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CM Chokes sometimes suck

hired an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The miniture CM chokes are 1k at 100MHz (1.6uH).

directly to ground, the other thru a Pi filter (220nF, 100K, 220nf) to a Hi Z ADC input. The caps are X7R. The signal BW < 1.0Hz. The noise bandwidth is >10 MHz but the Pi filter has a BW of >> Regards, Harry

then

is

range.

this

sold

Jfets are pretty good, too. Somebody, National maybe?, did a paper on opamp RF sensitivity. And somebody makes an IA that has explicit RF rectification protection.

Some opamps make damned fine RF detectors.

We have another recent bead application. We make a pair of VME and VXI crate controllers, and we have to drive a shipload of address and data lines, and VME is a messy bus, impedance-wise. We were seeing some serious undershoots along the bus, -1.5 or so, because modern bus drivers are screaming fast. And we had crosstalk concerns. We are spinning the board layouts anyhow, so we put a mess of tiny quad beads on the board, in series with all the VMEbus lines. So now we have a nice 3 ns falling edge and about half a volt of max undershoot, a downright purty waveform.

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John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin
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Yeah, if you're wrong when you say to take them out. It's never going to be a 'no choice now' situation, though: stand and fight. The defensive "do EVERYTHING we can" style is bad engineering, bad tactics. Your job is to use planning time and resources to good effect, and project hardware bloat is NOT a good effect.

"No evil, no matter how small, ought to be tolerated." -- Aesop

Reply to
whit3rd

============ we are talking about a few pennies worth of parts, i'm not sure I would use the word evil.

if the EMI guy is responsible for the EMI compatibility issues and he wants the CM filters in, I would let it go. It's his contribution to the effort, do not minimize it.

You never know, there may be 5 GHz RF fields and the CM filters help.

You never know, some issue, EMI or otherwise, may come up in the future and having those CM filter pads on the PWB may be helpful. It's cheap insurance.

Most importantly, maintaining a good working relationship with other members of the team is worth ,much more then the few cents for the parts. If the customer insists on the EMI guy being part of the team, I would even suggest soliciting the EMI guys input at the start of the next project.

The real issues here are much larger then engineering.

Mark

Reply to
Mark

Clearly Aesop wasn't an engineer.

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John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

hired an EMI guru. He insists on placing CM chokes on every single ended I/O signal line. These signals can be floating 100K thermistors that are received by a Pi filter (100K resistor and two 1.0uF caps to ground). The miniture CM chokes are 1k at 100MHz (1.6uH).

Have you killfiled google?

--
?? 100% natural 

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Reply to
Jasen Betts

If the level of evil is guaranteed to be substantially less than the specified limit, it's probably overdesigned.

Reply to
Spehro Pefhany

But they do hurt. They cost money. They do not provide the intended value. And worst of all they teach erroneous design techniques to the easily swayed. Costing lots of money permanently.

A far better design would use a tapped inductor 2 turns to the outside world per turn to inside on the two sides of the tap; then place a cap to ground at the tap. Toroids or closed pot cores preferred.

Measure the performance difference in front of management.

?-)

Reply to
josephkk

signal is "single ended".

That is a very different case. The CM choke might actually help. Drawing the circuit diagram.

___________@@@@___________> to measuring circuit Therm O=======CMchoke===

Reply to
josephkk

Now what's a deecee-bel? Our MBA professors never mentioned ... oh wait, it's a cheese, right? Yes, it is a cheese!

formatting link 

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Custom inductors to save money?

Why not a 5-cent dual ferrite bead?

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John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin

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