Do I neeed to connect a 50ohm resistor in series with an SMA connector

Thread Starter

hoyyoth

Joined Mar 21, 2020
192
Dear Team,

I have a circuit that accepts I2C SDA coming from another board via SMA cable.Do I need to put a 50ohm series resistor in between J27 and J26.
1636184255339.png
The signal from J27 will be given to a SPST switch and Switch output is given a level translator (1.8V to 5V) and the output of the level translator will be connected to an Arduino board(This is my signal Chain).

Regards
HARI
 

Ian0

Joined Aug 7, 2020
4,119
I2C is a short length transmission system which doesn't need matched impedances, (Maximum speed = 400kHz, maximum distance = 1m) λ/4 at 400kHz is 187m, so it is so much less than quarter of a wavelength that transmission line theory doesn't apply and you can regard it as bulk capacitance and bulk inductance.

So you don't need a 50Ω resistor, but why are you using a SMA connector, when you really need a 3 pin connector to connect SDA, SCL and ground? SMA would tend to indicate a screened cable, and really I2C could do without the extra capacitance.
 

Thread Starter

hoyyoth

Joined Mar 21, 2020
192
I2C is a short length transmission system which doesn't need matched impedances, (Maximum speed = 400kHz, maximum distance = 1m) λ/4 at 400kHz is 187m, so it is so much less than quarter of a wavelength that transmission line theory doesn't apply and you can regard it as bulk capacitance and bulk inductance.

So you don't need a 50Ω resistor, but why are you using a SMA connector, when you really need a 3 pin connector to connect SDA, SCL and ground? SMA would tend to indicate a screened cable, and really I2C could do without the extra capacitance.
Hi Ian,

Thank you very much.
The other board where the I2C signal coming from is not designed by me.The other team which designed
it already kept SMA connectors in that Board.
That board is an ASIC evaluation board . Most of the time it will be inside a thermal chamber.
My board will be sitting outside.

Regards
HARI
 

Ian0

Joined Aug 7, 2020
4,119
OK. Make sure that you label which is SDA and which is SDL, and don't run the I2C too fast and it will work. Just check the SCL waveform on a scope and make sure its risetime isn't too slow for the frequency you are using.
 

Thread Starter

hoyyoth

Joined Mar 21, 2020
192
OK. Make sure that you label which is SDA and which is SDL, and don't run the I2C too fast and it will work. Just check the SCL waveform on a scope and make sure its risetime isn't too slow for the frequency you are using.
Hi Ian,

Than you .I will take care of the rise time.
Our CLK frequency is very low .It is less than 100Khz.

Regards
HARI
 

MrChips

Joined Oct 2, 2009
25,036
You are showing J26 and J27 as mechanical connectors. 50Ω series resistor does not apply in this case.
What you need to examine is the input requirements of the receiver chip that receives the signal.

If the cable is a 50Ω coax cable, then the center pin at J26 needs to be terminated with 50Ω impedance to ground in order to minimize signal reflection. You need to look at the datasheet of the receiver and determine how this terminator resistor will affect the performance of the receiver.

Show us the circuit is connect to J27.
 
Top