To choke or not to choke?

[Richard Rasker]

Hello,

I regularly design relatively simple stuff with PIC and Atmel controllers
and the likes, and when I need to interface with the outside world by means
of switches or other slow interfaces, I simply use a series resistor
between 100R and 1K, followed by a capacitor to ground, value between 1nF
and 100nF (depending whether it's a scanning matrix or not, and the desired
speed).

Now I ran into a bit of a dispute with another designer who says that this
is a totally wrong approach, and that using those 3-terminal chokes instead
of series resistors and ground capacitors is far better (also when driving
any significant current).
Indeed I often see 2- or 3-terminal chokes in series with supply and I/O
lines of existing digital stuff, but even more often, they're nowhere to be
seen.

My question: what advantage has a microhenry (or even less) choke compared
to a well-defined RC filter? I understand the need to prevent HF from both
entering and leaving the circuitry, and compliance with EMC regulations is
very important, but do those little chokes actually do a better job in this
respect than simple RC?

Thanks for any leads,

Best regards,

Richard Rasker

 

[Joerg]

Hello,

I regularly design relatively simple stuff with PIC and Atmel controllers
and the likes, and when I need to interface with the outside world by means
of switches or other slow interfaces, I simply use a series resistor
between 100R and 1K, followed by a capacitor to ground, value between 1nF
and 100nF (depending whether it's a scanning matrix or not, and the desired
speed).

Now I ran into a bit of a dispute with another designer who says that this
is a totally wrong approach, and that using those 3-terminal chokes instead
of series resistors and ground capacitors is far better (also when driving
any significant current).
Indeed I often see 2- or 3-terminal chokes in series with supply and I/O
lines of existing digital stuff, but even more often, they're nowhere to be
seen.

My question: what advantage has a microhenry (or even less) choke compared
to a well-defined RC filter? I understand the need to prevent HF from both
entering and leaving the circuitry, and compliance with EMC regulations is
very important, but do those little chokes actually do a better job in this
respect than simple RC?

Chokes can be dangerous if the inductance is larger, because of
inductive kickback. The uC is usually going to be ok because it has
subtrate diodes but the stuff on the other side may not.

RC is just fine when the layout is clean. For super-sensitive apps (and
that's hardly a uC) the 3-terminal EMI filters from companies such as
Murata are nice because they efficiently suppress cell phone signal. But
I have needed those only in analog settings.

 

[Nemo]

I regularly design relatively simple stuff with PIC and Atmel controllers
and the likes, and when I need to interface with the outside world by means
of switches or other slow interfaces, I simply use a series resistor
between 100R and 1K, followed by a capacitor to ground, value between 1nF
and 100nF (depending whether it's a scanning matrix or not, and the desired
speed).

Now I ran into a bit of a dispute with another designer who says that this
is a totally wrong approach, and that using those 3-terminal chokes instead
of series resistors and ground capacitors is far better (also when driving
any significant current).
Indeed I often see 2- or 3-terminal chokes in series with supply and I/O
lines of existing digital stuff, but even more often, they're nowhere to be
seen.

My question: what advantage has a microhenry (or even less) choke compared
to a well-defined RC filter? I understand the need to prevent HF from both
entering and leaving the circuitry, and compliance with EMC regulations is
very important, but do those little chokes actually do a better job in this
respect than simple RC?

The other designer knows SOME of the story but lacks some of the
history. They simply allow plenty of current through. They were very
popular for a while for power rails. Also they let fast signals through
which 1kohm + 1nF will not.

Unfortunately they tend to resonate too. As the EMC standards increased
the upper frequency you had to test to, and as mobile phones became more
common, first at 800MHz then 1.6GHz then higher again, they became more
of a problem than a help - they actually amplified noise into sensitive
circuitry. So they fell out of favour.

The manufacturers caught up with the frequency requirements (1GHz) but
now the standards bodies test to 2GHz or more... the problems are not
apparent until you do a full EMC sweep over hundreds of MHz, so some
people think they are great.

The key parameter when choosing whether to use them is the speed of the
signal going through them. If you can use a 1k resistor and 1nF cap then
you are filtering much more effectively than one of these composite
filters can manage, until the RC begin resonating from their own self
inductance at hundreds of MHz (so use small surface mount ones). Another
reason not to use them is when you intend the product to still be
manufactured in 5 years' time. If however you need to filter USB signals
or get significant power through a line, then use a ferrite bead and a
capacitor, or one of these filters instead of an RC. Personally I prefer
a discrete L + discrete C instead of these filters as there are more
options if you need a second source or a change to the characteristics,
and because every time I've used a 3 terminal filter it has gone
obsolete within a few years or resonated when the EMC regs were extended.