ProtoDAC TDA1387 X8 project

[hifinet]

Looking at obtaining the parts for CMC filter. Everything is available except for the CMC. Parts cost would be $6.40. Mouser has the part on reorder with a long reorder time. The product is discontinued by the manufacturer. It might be available on the surplus market. 

The main problem with this design is that it is SMD, which locks you into specific size parts. This is critical with the SMD CMC. These parts have similar specs, but the footprints are different. Substituting through hole parts would make it easier. It could be constructed on a perf board. Soldering SMD is also very difficult with an iron.

UPDATE: I actually did find the CMC at LCSC (LCSC Part # C710586), which is the parts supplier for JLCPCB. They have 5 in stock. They are missing 4 parts from the BOM.

CMCs are also available on Ali. They are so cheap, they usually sell in multiples. A CMC is nothing more than two pieces of enameled wire wound around a core. The dot in the schematic indicates the top of the winding, and the lead numbers in the part diagram need to be followed. In this circuit, both wires start winding at the same end of the core. Common mode signals such as external EMI, RFI, etc. (identical signals in both + and -) cancel, and differential signals (5V) pass unimpeded. The core material and shape can affect the performance, as well as the windings, type of wire, etc. You just need to find one with similar specs that will fit the SMD pads on your PCB.

The inline choke (L2) helps to filter switching noise from the SMPS on the + line, along with the ceramic and tantalum caps on the output, which comprise an LC low pass filter.

It can help to wrap the SMPS lead with a clip-on ferrite bead or even better, wind it several times through a ferrite toroid. The long wire acts as an antenna, and it can feedback RFI into the SMPS error amplifier.