Why Series resistor (50ohm) placed at the I/P of SPDT switch IC (TMUX1119)

dcbingaman

Joined Jun 30, 2021
498
It appears they are trying to demonstrate typical 50 ohm impedance signal sources. These are common in RF designs and function generators also tend to have outputs of 50 ohms. Here we are transferring power to R3. Apparently the output on D could be a transmission line. In order to avoid reflections due to mismatched impedances, we keep the source impedance equal to the load impedance. The intrinsic impedance of typical coax cable is 50 ohms.

https://www.allaboutcircuits.com/te...-50-question-impedance-matching-in-rf-design/

This is application specific.
You do not need this for most switching applications. The other reference designs I saw did not use any series resistance to the inputs of the chip.
 

Thread Starter

hoyyoth

Joined Mar 21, 2020
170
Hi Dcb,

What I can see from my simulations is series resistor reduces the switching spikes,but don't know how.
Please see the below images.
WITHOUT USING 50ohm
1634661407288.png
WITH 50 ohm
1634661435385.png
MY CIRCUIT

1634661527377.png

Regards
HARI
 

dcbingaman

Joined Jun 30, 2021
498
Hi Dcb,

What I can see from my simulations is series resistor reduces the switching spikes,but don't know how.
Please see the below images.
WITHOUT USING 50ohm
View attachment 250623
WITH 50 ohm
View attachment 250624
MY CIRCUIT

View attachment 250625

Regards
HARI
The ringing is caused by the inductance of the wiring (normally). By using series resistors you create an RC time constant with the capacitance C1 with no series resistor is very small. By adding the series resistor you slow down the ramp rate. This can be useful when you have extremely fast edges. These can inductively couple to other parts of a circuit causing inductively coupled noise. By slowing down the ramp rate you take away a lot of the high frequencies that make up the square wave.
In many of the digital systems we developed it was common to place '0 ohm' resistors on signal trace paths on the board. If we had excessive ringing or cross talk we could replace them with some small resistance (usually only a few ohms) and it corrected these issues.
 

Thread Starter

hoyyoth

Joined Mar 21, 2020
170
Hi Dcb,

I kept a resistor of value 61.9 in series with S1 and S2 and solved the issue of overshoot and undershoot.

May I know how that resistor is reducing the overshoot and undershoot

Regards

HARI
 

DickCappels

Joined Aug 21, 2008
7,984
I sometimes do this to stop ringing from causing problems on hand-made boards where impedance is not well controlled. The ringin waveform is rolled off by the resistance and the input capacitance of the chip forming an RC low pass filters. This is especially useful when working with edge-sensitive inputs (only one edge for each transition).

1634809887270.png
 

nsaspook

Joined Aug 27, 2009
8,907
Hi Dcb,

I kept a resistor of value 61.9 in series with S1 and S2 and solved the issue of overshoot and undershoot.

May I know how that resistor is reducing the overshoot and undershoot

Regards

HARI
You can look that the reason why in several ways. Looking at the signal in the frequency domain is the common way but you can also use the time domain of reflections of energy to understand the overshoot and undershoot.
1634831816809.png
Frequency

1634831850227.png
Time, what you will see on an oscope. For high speed digital ringing signals on a PCB, transmission line analysis with propagation speeds, trace lengths and terminations are what provides the best intuition for ringing IMO.

https://www.diodes.com/assets/App-Note-Files/AN022-P.pdf

1634832594804.png
 
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