I am integrating a speed sensor (Honeywell SNDH-T4L-G01) which comprises 4 lines: VDD, VSS, A, B. The last two signals provide speed and direction information in a quadrature format.
These are open collector drivers, therefore in the control unit side a pull-up resistor shall be provided.
As the cable length to the sensor is around 3-4m, I am concerned about the capacitive coupling phenomena, as the quadrature signals are expected to switch at 7khz worst case in a quadrature fashion. I expect that both signals should cross-talk each other due to the weak pull-ups.
Cable shielding is mandatory, however the classical shielding schema (aluminium braid shielded cable with GND connection in one end) would not solve the crosstalking issue in my opinion, due to the capacitance of the line, the weak pull-up of the lines and the strong pull-down driver of the sensors.
Is it a good solution that I shield separately (VDD+A) and (GND+B) with the GND connection (and thus the connection between these shields, to avoid loops) being made at the control unit end?
I know trial and fine tuning is usual in these issues, but I want to start with the best shot possible.
Best Answer
Your main problem is that the relatively sharp high-to-low transition on an open-collector output could cause a false high-to-low transition on the other channel (when in the high state), by capacitive coupling between those two wires. Transition in the other direction is much less sharp and thus less likely to cause issue, as noted in that other answer.
Shielding each channel independently would greatly improve the problem. With two pairs, it is better from that standpoint to have a pair for Gnd and A, and another for Vcc and B (than it is to have a power pair and a signal pair). Individually shielded pairs will help, especially if the shielding is grounded.
There are options to be less dependent on cabling