TRS3122E: Source series resistors?

DIN is an input.

The ROUT output has a typical impedance of about 20 Ω. If the trace to your microcontroller is long enough so that you get transmission line effects, then adding a source termination resistor to match the characteristic impedance of the trace would be useful to prevent reflections.

Hi Jack,

They are not necessary - generally speaking you will not have a long enough trace from the microcontroller relative the maximum data-rate of the device for there to be major issues. Adding small resistors could help mitigate some higher frequency reflections - but generally its not a long enough bus to need it and it won't add much  benefit to the system.

Best,

Parker Dodson

Hi Jack

Normally it isn't needed - but it depends on the data-rate and rise time. In general how you approximate this is by looking at the relative wavelength of each harmonic of the incoming pulse train (worst case a string of alternating 0's and 1's i.e. 010101010) - generally worst case we only consider odd harmonics and the fundamental. If used I see usually up to the 7th or 9th harmonic - but each successive harmonic is going to much less energy where most of the energy on this pin would be DC, 1st, and 3rd harmonics and since this is a short bus in general I would most likely only focus on the fundemental and 3rd harmonic. 

For example if you use the 1Mbps data rate that is a base frequency of 500KHz 

Fo = 500KHz

F3 = 1.5MHz

F5 = 2.5MHz

F7 = 3.5MHz

F9 = 4.5MHz

If you divide the signal propagation speed (Vp = C * x ;  0 < x < 1) by the frequency you will get the wavelength (signal propagation speed is a fraction of C as conductors will not allow signal propagations up to the speed of light ) 

Assuming a VP of 0.78 * C

λ0 = 467.7 meters

λ3 = 155.9 meters

λ5 = 93.5 meters

λ7 = 66.8 meters

λ9 = 51.9 meters

Now reflections will become problematic when the electrical length of the circuit is greater than 1/10th of the wavelength.

So reflections could occur at bus lengths greater than:

46.77 meters for fundamental frequency

15.59 meters for 3rd harmonic

9.35 meters for 5th harmonic

6.68 meters for 7th harmonic

5.19 meters for 9th harmonic 

Please note how that even at 9th harmonic there is a 5 meter bus length before reflections. Considering RS-232 bus maxes out at 15 meters in the standard a 5 meter bus attached to a console facing pin would be extremely unusual/inadvisable as it should be much shorter. 

However since this isn't a perfect pulse train there is also higher frequency energy for transitions - which sets max length to transition_time / 10 * signal propagation speed (Vp). - however this device doesn't spec that time, but it should be fast enough that the approximations above are okay.

Please note that this example assumes the signal propagation speed is 0.78 * Speed of light - if the transmission line has a different propagation speed then calculations should be made to reflect that - but in general this shows that its not super likely that you will need to worry about reflections on these pins.

Best,

Parker Dodson