / min in


Provided both the driver and the receiver have a Vcom that's acceptable, ( see the data sheet ) , and a Vh and Vl thats compatible.

If they are not, then a way around is to AC couple the signals . This works for clocks, for data you have to endure there is no DC in the FFT of the signal.
 
Top