The PCB design given by NXP AN11154 will demonstrate the function of NXP’s hardware and software but you may want to consider a few modifications before deploying it in a typical DMX environment. The most significant consideration will be whether isolation is needed on the DMX interface.

In many cases, isolation is required for the DMX interface to keep the signals within the common mode range of the transceiver and prevent ground loops. I used the ADM2587 for its high level of integration and small size. The compromise is that this transceiver solution is costly, offers only standard transient, surge, and over voltage immunity, uses 3.3 volt signaling, and requires special layout attention to keep RF emissions within FCC limits.

The design includes a capacitor to bypass between the isolated and non-isolated commons. This will only be populated if the layout doesn’t keep the switching noise of the ADM2587 contained and will be no larger than necessary to keep emissions within limits. The amount of transient energy that can pass from the isolated side to the non-isolated is proportional to the amount of capacitance between the power planes.

I added a TVS diode to the data lines that the reference design doesn’t have. Many transceivers have some ESD and transient protection integrated. This might be sufficient for many applications. The external TVS will increase the amount of transient energy the design will withstand without damage. There are more components I could have added to further increase the designs ruggedness but I wasn’t willing to increase the board outline to accommodate them.

The common mode choke on the DMX input decreases RF emissions and increases transient immunity. The cables connected to an electronic assembly can be a good antenna for RF emissions. Reducing high-frequency noise in the cables is a good strategy for emissions reduction. The common mode choke will attenuate noise present on the data and power nets that pass through the choke to the cable. The choke will also increase the impedance to an incoming transient event reducing the amount of energy the TVS devices will have to absorb.

In the USB signal path, the TVS diodes are replaced with a lower capacitance option. The MMBZ9V1AL diodes have a capacitance of 280pF and load the USB data lines with 140pF since they are in series. This load capacitance in the reference design is acceptable for low-speed operation but expect signal degradation at full speed. The Semtech Rclamp0582 at 1.2pF each is suitable for high-speed USB connections.

Since the transceiver being used is more power hungry than in NXP’s AN11154 an increased current capability is necessary. The SA57000 SOT23 packaged regulator is rated for 150mA and the ADM2587 can consume more than that itself. There is more than enough capacity in the ZLDO1117 in SOT223 as it is rated for over 500mA. It also is specified for use with low ESR ceramic capacitors. C4 and C5 in NXP’s design are distributed as general bulk capacitance for the board. My modifications include bulk capacitors associated with parts that need them.

Avoid the typical common mode DMX on EIA-485 problems by using a board design with isolated transceiver modification outlined above.

NXP_DMX

Adding internal planes to the board doesn’t automatically reduce emissions.  The amount of RF radiation from the PCB is proportional to the loop area of signals with high-speed transitions. The loop area of a signal is the path from the signal source to the signal receivers and back. Usually, the signal return path is through circuit common. By adding a ground plane over which the signal trace will run a capacitively coupled return path is added throughout the signal’s run length. This is why switching a design from two to four layers usually will reduce emissions. Other layout practices will also reduce emissions. Examples are don’t cross high-speed nets with nets connected to wires that go off the board, reduce signal path lengths, add a circuit common return net adjacent to the signal net, and there are many others. If you have specific questions or problems and would like to know more contact me.