I am designing a dumb "isolation box" that is to sit between a computer and medical data acquisition equipment. The purpose of this box is to inject power and to provide communications and digital signal isolation. The overall cable length that the digital signals (short pulses >10µs) will travel is 10m or less, but edge integrity is critical.
On the isolated side of the box I only have access to 48V DC, and would like to avoid adding another over-specified switching power supply just to power the digital interface (I already need one for the non-isolated side). I would like to get away with just "passive" optocouplers and a minimum of additional circuitry on the data acquisition equipment side, but I am concerned with proper termination of the 100Ω differential pair.
The receiving side seems simple enough as it would be a relatively high-current signal driving the optocoupler LED (and a balancing diode) that can use a 100Ω resistor on each end of the line.
simulate this circuit – Schematic created using CircuitLab
The transmitting side seems more problematic as any solution I can think of would imply a reduced signal level that would then require a level shifter or comparator on the other side (such as this one) which would then require a few more components to function and to add proper ESD protection.
Am I over-complicating this? Should I just byte the bullet and add the wasteful power supply and a couple of RS-422 transceivers at each end?
Best Answer
Opto Isolators are non-ideal in many ways, including speed and difficulties with charging the driven cable capacitance.
You would be better to consider the new generation of digital isolators such as those made by TI.
For example if you are isolating at the send (Tx) side, the TI ISOW7821 is a two channel isolator that can be run from a 3.3 - 5V supply. It's good for 100Mbps so should be adequate for your 10us pulse/timing requirement. If you can't find a 3.3-5V supply then you'd have to generate this.
This application shows a CAN Bus being drivern, but the isolator could be used to drive your signal cable with almost any configuration you want.