Other Parts Discussed in Thread: SN65HVD24, THVD1450, STRIKE
Hi There
We have a project where we are trying to communicate between two nodes where the worst case distance is 1000m. We are using a cat5e cable and need a communication speed of 500kbps.
Looking at using the THVD1550 for this use case.
1. From section 7.7 in THVD1550, a rise time of 400ns gives me a signal BW of 875KHz.
2. The attenuation of cat5e cable at 850Khz is approx 2dB/100m. So for 1000m, the attenuation would be ~ 20dB.
http://www.interfacebus.com/24AWG_Attenuation-vs-Frequency.html
3. From section 7.6 The THVD1550 driver differential voltage is 2.7V. A 6dB attenuation would half the voltage. For 20dB, we would be halfing it 3 times atleast.
so the voltage at the end of 1000m would be ~ 337mV. Cat5e has a characteristic impedance of 100ohm, so If I have terminated the lines with 100ohm resistor to avoid reflections, I would again half my voltage to get 168.5mV
4. The sensitivity of the THVD1550 receiver is 200mV. So this design would not work for 1000m.
Is my understanding correct from the above math?
A follow up question I have is about the following TI document w.r.t THVD1550
Application Report SLLA375
The eye diagrams at section 4.3 (differential receiver input) does not show any loss of signal due to signal loss on a cat5e cable. Why is that? 3000ft at 2Mbps having a signal amplitude of around 2V pk-pk is quite hard to believe. The image seems to have a 1V/div on the amplitude.
Also, from table 1 3000ft at 2Mbps shows a jitter of 73.28% and the eye diagram shows a jitter of 5%, so there seems to be typo there or I am missing something critical. Hopefully someone can clear my confusion.
Thanks
Raghu