Ask Me Anything about Component Selection

Participate in our next Ask Me Anything about component selection where you can ask us questions on:

• Choosing components for reliability and performance
• Navigating supply chain challenges
• Ensuring compatibility with existing designs
• Cost-effective component selection
• And much more!

Post your component selection questions below before June 12th and our experts @steve.carney, @allank, and @atar.mittal will get you the answers!

How do you determine the best components for achieving controlled impedance?

Hi William!

As with any other project in life, we have some “ideal” choices and some “pragmatic” choices. If I could use 26.5 GHZ rated SMA connectors instead of less costly 18 GHz ones I would. If I could use JANTX components instead of inexpensive passives from China I would. And if I could make every board as big as I wanted and with as many layers as I wanted, I would. But, here in the real world we rarely have that free dom. The question itself is too broad to have a specific answer, sort of like “What’s the best car”, but I can give you some bit’s and pieces to point you in the right direction.

Let’s start with the simplest. Resistors. In the past we were often constrained to using ten percent and even 20 percent tolerance parts. Fifty ohm resistors, not being a standard value, weren’t even on the menu. When we needed a fifty ohm termination the closet standard value was 47 ohm. A 47 ohm resistor with a twenty percent tolerance could range from 37.6 ohms to 56.4 ohms. Not very encouraging. Today however there is almost no cost penalty for using one percent resistors. So use them everywhere.

Capacitors usually have three main issues – cost, size, and dielectric. These are in fact all related. One particular dielectric, say Y5V will offer capacitance values of a different range than for instance, an NPO or X7R. These differences are due to the ceramics used and those ceramics are primarily chosen for their behavior over temperature. The more behaved (or wider) the temperature range the less capacitance they can squeeze out of a particular ceramic. So the (limited) big picture is that NPO/COG ceramics (made from calcium zirconate) are very stable over temperature but only offer capacitance values of 0.5 pF to 0.1uF (and the higher values are likely only in larger case sizes). If you need less stability and more capacitance, you can use X7R for example (Barium Titanate) and get capacitance vales as high as 22uF. So for bypassing an IC, you would likely choose an X7R part, but in those critical tuning circuits or for coupling stages in a high frequency path stick to COG/NPO to keep your impedance constant.

Inductors might be the trickiest since it seems the differences are not as well documented or explained. I worked for a filter company who made pretty high performance filters. One of their customers insisted they weren’t giving out the correct schematic information because they could not duplicate the performance. What were they doing wrong? Well, they took regular old silver coated, 26 AWG buss wire and wound their coils around a form, measured the correct inductance, and essentially copied all the required components closely. Just not closely enough as we were using solid silver wire and thereby gaining a slightly improved Q in our coils. The devil is in the details.

Use the best you can, and think carefully through what you trying to do.

PS: There is a lot of information on the net, but one of the best I ran across was: https://www.kemet.com/en/us/technical-resources/heres-what-makes-mlcc-dielectrics-different.html

image002.jpg

3 Likes

Thanks a lot Allank for the detailed answer!