Anablog

I know Semtech are good people from meeting them when I was down in LA for an EDN editor’s conference. Not only that, my buddy Earl Craig from Building T works for them up hare in Silicon Valley. Semtech is doing more and more analog stuff like switching regulators. FET drivers and charge pumps but they also make some simple protection circuits that can save you a lot of grief. Because these filters, like ones STM makes, are so simple in concept, the press often overlooks them. Now it may seem pretty trivial to make a diode protection device, but to do it with really low stray capacitance (0.6pF) like ST does, or to combine R-C components like Semtech does in their EClamp™238xP, well that is really pretty high-tech, even if it does not involve any complex analog circuitry. It is a triumph of process engineering, physics and manufacturing, not circuit design. Semtech sent the press release for the Eclamp2384, Eclamp2384, Eclamp2388 (pdf) series of parts, the last digit indicating how many lines it protects.

Protecting your circuits from RF interference is getting to be a big deal. I called on a LAN VOIP phone maker when I worked for National Semi. They had interference getting into their audio circuit from a cell phone set down a foot away from their office phone. It took some board redesign and the addition of small capacitors on critical nodes in order to keep the cell phone RF out of the analog audio signal path. And one mistake engineers make is using big-value physically-big capacitors. You are trying to filter out RF at 900, 1,800 or 1,900 MHz. You need picofarads in a tiny 0402 or smaller case in order to have low impedance at those frequencies—be sure to look at the impedance plots for whatever cap you are thinking about using. Better yet, use parts like Semtech makes and rest assured that the parts will keep interfering RF signals out of your circuits.

I was trying to filter out RF form an office phone, if you are designing a cell phone or handheld it is even tougher because then you have inches, not feet of separation. Your product might have Bluetooth, 802.11 as well as camera and cell phone systems. Keeping the RF from all those interfering sources out of your circuitry can be tough. Like most EMI issues, this one comes up right before you want to ship, so be sure to have an understanding of these issues; they are coming up more and more often.